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Human Biology

Human Biology

Human Anatomy and Physiology

Christine Miller

Thompson Rivers University

Kamloops

Contents

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Chapter 1 - Nature and Processes of Science

1.1 Case Study: Why Should You Learn About Science?

Created by: CK-12/Adapted by: Christine Miller

1.1.1
Figure 1.1.1 Excitement!

Case Study: To Give a Shot or Not

Samantha and Aki are expecting their first child. They are excited for the baby to arrive, but they are nervous, too. Will the baby be healthy? Will they be good parents? It seems like there are a million decisions to make. Will Samantha breastfeed or will they use formula? Will they buy a crib or let the baby sleep in their bed? Samantha goes online to try to find some answers. She finds a website by an author who writes books about parenting. On this site, she reads an article that argues that children should not be given many standard childhood vaccines, including the measles, mumps, and rubella (MMR) vaccine.

The article claims that the MMR vaccine has been proven to cause autism. It gives examples of three children who came down with autism-like symptoms shortly after their first MMR vaccination at one year of age. The author believes that the recent increase in the incidence of children diagnosed with autism-spectrum disorders is due to the fact that childhood vaccinations have also increased.

Samantha is concerned. She does not want to create lifelong challenges for her child by increasing his risk of autism. Besides, aren’t diseases like measles, mumps, and rubella basically eradicated by now? Why should she endanger the health of her baby by injecting him with vaccines for diseases that are a thing of the past?

A sleeping infant swaddled in blankets.
Figure 1.1.2 When infants are too young to receive vaccinations, they are protected from contracting life-threatening diseases by the immunity of those around them who have received their vaccinations.

Once baby James is born, Samantha and Aki bring him to the pediatrician’s office. Dr. Rodriguez says James needs some shots. Samantha is reluctant and shares what she read online. Dr. Rodriguez assures Samantha that the study that originally claimed a link between the MMR vaccine and autism has been found to be fraudulent, and that vaccines have repeatedly been demonstrated safe and effective in peer-reviewed studies.

Although Samantha trusts her doctor, she is not fully convinced. What about the increase in the number of children with autism and the cases where symptoms of autism appeared after MMR vaccination?  Samantha and Aki have a tough decision to make, but a better understanding of science can help them. In this chapter, you will learn about what science is (and what it is not), how it works, and how it relates to human health. At the end of this chapter, you will learn how Samantha and Aki use scientific evidence and reasoning to help them decide whether they should vaccinate their baby.

As you read this chapter, think about the following questions:

  1. How do you think the quality of Samantha’s online source of information about vaccines compares to Dr. Rodriguez’s sources?
  2. Do you think the arguments presented here that claim that the MMR vaccine causes autism are scientifically valid? Could there be alternative explanations for the observations?
  3. Why do you think diseases like measles, polio, and mumps are rare these days? Why are we still vaccinating for these diseases?

Chapter Overview: Science

In this chapter, you will learn about the nature and process of science. Specifically, you will learn about:

  • What science is and the types of questions it can answer.
  • How scientific knowledge advances through systematic and repeated experimentation and testing.
  • How scientific ideas are open to revision, although sound scientific ideas can withstand repeated testing.
  • What a scientific theory is and how it differs from common usage of the word “theory.”
  • Examples of scientific breakthroughs in biology, including the development of the first vaccines, Mendel’s laws of inheritance, and the germ theory of disease.
  • The scientific method, how it is used to answer scientific questions, and how it is often a nonlinear and iterative process.
  • How scientific experiments are designed and carried out, including the use of controls, the manipulation of variables to test the effects on other variables, and ways to minimize sources of error.
  • The importance of Traditional Ecological Knowledge and the ways in which knowledge can be collected and handed down through many generations.
  • Characteristics of pseudoscience, which is defined as a claim, belief, or practice that is presented as scientific but does not adhere to scientific standards and methods.
Attributions

Figure 1.1.1

 Excitement by Randy Rooibaatjie on Unsplash, used under the Unsplash License (https://unsplash.com/license).

Figure 1.1.2

Baby in black and white swaddle blanket, by Julie Johnson on Unsplash, used under the Unsplash License (https://unsplash.com/license).

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1.2 What is Science?

Created by: CK-12/Adapted by: Christine Miller

1.2.1 Vaccine
Figure 1.2.1 Getting vaccinated.

Ouch!

The person in Figure 1.2.1 is getting a flu vaccine. You probably know that getting a vaccine can hurt — but it’s usually worth it. A vaccine contains dead or altered forms of germs that normally cause a disease, such as flu or measles. The germs in vaccines have been inactivated or weakened so they can no longer cause illness, but are still “noticed” by the immune system.

They stimulate the immune system to produce chemicals that can kill the actual germs if they enter the body, thus preventing future disease. How was such an ingenious way to prevent disease discovered? The short answer is more than two centuries of science.

A young child in Bangladesh is covered with skin lesions from smallpox. The scarring covers the child's face, including lips and eyelids, as well as the torso and arms.
Figure 1.2.2 A young child in Bangladesh is covered with skin lesions from smallpox. Until it was eradicated, this highly contagious infection caused many deaths, and those that survived were often severely scarred for life.

Science as Process

You may think of science as a large and detailed body of knowledge, but science is also the process by which this knowledge is obtained. Science uses experimentation, evidence, and logic to continuously test ideas. Over time and through repeated experimentation and testing, scientific knowledge advances.

We’ve been accumulating knowledge of vaccines for more than two centuries. The discovery of the first vaccine, as well as the process of vaccination, dates back to 1796. An English doctor named Edward Jenner observed that people who became infected with cowpox did not get sick from smallpox, a similar but much more severe disease (Figure 1.2.2). Jenner decided to transmit cowpox to a young boy to see if it would protect him from smallpox. He gave the boy cowpox by scratching liquid from cowpox sores into the boy’s skin. Then, six weeks later, he scratched liquid from smallpox sores into the boy’s skin. As Jenner predicted, the boy did not get sick from smallpox. Jenner had discovered the first vaccine, although additional testing was needed to show that it really was effective.

Almost a century passed before the next vaccine was discovered, a vaccine for cholera in 1879. Around the same time, French chemist Louis Pasteur found evidence that many human diseases are caused by germs, which earned him the title of “father of germ theory.” Since Pasteur’s time, vaccines have been discovered for scores of additional diseases caused by germs, and scientists are currently researching vaccines for many others.

Benefits of Science

Medical advances such as the discovery of vaccines are one of the most important benefits of science, but science and scientific knowledge are also crucial for most other human endeavors. Science is needed to design safe cars, predict storms, control global warming, develop new technologies of many kinds, help couples have children, and put humans on the moon. Clearly, the diversity of applications of scientific knowledge is vast!

 

1.2 Summary

  • Science is a large and detailed body of knowledge. It is also the process by which this knowledge is obtained.
  • Science uses experimentation, evidence, and logic to continuously test ideas. Over time and through repeated experimentation and testing, scientific knowledge advances.
  • Medical advances such as the development of vaccines are one of the most important benefits of science, but science and scientific knowledge are also crucial for most other human endeavors.

1.2 Review Questions

  1. Explain why science is considered both a process and a body of knowledge.
  2. State three specific examples of human endeavors that are based on scientific knowledge.
  3. How does science influence your daily life?
  4. Jenner used a young boy as a research subject in his smallpox vaccine research. Today, scientists must follow strict guidelines when using human subjects in their research. What unique concerns do you think might arise when human beings are used as research subjects?
  5. What gave Jenner the idea to develop a vaccine for smallpox?
  6. Why do you think almost a century passed between the development of the first vaccine (for smallpox) and the development of the next vaccine (for cholera)

1.2 Explore More

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=463

How we conquered the deadly smallpox virus – Simona Zompi, TED-Ed, 2013.

Attributions

Figure 1.2.1

Vacina, centro de vacinação, by Hyttalo Souza on Unsplash, used under the Unsplash License (https://unsplash.com/license). 

Figure 1.2.2

Child with Smallpox/ID#3265, by CDC/ James Hicks, from the Centre for Disease Control and Prevention, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

TED-Ed. (2013, October 28). How we conquered the deadly smallpox virus – Simona Zompi.  YouTube. https://www.youtube.com/watch?v=yqUFy-t4MlQ&feature=youtu.be

Wikipedia contributors. (2020, August 9). Edward Jenner. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Edward_Jenner&oldid=971970576

Wikipedia contributors. (2020, August 5). Louis Pasteur. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Louis_Pasteur&oldid=971330056

 

 

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1.3 The Nature of Science

Created by: CK-12/Adapted by: Christine Miller

Clip art of a person thinking with a thought bubble filled with question marks.

Defining Science

Science is a distinctive way of gaining knowledge about the natural world that starts with a question and then tries to answer the question using evidence and logic. It is an exciting exploration of all the whys and hows that any curious person might ask about the world. You can be part of that exploration! Besides your curiosity, all you need is a basic understanding of how scientists think and how science is done. In this concept, you’ll learn how to think like a scientist.

Thinking Like a Scientist

Thinking like a scientist rests on certain underlying assumptions. Scientists assume that:

 

Nature Is Understandable

Scientists think of nature as a single system controlled by natural laws. By discovering natural laws, scientists strive to increase their understanding of the natural world. Laws of nature are expressed as scientific laws. A scientific law is a statement that describes what always happens under certain conditions in nature.

Scientific Ideas Are Open to Change

Science is both a process and body of knowledge. Scientific knowledge is generated through systematic processes, such as observation and experimentation. Scientists are always testing and revising their ideas, and as new observations are made, existing ideas may be challenged. Ideas may be replaced with new ideas that better fit the facts, but more often, existing ideas are simply revised. Through many new discoveries over time, scientists gradually build an increasingly accurate and detailed understanding of the natural world.

Scientific Knowledge May Be Long Lasting

Many scientific ideas have stood the test of time. About 200 years ago, the scientist John Dalton proposed atomic theory — the theory that all matter is made of tiny particles called atoms. This theory is still valid today. During the two centuries since the theory was first proposed, scientists have learned a lot more about atoms and the even smaller particles that compose them. Nonetheless, the idea that all matter consists of atoms remains valid. There are many other examples of basic scientific ideas that have been tested repeatedly and proven sound. You will learn about many of them as you study human biology.

Not All Questions Can be Answered by Science

Science rests on evidence and logic, and evidence comes from observations. Therefore, science deals only with things that can be observed. An observation is anything that is detected through human senses or with instruments or measuring devices that extend human senses. Things that cannot be observed or measured by current means — such as supernatural beings or events — are outside the bounds of science. Consider these two questions about life on Earth:

The first question can be answered by science on the basis of scientific evidence (such as fossils and logical arguments). The second question could be a matter of belief, but no evidence can be gathered to support or refute it. Therefore, it is outside the realm of science.

1.3 Summary

  • Science is a distinctive way of gaining knowledge about the natural world that tries to answer questions using evidence and logic.
  • Scientists assume that nature can be understood through systematic study.
  • Scientific ideas are open to revision.
  • Sound scientific ideas withstand the test of time.
  • Science cannot provide answers to all of our questions.

1.3 Review Questions

  1. Define science.
  2. What is the general goal of science?
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    http://humanbiology.pressbooks.tru.ca/?p=467

  4. Identify four basic assumptions that scientists make when they study the natural world.
  5. Do observations in science have to be made by the naked eye? Can you think of a way in which scientists might be able to make observations about something they cannot directly see?
  6. If something cannot be observed, can it be tested scientifically? Explain your reasoning.
  7. Scientific knowledge builds upon itself. Give an example of a scientific idea from the reading where the initial idea developed further as science advanced.
  8. Discuss this statement: “Scientific ideas are always changing, so they can’t be trusted.” Do you think this is true?
  9. Why do you think that scientific knowledge expands as technology becomes more advanced?

1.3 Explore More

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Nature of Science with the Ameoba Sisters, 2019.

 

References

Amoeba Sisters. (2019, Jun 6). Nature of science with Ameoba Sisters. YouTube. https://www.youtube.com/watch?v=3nAETHZTObk

Wikipedia contributors. (2020, July 25). John Dalton. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=John_Dalton&oldid=969425891

 

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1.4 Scientific Investigations

Created by: CK-12/Adapted by: Christine Miller

“Doing” Science

Science is as much about doing as knowing. Scientists are always trying to learn more and gain a better understanding of the natural world. There are basic methods of gaining knowledge that are common to all of science. At the heart of science is the scientific investigation. A scientific investigation is a systematic approach to answering questions about the physical and natural world. Scientific investigations can be observational —  for example, observing a cell under a microscope and recording detailed descriptions. Other scientific investigations are experimental — for example, treating a cell with a drug while recording changes in the behavior of the cell.

The flow chart below shows the typical steps followed in an experimental scientific investigation. The series of steps shown in the flow chart is frequently referred to as the scientific method. Science textbooks often present this simple, linear “recipe” for a scientific investigation. This is an oversimplification of how science is actually done, but it does highlight the basic plan and purpose of an experimental scientific investigation: testing ideas with evidence. Each of the steps in the flow chart is discussed in greater detail below.

Diagram shows the scientific cycle arranged in a circular formation: Observation, questions, hypothesis, experiment, analysis, conclusion and then returning to observation again.
Figure 1.4.1 The Scientific Method is a never ending cycle.

Science is actually a complex endeavor that cannot be reduced to a single, linear sequence of steps, like the instructions on a package of cake mix. Real science is nonlinear, iterative (repetitive), creative, unpredictable, and exciting. Scientists often undertake the steps of an investigation in a different sequence, or they repeat the same steps many times as they gain more information and develop new ideas. Scientific investigations often raise new questions as old ones are answered. Successive investigations may address the same questions, but at ever deeper levels. Alternatively, an investigation might lead to an unexpected observation that sparks a new question and takes the research in a completely different direction.

Knowing how scientists “do” science can help you in your everyday life, even if you aren’t a scientist. Some steps of the scientific process — such as asking questions and evaluating evidence — can be applied to answering real-life questions and solving practical problems.

Making Observations

A health professional viewing an xray.
Figure 1.4.2 Health professionals use many tools in order to make observations.

Testing an idea typically begins with observations. An observation is anything that is detected through human senses or with instruments or measuring devices that enhance human senses. We usually think of observations as things we see with our eyes, but we can also make observations with our sense of touch, smell, taste, or hearing. In addition, we can extend and improve our own senses with instruments such as thermometers and microscopes. Other instruments can be used to sense things that human senses cannot detect at all, such as ultraviolet light or radio waves.

A black and white photo of Alexander Fleming examining bacterial growth on a petri dish.
Figure 1.4.3 Alexander Fleming examining bacterial growth.

Sometimes, chance observations lead to important scientific discoveries. One such observation was made by the Scottish biologist Alexander Fleming (pictured below) in the 1920s. Fleming’s name may sound familiar to you because he is famous for a major discovery. Fleming had been growing a certain type of bacteria on glass plates in his lab when he noticed that one of the plates was contaminated with mold. On closer examination, Fleming observed that the area around the mold was free of bacteria.

Asking Questions

Observations often lead to interesting questions. This is especially true if the observer is thinking like a scientist. Having scientific training and knowledge is also useful. Relevant background knowledge and logical thinking help make sense of observations so the observer can form particularly salient questions. Fleming, for example, wondered whether the mold — or some substance it produced — had killed bacteria on the plate. Fortunately for us, Fleming didn’t just throw out the mold-contaminated plate. Instead, he investigated his question and in so doing, discovered the antibiotic penicillin.

Hypothesis Formation

Typically, the next step in a scientific investigation is to form a hypothesis. A hypothesis is a possible answer to a scientific question. But it isn’t just any answer. A hypothesis must be based on scientific knowledge. In other words, it shouldn’t be at odds with what is already known about the natural world. A hypothesis also must be logical, and it is beneficial if the hypothesis is relatively simple. In addition, to be useful in science, a hypothesis must be testable and falsifiable. In other words, it must be possible to subject the hypothesis to a test that generates evidence for or against it. It must also be possible to make observations that would disprove the hypothesis if it really is false.

For example, Fleming’s hypothesis might have been: “A particular kind of bacteria growing on a plate will die when exposed to a particular kind of mold.” The hypothesis is logical and based directly on observations. The hypothesis is also simple, involving just one type each of mold and bacteria growing on a plate. In addition, hypotheses are subject to “if/then” conditions. Thus, Fleming might have stated, “If a certain type of mold is introduced to a particular kind of bacteria growing on a plate, then the bacteria will die.” This makes the hypothesis easy to test and ensures that it is falsifiable. If the bacteria were to grow in the presence of the mold, it would disprove the hypothesis (assuming the hypothesis is really false).

Hypothesis Testing

Hypothesis testing is at the heart of the scientific method. How would Fleming test his hypothesis? He would gather relevant data as evidence. Evidence is any type of data that may be used to test a hypothesis. Data (singular, datum) are essentially just observations. The observations may be measurements in an experiment or just something the researcher notices. Testing a hypothesis then involves using the data to answer two basic questions:

  1. If my hypothesis is true, what would I expect to observe?
  2. Does what I actually observe match what I expected to observe?

A hypothesis is supported if the actual observations (data) match the expected observations. A hypothesis is refuted if the actual observations differ from the expected observations.

The scientific method is employed by scientists around the world, but it is not always conducted in the order above. Sometimes, hypothesis are formulated before observations are collected; sometimes observations are made before hypothesis are created. Regardless, it is important that scientists record their procedures carefully, allowing others to reproduce and verify the experimental data and results. After many experiments provide results supporting a hypothesis, the hypothesis becomes a theory. Theories remain theories forever, and are constantly being retested with every experiment and observation. Theories can never become fact or law.

In science, a law is a mathematical relationship that exists between observations under a given set of conditions. There is a fundamental difference between observations of the physical world and explanations of the nature of the physical world. Hypotheses and theories are explanations, whereas laws and measurements are observational

1.4 Summary

  • The scientific method consists of making observations, formulating a hypothesis, testing the hypothesis with new observations, making a new hypothesis if the new observations contradict the old hypothesis, or continuing to test the hypothesis if the observations agree.
  • A hypothesis is a tentative explanation that can be tested by further observation.
  • A theory is a hypothesis that has been supported with repeated testing.
  • A scientific law is a statement that summarizes the results of many observations.
  • Experimental data must be verified by reproduction from other scientists.
  • Theories must agree with all observations made on the phenomenon under study.
  • Theories are continually tested, forever.

1.4 Review Questions

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1.4 Explore More

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How simple ideas lead to scientific discoveries, TED-Ed,  2012.

Attributions

Figure 1.4.1

The Scientific Method (simple), by Thebiologyprimer on Wikimedia Commons is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).

Figure 1.4.2

Anatomy Bone Bones Check Doctor Examine Film, by rawpixel on Pixabay, used under the Pixabay License (https://pixabay.com/de/service/license/).

Figure 1.4.3

Penicillin Past, Present and Future- the Development and Production of Penicillin, England, 1944, by Ministry of Information Photo Division Photographer. This photograph was scanned and released by the Imperial War Museum on the IWM Non Commercial Licence. It is now in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

TED-Ed. (2012, Mar 13). How simple ideas lead to scientific discoveries. YouTube. https://www.youtube.com/watch?v=F8UFGu2M2gM

Wikipedia contributors. (2020, July 7). Alexander Fleming. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Alexander_Fleming&oldid=966489433

 

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1.5 Theories in Science

Created by: CK-12/Adapted by: Christine Miller

What Is a Scientific Theory?

Germ theory, which is described in detail below, is one of several scientific theories you will read about in human biology. A scientific theory is a broad explanation for events. Scientific theories are widely accepted by the scientific community. To become a theory, an explanation must be strongly supported by a great deal of evidence.

People commonly use the word theory to describe a guess or hunch about how or why something happens. For example, you might say, “I think a woodchuck dug this hole in the ground, but it’s just a theory.” Using the word theory in this way is different from the way it is used in science. A scientific theory is not just a guess or hunch that may or may not be true. In science, a theory is an explanation that has a high likelihood of being correct because it is so well supported by evidence.

 

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What is the difference between a scientific law and theory? by Matt Anticole, TEDEd, 2015

Germ Theory: A Human Biology Example

A black and white side-profile caricature of Girolamo Fracastoro wearing tradition middle-18th century attire.
Figure 1.5.1 Girolamo Fracastoro made the first clear statement of the germ theory of disease.

The germ theory of disease states that contagious diseases are caused by germs, or microorganisms, which are organisms that are too small to be seen without magnification. Microorganisms which cause disease are called pathogens. Human pathogens include bacteria and viruses, among other microscopic entities. When pathogens invade humans or other living hosts, they grow, reproduce, and make their hosts sick. Diseases caused by germs are contagious because the microorganisms that cause them can spread from person to person.

First Statement of Germ Theory

Germ theory was first clearly stated by an Italian physician named Girolamo Fracastoro (pictured in Figure 1.5.1) in the mid-1500s. Fracastoro proposed that contagious diseases are caused by transferable “seed-like entities,” which we now call germs. According to Fracastoro, germs spread through populations through direct or indirect contact between individuals, making people sick.

Fracastoro’s idea, though essentially correct, was disregarded by other physicians. Instead, Hippocrates‘ and Galen’s idea of miasma remained the accepted explanation for the spread of disease for another 300 years. However, evidence for Fracastoro’s idea accumulated during that time. Some of the earliest evidence was provided by the Dutch lens and microscope maker Anton van Leeuwenhoek, who is considered by many to be the father of microbiology. By the 1670s, van Leeuwenhoek had directly observed many different types of microorganisms, including bacteria.

Evidence from Puerperal Fever

One of the first physicians to demonstrate that a microorganism is the cause of a specific human disease was the Hungarian obstetrician Ignaz Semmelweis in the 1840s. The disease was puerperal fever, an often-fatal infection of the female reproductive organs. Puerperal fever is also called childbed fever, because it usually affects women who have just given birth.

Figure 1.5.2 Semmelweis showed how deaths from puerperal fever increased after doctors began doing autopsies at Wien Maternity Clinic (first vertical line) and decreased after doctors started disinfecting their hands (red box).

Semmelweis observed that deaths from puerperal fever occurred much more often when women had been attended by doctors at his hospital than by midwives at home. Semmelweis also noticed that doctors often came directly from autopsies to the beds of women about to give birth. From his observations, Semmelweis inferred that puerperal fever was a contagious disease caused by some type of matter carried to pregnant patients on the hands of doctors from autopsied bodies. As a consequence, Semmelweis urged doctors and medical students at his hospital to wash their hands with chlorinated lime water before examining pregnant women. After this change, the hospital’s death rate for women who had just given birth fell from 18 to 2 per cent, which was a 90 per cent reduction. Some of Semmelweis’ findings are presented in the graph above-right.

Semmelweis published his results, but they were derided by the medical profession. The idea that doctors themselves were the carriers of a fatal disease was taken as a personal affront by his fellow physicians. One of Semmelweis’ peers protested indignantly that doctors are gentlemen and that gentlemen’s hands are always clean. As a result of attitudes such as this, Semmelweis became the target of a vicious smear campaign. Eventually, Semmelweis had a mental breakdown and was committed to a mental hospital, where he died.

Father of Germ Theory

A view through a microscope showing larger irregularly oval blue cells, and strings of smaller yellow round cells. The chains of small yellow cells are the Streptococcus pyogenes.
Figure 1.5.3 Pasteur discovered that the bacterium Streptococcus pyogenes causes puerperal fever.
A painting showing Louis Pasteur sitting in his lab examining a substance in a bottle
Figure 1.5.4 Louis Pasteur investigated the causes of diseases, such as puerperal fever.

Throughout the later 1800s, more formal investigations were conducted about the relationship between germs and disease. Some of the most important were undertaken by Louis Pasteur. Pasteur (right) was a French chemist who did careful experiments to show that fermentation, food spoilage, and certain diseases are caused by microorganisms. He discovered the cause of puerperal fever in 1879. He determined it was an infection caused by the bacterium Streptococcus pyogenes, shown under magnification (Figure 1.5.3).

 

Although Pasteur was not the first person to propose germ theory, his investigations clearly supported it. He also became a strong proponent of the theory and managed to convince most of the scientific community of its validity. For these reasons, Pasteur is often regarded as the father of germ theory.

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1.5 Summary

  •  A scientific theory is a broad explanation that is widely accepted because it is strongly supported by a great deal of evidence.
  • An example of a scientific theory is the germ theory of disease. According to this theory, contagious diseases are caused by germs, or microorganisms.
  • The germ theory of disease was first proposed in the mid-1500s. It was not widely accepted until the late 1800s, when it was strongly supported by experimental evidence from Louis Pasteur.

1.5 Review Questions

  1. Define scientific theory.
  2. Compare the way the word theory is used in science versus in everyday language.
  3. What is the germ theory of disease? How did it develop?
  4. Explain why Pasteur, rather than Fracastoro or Semmelweis, is called the father of germ theory.
  5. Galen and Fracastoro may have come up with different explanations for how disease is spread, but what observations do you think they made that were similar?
  6. Use the explanation of Semmelweis’ research and the graph in Figure 1.9 to answer the following questions:
    • What was Semmelweis’ observation that led him to undertake this study? What question was he trying to answer?
    • What was the hypothesis (i.e. proposed answer for a scientific question) that Semmelweis was testing?
    • Why did Semmelweis track death rates from puerperal fever at Dublin Maternity Hospital, where autopsies were not performed?
    • What were two pieces of evidence shown in the graph that supported Semmelweis’ hypothesis?
    • Why do you think it was important that Semmelweis compared Dublin Maternity Hospital and Wien Maternity Clinic over the same years?
  7. What is the difference between a microorganism and a pathogen?
  8. Explain why the development of the microscope lent support to the germ theory of disease.
  9. Does the observation of microorganisms alone conclusively prove that germ theory is correct? Why or why not?
  10. Who do you think was using more scientific reasoning: Semmelweis or the physicians that derided his results? Explain your answer.

1.5 Explore More

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Sammelweis – USA/ Austria Film Belvedere Film, Semmelweis Orvostörténeti Múzeum, 2013

Attributions

Figure 1.5.1

Fracastoro, Girolamo, 1478-1553,. by Francesco Redenti 1820-1876, from Wellcome Library Record no. 3120i, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 1.5.2

Puerperal fever yearly mortality rates, 1833-1858, by Power.corrupts, has been released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 1.5.3

Streptococcus pyogenes 01, from Centers for Disease Control and Prevention’s Public Health Image Library (PHIL), ID #2110is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 1.5.4

Albert Edelfelt – Louis Pasteur – 1885, photograph by Ondra Havala, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Semmelweis Orvostörténeti Múzeum. (2013, October 31). Sammelweiz. YouTube. https://www.youtube.com/watch?v=rPiW6Y_oDJo&feature=emb_logo

TEDEd. (2015). What’s the difference between a scientific law and a theory? – Matt Anticole. YouTube. https://www.youtube.com/watch?v=GyN2RhbhiEU&t=91s

Wikipedia contributors. (2020, August 3). Antonie van Leeuwenhoek. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Antonie_van_Leeuwenhoek&oldid=970998908

Wikipedia contributors. (2020, July 28). Galen. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Galen&oldid=969901897

Wikipedia contributors. (2020, July 1). Girolamo Fracastoro. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Girolamo_Fracastoro&oldid=965417568

Wikipedia contributors. (2020, July 30). Hippocrates. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Hippocrates&oldid=970254565

Wikipedia contributors. (2020, July 21). Ignaz Semmelweis. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ignaz_Semmelweis&oldid=968773367

Wikipedia contributors. (2020, August 5). Louis Pasteur. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Louis_Pasteur&oldid=971330056

Wikipedia contributors. (2020, August 5). Miasma theory. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Miasma_theory&oldid=971286379

 

 

 

5

1.6 Traditional Ecological Knowledge

Created by: Christine Miller

Definition

In order to truly understand the concept of  Traditional Ecological Knowledge (TEK), it is important to gather as many definitions as possible- this gives us an accurate breadth of the term with all its nuances.  Click through the images below to read several descriptions of TEK.

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=473

Value

People who have lived in a community for generations are often the first to notice any signs of environmental change.  Information about a particular region’s climate and ecology is retained when the people from the region take on location as part of their cultural identity across generations.  This traditional knowledge is passed from generation to generation through story telling and mentorship.

TEK shares some similarities with what is termed “Western Science”.  Both recognize that knowledge is always growing and changing and that observations are critical to recognizing patterns and causalities in nature.  In addition, both TEK and Western Science recognize interdependence in biological systems and the need to treat ecology as  a complex system.  TEK differs in some ways from Western Science: knowledge is passed on orally, partly through metaphor and story, and this learned knowledge is embedded into daily living.  TEK also differs from Western Science in that TEK is tied in to morality, spirituality and individual identity, making it more than just knowledge; it is sacred knowledge.

Examples

An Avalanche Lily in Bloom. The plant has two wide oval shaped leaves growing from the base of the plant. A single slender stem suspends a yellow flower with six yellow petals. The flower is tilted towards the ground and the anthers and stamen hang below the petals.
Figure 1.6.1 Avalanche Lily in bloom.

People who are indigenous to the province of British Columbia have been managing natural resources in this area for time immemorial.  Numerous examples of sustainable harvesting methods can be found across the province, but our example, harvesting and management of the Avalanche Lily, comes from the Secwepemc peoples of the interior of British Columbia.  Many thanks to Nancy Turner, Marianne Boelscher Ignace and Ronald Ignace for their documentation of these practices in their paper: Traditional Ecological Knowledge and Wisdom of Aboriginal Peoples in British Columbia

The Avalanche Lily is a yellow-flowered member of the lily family native to Western North America.  This flower grows from an edible  bulb which ranges in size from 3-5 centimetres.  The Secwepemc people have harvested these bulbs as an important food source for generations.  Oral transmission of knowledge allowed the Secwepemc people to use thousands of years of accumulated data around growing cycles, seasons and management practices to harvest these plants effectively while maintaining a healthy population of lilies for future use.  Very intentional conservation strategies were/are practiced when harvesting the bulbs:

The two videos below show how knowledge of a particular ecosystem is handed down through the traditions of mentorship and storytelling:

 

Thumbnail for the embedded element "Modern Science, Native Knowledge"

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Modern science, native knowledge, by The Nature Conservancy, 2015

First Stories – Nganawendaanan Nde’ing (I Keep Them in My Heart), by Shannon Letande, 2006

TEK is Part of Place

The Traditional Ecological Knowledge held by Indigenous communities often includes very location specific knowledge.  There are many diverse groups of First Peoples in British Columbia, each with expert knowledge about the ecology of their specific ancestral regions.  The link to the map from the Native Land Digital website shows some of the traditional boundaries of the Indigenous people in British Columbia. Click on the areas to see where First Nations communities are located.

1.6 Summary

  • Traditional Ecological Knowledge (TEK) is an important and valuable body of knowledge
  • People groups who have lived in an area over generations pass down TEK through storytelling and mentorship
  • TEK and Western Science share certain characteristics, including use of observations, and identification of patterns and causalities in nature
  • TEK and Western Science differ in that TEK is passed down through oral storytelling and is deeply rooted in morality, spirituality and individual identity

 

1.6 Review Questions

Type your exercises here.

  1. Define Traditional Ecological Knowledge.
  2. How is TEK passed down through generations?
  3. How does TEK differ from Western Science?
  4. What are some ways in which TEK can inform resource management?
  5. What are some of the ramifications of loss of TEK?  How can TEK be maintained?

1.6 Explore More

Thumbnail for the embedded element "TEDxTC - Winona LaDuke - Seeds of Our Ancestors, Seeds of Life"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=473

TEDxTC – Winona LaDuke – Seeds of Our Ancestors, Seeds of Life, by TEDx-TC, 2012

Attributions

Definitions of Traditional Ecological Knowledge

Figure 1.6.1

Erythronium grandiflorum 5077, by Walter Siegmund on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

References

Herkes, J. (n.d.). Continuing studies: Traditional ecological knowledge. University of Northern British Columbia. Retrieved from https://www.unbc.ca/continuing-studies/courses/traditional-ecological-knowledge

Inglis, J. T. (1993). Traditional ecological knowledge: Concepts and cases (p. vi). Canadian Museum of Nature, Ottawa, Ontario, Canada.

Letande, S. (2006). First Stories – Nganawendaanan Nde’ing (I keep them in my heart). https://www.nfb.ca/film/first_stories_nganawendaanan_ndeing/

Minerals Management Service (n.d.). What is Traditional Knowledge [online]. Government of Alaska.https://web.archive.org/web/20030328053734/http://www.mms.gov/alaska/native/tradknow/tk_mms2.htm

TEDx-TC. (2012, March 4). TEDxTC – Winona LaDuke – Seeds of our ancestors, seeds of life. https://www.youtube.com/watch?v=pHNlel72eQc&feature=youtu.be

The Nature Conservancy. (2015, February 25). Modern science, native knowledge. YouTube. https://www.youtube.com/watch?v=1QRpnHoGivk

Turner, N., Ignace, M., & Ignace, R. (2000, October 1). Traditional ecological knowledge and wisdom of Aboriginal peoples in British Columbia. Ecological Applications, 10(5), 1275-1287. doi:10.2307/2641283

Wakefield, A.J. (1999, September 11). MMR vaccination and autism. Lancet, 354(9182), 949-950.  https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(05)75696-8/fulltext doi:https://doi.org/10.1016/S0140-6736(05)75696-8

 

 

6

1.7 Pseudoscience and Other Misuses of Science

Created by: CK-12/Adapted by: Christine Miller

What Is Pseudoscience?

Pseudoscience is a claim, belief, or practice that is presented as scientific but does not adhere to the standards and methods of science. True science is based on repeated evidence-gathering and testing of falsifiable hypotheses. Pseudoscience does not adhere to these criteria. In addition to phrenology, some other examples of pseudoscience include astrology, extrasensory perception (ESP), reflexology, reincarnation, and Scientology,

Characteristics of Pseudoscience

Whether a field is actually science or just pseudoscience is not always clear. However, pseudoscience generally exhibits certain common characteristics. Indicators of pseudoscience include:

Persistence of Pseudoscience

Despite failing to meet scientific standards, many pseudosciences survive. Some pseudosciences remain very popular with large numbers of believers. A good example is astrology.

Astrology is the study of the movements and relative positions of celestial objects as a means for divining information about human affairs and terrestrial events. Many ancient cultures attached importance to astronomical events, and some developed elaborate systems for predicting terrestrial events from celestial observations. Throughout most of its history in the West, astrology was considered a scholarly tradition and was common in academic circles. With the advent of modern Western science, astrology was called into question. It was challenged on both theoretical and experimental grounds, and it was eventually shown to have no scientific validity or explanatory power.

Figure 1.7.1 Zodiac signs.

Today, astrology is considered a pseudoscience, yet it continues to have many devotees. Most people know their astrological sign, and many people are familiar with the personality traits supposedly associated with their sign. Astrological readings and horoscopes are readily available online and in print media, and a lot of people read them, even if only occasionally. About a third of all adult Americans actually believe that astrology is scientific. Studies suggest that the persistent popularity of pseudosciences such as astrology reflects a high level of scientific illiteracy. It seems that many Americans do not have an accurate understanding of scientific principles and methodology. They are not convinced by scientific arguments against their beliefs.

Dangers of Pseudoscience

Belief in astrology is unlikely to cause a person harm, but belief in some other pseudosciences might — especially in health care-related areas. Treatments that seem scientific but are not may be ineffective, expensive, and even dangerous to patients. Seeking out pseudoscientific treatments may also delay or preclude patients from seeking scientifically-based medical treatments that have been tested and found safe and effective. In short, irrational health care may not be harmless.

Scientific Hoaxes, Frauds, and Fallacies

Pseudoscience is not the only way that science may be misused. Scientific hoaxes, frauds, and fallacies may misdirect the pursuit of science, put patients at risk, or mislead and confuse the public. An example of each of these misuses of science and its negative effects is described below.

The Piltdown Hoax

A side profile view of an artists rendition of what the Piltdown Man may have looked like, had he been real.
Figure 1.7.2 This reconstruction of Piltdown Man’s head was based on jaw and skull bone fragments.

Piltdown Man (see picture left) was a paleontological hoax in which bone fragments were presented as the fossilized remains of a previously unknown early human. These fragments consisted of parts of a skull and jawbone, reported to have been found in 1908 in a gravel pit at Piltdown, East Sussex, England. The significance of the specimen remained the subject of controversy until it was exposed in 1953 as a hoax. It eventually came to light that the specimen consisted of the lower jawbone of an orangutan deliberately combined with skull bones of a modern human. The Piltdown hoax is perhaps the most infamous paleontological hoax ever perpetrated, both for its impact on the direction of research on human evolution and for the length of time between its “discovery” and its full exposure as a forgery.

A replica of the infamous Piltdown skull. The skull is encased in a glass sphere. The replica shows portions of the skull which were bone in white, and the portions of the skull which were inferred in black.
Figure 1.7.3 A replica of the infamous Piltdown skull.

In 1912, the head of the geological department at the British Museum proposed that Piltdown man represented an evolutionary missing link between apes and humans. With its human-like cranium and ape-like jaw, it seemed to support the idea then prevailing in England that human evolution began with the brain. The Piltdown specimen led scientists down a blind alley in the belief that the human brain increased in size before the jaw underwent size reductions to become more like the modern human jaw. This belief confused and misdirected the study of human evolution for decades, and actual fossils of early humans were ignored because they didn’t support the accepted paradigm.

The Vaccine-Autism Fraud

You may have heard that certain vaccines put the health of young children at risk. This persistent idea is not supported by scientific evidence or accepted by the vast majority of experts in the field. It stems largely from an elaborate medical research fraud that was reported in a 1998 article published in the respected British medical journal, The Lancet. The main author of the article was a British physician named Andrew Wakefield. In the article, Wakefield and his colleagues described case histories of 12 children, most of whom were reported to have developed autism soon after the administration of the MMR (measles, mumps, rubella) vaccine.

Several subsequent peer-reviewed studies failed to show any association between the MMR vaccine and autism. It also later emerged that Wakefield had received research funding from a group of people who were suing vaccine manufacturers. In 2004, ten of Wakefield’s 12 coauthors formally retracted the conclusions in their paper. In 2010, editors of The Lancetretracted the entire paper. That same year, Wakefield was charged with deliberate falsification of research and barred from practicing medicine in the United Kingdom. Unfortunately, by then, the damage had already been done. Parents afraid that their children would develop autism had refrained from having them vaccinated. British MMR vaccination rates fell from nearly 100 per cent to 80 per cent in the years following the study. The consensus of medical experts today is that Wakefield’s fraud put hundreds of thousands of children at risk because of the lower vaccination rates and also diverted research efforts and funding away from finding the true cause of autism.

Correlation-Causation Fallacy

Many statistical tests used in scientific research calculate correlations between variables. Correlation refers to how closely related two data sets are, which may be a useful starting point for further investigation. Correlation, however, is also one of the most misused types of evidence, primarily because of the logical fallacy that correlation implies causation. In reality, just because two variables are correlated does not necessarily mean that either variable causes the other.

A few simple examples, illustrated by the graphs below, can be used to demonstrate the correlation-causation fallacy. Assume a study found that both per capita consumption of mozzarella cheese and the number of Civil Engineering doctorates awarded are correlated; that is, rates of both events increase together. If correlation really did imply causation, then you could conclude from the second example that the increase in age of Miss America causes an increase in murders of a specific type or vice versa.

A chart showing the correlation between per capita consumption of mozzarella cheese, and the number of civil engineering doctorates awarded.
Figure 1.7.4 Spurious Correlations [Causation Fallacy] – Consumption of mozzarella cheese and awarded Doctorates
A chart showing a correlation between the age of Miss America, and the number of Murders by steam, hot vapours, and hot objects.
Figure 1.7.5 Spurious Correlations (Causation Fallacy)- Miss America and Murder

An actual example of the correlation-causation fallacy occurred during the latter half of the 20th century. Numerous studies showed that women taking hormone replacement therapy (HRT) to treat menopausal symptoms also had a lower-than-average incidence of coronary heart disease (CHD). This correlation was misinterpreted as evidence that HRT protects women against CHD. Subsequent studies that controlled other factors related to CHD disproved this presumed causal connection. The studies found that women taking HRT were more likely to come from higher socio-economic groups, with better-than-average diets and exercise regimens. Rather than HRT causing lower CHD incidence, these studies concluded that HRT and lower CHD were both effects of higher socio-economic status and related lifestyle factors.

Check out this “Rough Guide to Spotting Bad Science” infographic from Compound Interest:

Figure 1.7.6 A Rough Guide to Spotting Bad Science.

1.7 Summary

  • Pseudoscience is a claim, belief, or practice that is presented as scientific, but does not adhere to scientific standards and methods.
  • Indicators of pseudoscience include untestable claims, lack of openness to testing by experts, absence of progress in advancing knowledge, and attacks on the motives and character of critics.
  • Some pseudosciences, including astrology, remain popular. This suggests that many people do not possess the scientific literacy needed to distinguish pseudoscience from true science, or to be convinced by scientific arguments against them.
  • Belief in a pseudoscience such as astrology is unlikely to cause harm, but belief in pseudoscientific medical treatments may be harmful.
  • In addition to pseudoscience, other examples of the misuse of science include scientific hoaxes (such as the Piltdown hoax), scientific frauds (such as the MMR vaccine-autism fraud), and scientific fallacies (such as the correlation-causation fallacy).

1.7 Review Questions

  1. Define pseudoscience. Give three examples.
  2. What are some indicators that a claim, belief, or practice might be pseudoscience rather than true science?
  3. Astrology was once considered a science, and it was common in academic circles. Why did its status change from a science to a pseudoscience?
  4. What are possible reasons that some pseudosciences remain popular even after they have been shown to have no scientific validity or explanatory power?
  5. List three other ways besides pseudoscience that science can be misused, and identify an example of each.
  6. Explain how misuses of science may waste money and effort. How can they potentially cause harm to the public?
  7. Many claims made by pseudoscience cannot be tested with evidence. From a scientific perspective, why is it important that claims be testable?
  8. What do you think is the difference between pseudoscience and belief?
  9. If you see a website that claims that an herbal supplement causes weight loss and they use a lot of scientific terms to explain how it works, can you be assured that the drug is scientifically proven to work? If not, what are some steps you can take to determine whether or not the drug does in fact work?
  10. Why do you think it was problematic that Andrew Wakefield received funding from a group of people who were suing vaccine manufacturers?
  11. What do you think it says about the 1998 Wakefield paper that ten of the 12 coauthors formally retracted their conclusions?

1.7 Explore More

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How to spot a misleading graph – Lea Gaslowitz, TED-Ed, 2017.

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How statistics can be misleading – Mark Liddell, TED-Ed, 2016.

 

Attributions

Figure 1.7.1

Zodiac Signs Cancer Aquarius Aries Gemini Leo from Max Pixel, is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).

Figure 1.7.2

Piltdown Man – McGregor model, by James Howard McGregor on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 1.7.3

Sterkfontein Piltdown man, by Anrie  on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 1.7.4

Spurious Correlations (Causation Fallacy) – Consumption of mozzarella cheese and awarded Doctorates by Tyler Vigen on Tylervigen.com is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 1.7.5

Spurious Correlations (Causation Fallacy) – Miss America and Murder, by Tyler Vigen, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 1.7.6

A rough guide to spotting bad science, by Compound Interest, is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/ca/) license

References

TED-Ed. (2017, July 6). How to spot a misleading graph – Lea Gaslowitz. YouTube. https://www.youtube.com/watch?v=E91bGT9BjYk&feature=youtu.be

Wakefield, A.J., Murch, S.H., Anthony, A., Linnell, J., Casson, D.M., Malik, M., et al. (1998). Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. Lancet, 351: 637–41.

Wikipedia contributors. (2020, June 18). Andrew Wakefield. Wikipedia. https://en.wikipedia.org/w/index.php?title=Andrew_Wakefield&oldid=963243135

7

1.8 Case Study Conclusion: To Give a Shot or Not

Created by CK-12/Adapted by Christine Miller

As you read in the beginning of this chapter, new parents Samantha and Aki left their pediatrician’s office still unsure whether or not to vaccinate baby James. Dr. Rodriguez gave them a list of reputable sources where they could look up information about the safety of vaccines, including the Centers for Disease Control and Prevention (CDC). Samantha and Aki read that the consensus within the scientific community is that there is no link between vaccines and autism. They find a long list of studies published in peer-reviewed scientific journals that disprove any link. Additionally, some of the studies are “meta-analyses” that analyzed the findings from many individual studies. The new parents are reassured by the fact that many different researchers, using a large number of subjects in numerous well-controlled and well-reviewed studies, all came to the same conclusion.

Figure 1.8.1 Do your research!

Samantha also went back to the web page that originally scared her about the safety of vaccines. She found that the author was not a medical doctor or scientific researcher, but rather a self-proclaimed “child wellness expert.” He sold books and advertising on his site, some of which were related to claims of vaccine injury. She realized that he was both an unqualified and potentially biased source of information.

Samantha also realized that some of his arguments were based on correlations between autism and vaccines, but, as the saying goes, “correlation does not imply causation.” For instance, the recent rise in autism rates may have occurred during the same time period as an increase in the number of vaccines given in childhood, but Samantha could think of many other environmental and social factors that have also changed during this time period. There are just too many variables to come to the conclusion that vaccines, or anything else, are the cause of the rise in autism rates based on that type of argument alone. Also, she learned that the age of onset of autism symptoms happens to typically be around the time that the MMR vaccine is first given, so the apparent association in the timing may just be a coincidence.

Finally, Samantha came across news about  a measles outbreak in Vancouver, British Columbia in the winter of 2019. Measles wasn’t just a disease of the past! She learned that measles and whooping cough, which had previously been rare thanks to widespread vaccinations, are now on the rise, and that people choosing not to vaccinate their children seems to be one of the contributing factors. She realized that it is important to vaccinate her baby against these diseases, not only to protect him from their potentially deadly effects, but also to protect others in the population.

In their reading, Samantha and Aki learn that scientists do not yet know the causes of autism, but they feels reassured by the abundance of data that disproves any link with vaccines. Both parents think that the potential benefits of protecting their baby’s health against deadly diseases outweighs any unsubstantiated claims about vaccines. They will be making an appointment to get baby James his shots soon.

Chapter 1 Summary

In this chapter, you learned about some of the same concepts that helped Samantha and Aki make an informed decision. Specifically:

  • Science is a distinctive way of gaining knowledge about the natural world that is based on the use of evidence to logically test ideas. As such, science is a process, as well as a body of knowledge.
  • A scientific theory, such as the germ theory of disease, is the highest level of explanation in science. A theory is a broad explanation for many phenomena that is widely accepted because it is supported by a great deal of evidence.
  • The scientific investigation is the cornerstone of science as a process. A scientific investigation is a systematic approach to answering questions about the physical and natural world. An investigation may be observational or experimental.
  • A scientific experiment is a type of scientific investigation in which the researcher manipulates variables under controlled conditions to test expected outcomes. Experiments are the gold standard for scientific investigations and can establish causation between variables.
  • Nonexperimental scientific investigations such as observational studies and modeling may be undertaken when experiments are impractical, unethical, or impossible. Observational studies generally can establish correlation — but not causation — between variables.
  • A pseudoscience, such as astrology, is a field that is presented as scientific but that does not adhere to scientific standards and methods. Other misuses of science include deliberate hoaxes, frauds, and fallacies made by researchers.
  • Strict guidelines must be followed when using human subjects in scientific research. Among the most important protections is the requirement for informed consent.

Now that you know about the nature and process of science, you can apply these concepts in the next chapter to the study of human biology.

Chapter 1 Review

  1. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=477

  2. Why does a good hypothesis have to be falsifiable?
  3. Name one scientific law.
  4. Name one scientific theory.
  5. Give an example of a scientific idea that was later discredited.
  6. A statistical measurement called a P-value is often used in science to determine whether or not a difference between two groups is actually significant or simply due to chance. A P-value of 0.03 means that there is a 3% chance that the difference is due to chance alone. Do you think a P-value of 0.03 would indicate that the difference is likely to be significant? Why or why not?
  7. Why is it important that scientists communicate their findings to others? How do they usually do this?
  8. What is a “control group” in science?
  9. In a scientific experiment, why is it important to only change one variable at a time?
  10. Which is the dependent variable – the variable that is manipulated or the variable that is being affected by the change?
  11. You see an ad for a “miracle supplement” called NQP3 that claims the supplement will reduce belly fat. They say it works by reducing the hormone cortisol and by providing your body with missing unspecified “nutrients”, but they do not cite any peer-reviewed clinical studies. They show photographs of three people who appear slimmer after taking the product. A board-certified plastic surgeon endorses the product on television. Answer the following questions about this product.

a. Do you think that because a doctor endorsed the product, it really works? Explain your answer.

b. What are two signs that these claims could actually be pseudoscience instead of true science?

c. Do you think the photographs are good evidence that the product works? Why or why not?

d. If you wanted to do a strong scientific study of whether this supplement does what it claims, what would you do? Be specific about the subjects, data collected, how you would control variables, and how you would analyze the data.

e. What are some ways that you would ensure that the subjects in your experiment in part d are treated ethically and according to human subjects protections regulations?

Attribution

Figure 1.8.1

[Photo of person sitting in front of personal computer] by Avel Chuklanov on Unsplash is used under the Unsplash License (https://unsplash.com/license).

II

Chapter 2 - Biology: The Study of Life

8

2.1 Case Study: Why Should You Study Human Biology?

Created by CK-12/Adapted by Christine Miller

Case Study: Our Invisible Inhabitants

Figure 2.1.1 Lanying has the flu. Can she stop taking her antibiotics once she starts feeling better?

Lanying is suffering from a fever, body aches, and a painful sore throat that feels worse when she swallows. She visits her doctor, who examines her and performs a throat culture. When the results come back, he tells her that she has strep throat, which is caused by the bacteria Streptococcus pyogenes. He prescribes an antibiotic that will either kill the bacteria or stop it from reproducing, and advises her to take the full course of the treatment even if she is feeling better earlier. Stopping early can cause an increase in bacteria that are resistant to antibiotics.

Lanying takes the antibiotic as prescribed. Toward the end of the course, her throat is feeling much better — but she can’t say the same for other parts of her body! She has developed diarrhea and an itchy vaginal yeast infection. She calls her doctor, who suspects that the antibiotic treatment has caused both the digestive distress and the yeast infection. He explains that our bodies are home to many different kinds of microorganisms, some of which are actually beneficial to us because they help us digest our food and minimize the population of harmful microorganisms. When we take an antibiotic, many of these “good” bacteria are killed along with the “bad,” disease-causing bacteria, which can result in diarrhea and yeast infections.

Lanying’s doctor prescribes an antifungal medication for her yeast infection. He also recommends that she eat yogurt with live cultures, which will help replace the beneficial bacteria in her gut. Our bodies contain a delicate balance of inhabitants that are invisible without a microscope, and changes in that balance can cause unpleasant health effects.

What Is Human Biology?

As you read the rest of this book, you’ll learn more amazing facts about the human organism, and you’ll get a better sense of how biology relates to your health. Human biology is the scientific study of the human species, which includes the fascinating story of human evolution and a detailed account of our genetics, anatomy, physiology, and ecology. In short, the study focuses on how we got here, how we function, and the role we play in the natural world. This helps us to better understand human health, because we can learn how to stay healthy and how diseases and injuries can be treated. Human biology should be of personal interest to you to the extent that it can benefit your own health, as well as the health of your friends and family. This branch of science also has broader implications for society and the human species as a whole.

 As you continue reading, think about what you want to learn about your own body. What questions or concerns do you have? Make a list of them and use it to guide your study of human biology. You can revisit the list throughout the course to see if your questions have been answered. If not, you’ll have the tools you need to find the answers. You will have learned how to find sources of information about human biology, and you’ll be able to judge which sources are most reliable.

 

Chapter Overview: Living Organisms and Human Biology

In the rest of this chapter, you’ll learn about the traits shared by all living things, the basic principles that underlie all of biology, the vast diversity of living organisms, what it means to be human, and our place in the animal kingdom. Specifically, you’ll learn:

  • The seven traits shared by all living things: homeostasis, or the maintenance of a more-or-less constant internal environment; multiple levels of organization consisting of one or more cells; the use of energy and metabolism; the ability to grow and develop; the ability to evolve adaptations to the environment; the ability to detect and respond to environmental stimuli; and the ability to reproduce.
  • The basic principles that unify all fields of biology, including gene theory, homeostasis, and evolutionary theory.
  • The diversity of life (including the different kinds of biodiversity), the definition of a species, the classification and naming systems for living organisms, and how evolutionary relationships can be represented through diagrams, such as phylogenetic trees.
  • How the human species is classified and how we’ve evolved from our close relatives and ancestors.
  • The physical traits and social behaviors that humans share with other primates.

As you read this chapter, consider the following questions about Lanying’s situation:

  1. What do single-celled organisms (such as the bacteria and yeast living in and on Lanying) have in common with humans?
  2. How are bacteria, yeast, and humans classified?
  3. How do the concepts of homeostasis and biodiversity apply to Lanying’s situation?
  4. Why can stopping antibiotics early cause the development of antibiotic-resistant bacteria?

 

Attribution

Figure 2.1.1

Photo (face mask) by Michael Amadeus, on Unsplash is used under the Unsplash license (https://unsplash.com/license).

Reference

Mayo Clinic Staff (n.d.). Strep throat [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/strep-throat/symptoms-causes/syc-20350338

9

2.2 Shared Traits of All Living Things

Created by CK-12/Adapted by Christine Miller

The Thinker

The Thinker (French: Le Penseur) is a bronze sculpture by Auguste Rodin, usually placed on a stone pedestal. The work shows a nude male figure of over life-size sitting on a rock with his chin resting on one hand as though deep in thought, often used as an image to represent philosophy.
Figure 2.2.1 The Thinker by Auguste Rodin.

You’ve probably seen this famous statue created by the French sculptor Auguste Rodin. Rodin’s skill as a sculptor is especially evident here because the statue — which is made of bronze — looks so lifelike. How does a bronze statue differ from a living, breathing human being or other living organism? What is life? What does it mean to be alive? Science has answers to these questions.

Characteristics of Living Things

To be classified as a living thing, most scientists agree that an object must have all seven of the traits listed below. Humans share these characteristics with other living things.

  1. Homeostasis
  2. Organization
  3. Metabolism
  4. Growth
  5. Adaptation
  6. Response to stimuli
  7. Reproduction

Homeostasis

All living things are able to maintain a more-or-less constant internal environment. Regardless of the conditions around them, they can keep things relatively stable on the inside. The condition in which a system is maintained in a more-or-less steady state is called homeostasis. Human beings, for example, maintain a stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn’t freeze. Instead, by shivering and other means, it maintains a stable internal temperature.

Figure 2.2.2 Homeostasis of body temperature.

Organization

Living things have multiple levels of organization. Their molecules are organized into one or more cells. A cell is the basic unit of the structure and function of living things. Cells are the building blocks of living organisms. An average adult human being, for example, consists of trillions of cells. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells and onion cells in Figures 2.2.3 and 2.2.4. What similarities do you see?

 

Shows the image through a microscope of human cheek cells. The cells are oval in shape and light blue, with a darker blue spot close to the centre. The light blue shows the cell membrane and cytoplasm and the darker blue shows the nucleus of the cell.
Figure 2.2.3 Human cheek cells.
Shows an image through a microscope of onion cells. The cells are packed together and are rectangular in shape. Their cell walls and nuclei are stained a darker blue and the cytoplasm is whitish.
Figure 2.2.4 Onion cells.

Metabolism

All living things can use energy. They require energy to maintain internal conditions (homeostasis), to grow, and to execute other processes. Living cells use the “machinery” of metabolism, which is the building up and breaking down of chemical compounds. Living things can transform energy by building up large molecules from smaller ones. This form of metabolism is called anabolism. Living things can also break down, or decompose, large organic molecules into smaller ones. This form of metabolism is called catabolism.

Consider weight lifters who eat high-protein diets. A protein is a large molecule made up of several small amino acids. When we eat proteins, our digestive system breaks them down into amino acids (catabolism), so that they are small enough to be absorbed by the digestive system and into the blood. From there, amino acids are transported to muscles, where they are converted back to proteins (anabolism).

Image shows a man and woman holding hands with a toddler between them. All three are walking down a grassy path in their bare feet.
Figure 2.2.5 Humans grow and develop.

Growth

All living things have the capacity for growth. Growth is an increase in size that occurs when there is a higher rate of anabolism than catabolism. A human infant, for example, has changed dramatically in size by the time it reaches adulthood, as is apparent from the image below. In what other ways do we change as we grow from infancy to adulthood?

A human infant has a lot of growing to do before adulthood.

 Adaptations and Evolution

An adaptation is a characteristic that helps living things survive and reproduce in a given environment. It comes about because living things have the ability to change over time in response to the environment. A change in the characteristics of living things over time is called evolution. It develops in a population of organisms through random genetic mutations and natural selection.

Response to Stimuli

All living things detect changes in their environment and respond to them. These stimuli can be internal or external, and the response can take many forms, from the movement of a unicellular organism in response to external chemicals (called chemotaxis) to complex reactions involving all the senses of a multicellular organism. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (called phototropism).

Click through the images below: the venus fly trap, the cat, and the flower are all showing response to a stimuli.

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Figure 2.2.6 Examples of responses to environmental stimuli. 

Reproduction

All living things are capable of reproduction, the process by which living things give rise to offspring. Reproduction may be as simple as a single cell dividing to form two daughter cells, which is how bacteria reproduce. Reproduction in human beings and many other organisms, of course, is much more complicated. Nonetheless, whether a living thing is a human being or a bacterium, it is normally capable of reproduction.

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Feature: Myth vs. Reality

Myth: Viruses are living things.

Reality: The traditional scientific view of viruses is that they originate from bits of DNA or RNA shed from the cells of living things, but that they are not living things themselves. Scientists have long argued that viruses are not living things because they do not exhibit most of the defining traits of living organisms. A single virus, called a virion, consists of a set of genes (DNA or RNA) inside a protective protein coat, called a capsid. Viruses have organization, but they are not cells, and they do not possess the cellular “machinery” that living things use to carry out life processes. As a result, viruses cannot undertake metabolism, maintain homeostasis, or grow.

Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall; the magnification is approximately 200,000
Figure 2.2.7 Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall; the magnification is approximately 200,000.

They do not seem to respond to their environment, and they can reproduce only by invading and using “tools” inside host cells to produce more virions. The only traits viruses seem to share with living things is the ability to evolve adaptations to their environment. In fact, some viruses evolve so quickly that it is difficult to design drugs and vaccines against them! That’s why maintaining protection from the viral disease influenza, for example, requires a new flu vaccine each year.

Within the last decade, new discoveries in virology (the study of viruses) suggest that this traditional view about viruses may be incorrect, and that the “myth” that viruses are living things may be the reality. Researchers have discovered giant viruses that contain more genes than cellular life forms, such as bacteria. Some of the genes code for proteins needed to build new viruses, which suggests that these giant viruses may be able — or were once able — to reproduce without a host cell. Some of the strongest evidence that viruses are living things comes from studies of their proteins, which show that viruses and cellular life share a common ancestor in the distant past. Viruses may have once existed as primitive cells, but at some point they lost their cellular nature and became modern viruses that require host cells to reproduce. This idea is not so far-fetched when you consider that many other species require a host to complete their life cycle.

 

2.2 Summary

  • To be classified as a living thing, most scientists agree that an object must exhibit seven characteristics. Humans share these traits with all other living things.
  • All living things:
    • Can maintain a more-or-less constant internal environment, which is called homeostasis.
    • Have multiple levels of organization and consist of one or more cells.
    • Can use energy and are capable of metabolism.
    • Grow and develop.
    • Can evolve adaptations to their environment.
    • Can detect and respond to environmental stimuli.
    • Are capable of reproduction, which is the process by which living things give rise to offspring.

2.2 Review Questions

  1. Identify the seven traits that most scientists agree are shared by all living things.
  2. What is homeostasis? What is one way humans fulfill this criterion of living things?
  3. Define reproduction and describe two different examples.
  4. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you ethically determine if it is a living thing?
  5. Describe viruses and which traits they do and do not share with living things. Do you think viruses should be considered living things? Why or why not?
  6. People who are biologically unable to reproduce are certainly still considered alive. Discuss why this situation does not invalidate the criteria that living things must be capable of reproduction.
  7. What are the two types of metabolism described here. What are their differences?
  8. What are some similarities between the cells of different organisms? If you are not familiar with the specifics of cells, simply describe the similarities you see in the pictures above.
  9. What are two processes in a living thing that use energy?
  10. Give an example of a response to stimuli in humans.
  11. Do unicellular organisms (such as bacteria) have an internal environment that they maintain through homeostasis? Why or why not?
  12. Evolution occurs through natural ____________ .
  13. If alien life is found on other planets, do you think the aliens will have cells? Discuss your answer.
  14. Movement in response to an external chemical is called ___________, while movement towards light is called ___________ .

2.2 Explore More

Thumbnail for the embedded element "Characteristics of Life"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=419

Characteristics of Life, Ameoba Sisters, 2017.

Attributions

Figure 2.2.1

The Thinker MET 131262, by Auguste Rodin, 1910, from the Metropolitan Museum of Art, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 2.2.2

Homeostasis: Figure 4, by OpenStax College, Biology is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license. Download for free at http://cnx.org/contents/04fdb865-17a1-43d8-bb33-36f821ddd119@7.

Figure 2.2.3

Human cheek cells, by Joseph Elsbernd, 2012, on Flickr, is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.

Figure 2.2.4

Onion cells 2, by Umberto Salvagnin, 2009, on Flickr, is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 2.2.5

Photo (family) by Jakob Owens on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 2.2.6

Figure 2.2.7

Bacteriophages, by Dr. Graham Beards, is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

References

Ameoba Sisters. (2017, October 26). Characteristics of life. YouTube. https://www.youtube.com/watch?v=cQPVXrV0GNA&feature=youtu.be

OpenStax. (2016, March 23). Figure 4 The body is able to regulate temperature in response to signals from the nervous system. In OpenStax, Biology (Section 33.3). OpenStax CNX. http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8.

Wikipedia contributors. (2020, June 14). Adaptation. Wikipedia. https://en.wikipedia.org/w/index.php?title=Adaptation&oldid=962556016

Wikipedia contributors. (2020, June 21). Auguste Rodin. Wikipedia. https://en.wikipedia.org/w/index.php?title=Auguste_Rodin&oldid=963668399

Wikipedia contributors. (2020, June 22). Chemotaxis. Wikipedia. https://en.wikipedia.org/w/index.php?title=Chemotaxis&oldid=963884872

Wikipedia contributors. (2020, June 22). Evolution. Wikipedia. https://en.wikipedia.org/w/index.php?title=Evolution&oldid=963929880

Wikipedia contributors. (2020, June 20). Phototropism. Wikipedia. https://en.wikipedia.org/w/index.php?title=Phototropism&oldid=963567791

Wikipedia contributors. (2020, June 22). Virus. Wikipedia. https://en.wikipedia.org/w/index.php?title=Virus&oldid=963829311

 

10

2.3 Basic Principles of Biology

Created by CK-12/Adapted by Christine Miller

Why Are Humans Such Sweaty Animals?

Image shows a close-up view of the upper portion of a person's face. The person's skin shows redness due to heat and beads of sweat on their brow.
Figure 2.3.1 Humans sweat to lower their body temperature.

Combine exercise and a hot day, and you get sweat — and lots of it. Sweating is one of the adaptations humans have evolved to maintain homeostasis, or a constant internal environment. When sweat evaporates from the skin, it uses up some of the excess heat energy on the skin, thus helping to reduce the body’s temperature. Humans are among the sweatiest of all species, with a fine-tuned ability to maintain a steady internal temperature, even at very high outside temperatures.

Unifying Principles of Biology

All living things have mechanisms for homeostasis. Homeostasis is one of four basic principles or theories that explain the structure and function of all species (including our own). Whether biologists are interested in ancient life, the life of bacteria, or how humans could live on Mars, they base their understanding of biology on these unifying principles:

Cell Theory

According to cell theory, all living things are made of cells, and living cells come only from other living cells. Each living thing begins life as a single cell. Some living things, including bacteria, remain single-celled. Other living things, including plants and animals, grow and develop into many cells. Your own body is made up of an amazing 100 trillion cells. But even you — like all other living things — began life as a single cell.

Watch this TED-Ed video about the origin of cell theory:

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=421

The Wacky History of Cell Theory – Lauren Royal-Woods, TED-Ed, 2012

Gene Theory

Gene theory is the idea that the characteristics of living things are controlled by genes, which are passed from parents to their offspring. Genes are located on larger structures called chromosomes. Chromosomes are found inside every cell, and they consist of molecules of DNA (deoxyribonucleic acid). Those molecules of DNA are encoded with instructions that “tell” cells how to behave.

Homeostasis

Homeostasis, or the condition in which a system is maintained in a more-or-less steady state, is a characteristic of individual living things, like the human ability to sweat. Homeostasis also applies to the entire biosphere, wherever life is found on Earth. Consider the concentration of oxygen in Earth’s atmosphere. Oxygen makes up 21 per cent of the atmosphere, and this concentration is fairly constant. What maintains this homeostasis in the atmosphere? The answer is living things.

Most living things need oxygen to survive, so they remove oxygen from the air. On the other hand, many living things, including plants, give off oxygen when they convert carbon dioxide and water to food in the process of photosynthesis. These two processes balance out so the air maintains a constant level of oxygen.

Evolutionary Theory

A chameleon on a branch, surrounded by foliage. The chameleon is camouflaged to blend into its surroundings.
Figure 2.3.2 A chameleon exhibits its colour changing adaptation to match its background.

Evolution is a change in the characteristics of populations of living things over time. Evolution can occur by a process called natural selectionwhich results from random genetic mutations in a population. If these mutations lead to changes that allow the living things to better survive, then their chances of surviving and reproducing in a given environment increase. They will then pass more genes to the next generation. Over many generations, this can lead to major changes in the characteristics of those living things. Evolution explains how living things are changing today, as well as how modern living things descended from ancient life forms that no longer exist on Earth.

Traits that help living things survive and reproduce in a given environment are called adaptations. You can see an obvious adaptation in the image below. The chameleon is famous for its ability to change its colour to match its background as camouflage. Using camouflage, the chameleon can hide in plain sight.

Feature: Myth vs. Reality

Misconceptions about evolution are common. They include the following myths:

Myth

Reality

“Evolution is “just” a theory or educated guess.” Scientists accept evolutionary theory as the best explanation for the diversity of life on Earth because of the large body of scientific evidence supporting it. Like any scientific theory, evolution is a broad, evidence-supported explanation for multiple phenomena.
“The theory of evolution explains how life on Earth began.” The theory of evolution explains how life changed on Earth after it began.
“The theory of evolution means that humans evolved from apes like those in zoos.” Humans and modern apes both evolved from a common ape-like ancestor millions of years ago.

2.3 Summary

  • Four basic principles or theories unify all fields of biology: cell theory, gene theory, homeostasis, and evolutionary theory.
  • According to cell theory, all living things are made of cells and come from other living cells.
  • Gene theory states that the characteristics of living things are controlled by genes that pass from parents to offspring.
  • All living things strive to maintain internal balance, or homeostasis.
  • The characteristics of populations of living things change over time through the process of micro-evolution as organisms acquire adaptations, or traits that better suit them to a given environment.

Use the flashcards below to review the four principles:

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=421

2.3 Review Questions

  1. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=421

  2. How does sweating help the human body maintain homeostasis?
  3. Explain cell theory and gene theory.
  4. Describe an example of homeostasis in the atmosphere.
  5. Describe how you can apply the concepts of evolution,natural selection, adaptation, and homeostasis to the human ability to sweat.
  6. Which of the four unifying principles of biology is primarily concerned with:
    • how DNA is passed down to offspring?
    • how internal balance is maintained?
  7. _____________ are located on ______________.
    • chromosomes; genes
    • genes;chromosomes
    • genes; traits
    • none of the above
  8. Define an adaptation and give one example.
  9. Explain how gene theory and evolutionary theory relate to each other.
  10. Does evolution by natural selection occur within one generation? Why or why not?
  11. Explain why you think chameleons evolved the ability to change their colour to match their background, as well as how natural selection may have acted on the ancestors of chameleons to produce this adaptation.

2.3 Explore More

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Myths and misconceptions about evolution – Alex Gendler, TEDEd, 2013

Attributions

Figure 2.3.1

Photo(perspiration), by Hans Reniers on Unsplash. is used under the Unsplash license (https://unsplash.com/license).

Figure 2.3.2

Mediterranean Chameleon Reptile Lizard, by user:1588877 on Pixabay, is used under the Pixabay license (https://pixabay.com/de/service/license/).

References

TED-Ed. (2012, June 4). The wacky history of cell theory – Lauren Royal-Woods. YouTube. https://www.youtube.com/watch?v=4OpBylwH9DU&feature=youtu.be

TED-Ed. (2013, July 8). Myths and misconceptions about evolution – Alex Gendler. YouTube. https://www.youtube.com/watch?v=mZt1Gn0R22Q&t=10s

 

11

2.4 Diversity of Life

Created by CK-12/Adapted by Christine Miller

So Many Species!

Figure 2.4.1 The classification of species from each of the six kingdoms.

The collage shows a single species in each of the six kingdoms into which all of Earth’s living things are commonly classified. How many species are there in each kingdom? In a word: millions. A total of almost two million living species have already been identified, and new species are being discovered all the time. Scientists estimate that there may be as many as 30 million unique species alive on Earth today! Clearly, there is a tremendous variety of life on Earth.

What Is Biodiversity?

Biological diversity, or biodiversity, refers to all of the variety of life that exists on Earth. Biodiversity can be described and measured at three different levels: species diversity, genetic diversity, and ecosystem diversity.

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Why is Biodiversity So Important? – Kim Preshoff, TEDEd, 2015

Defining a Species

Biodiversity is most often measured by counting species, but what is a species? The answer to that question is not as straightforward as you might think. Formally, a species is defined as a group of actually or potentially interbreeding organisms. This means that members of the same species are similar enough to each other to produce fertile offspring together. By this definition of species, all human beings alive today belong to one species, Homo sapiens. All humans can potentially interbreed with each other, but not with members of any other species.

In the real world, it isn’t always possible to make the observations necessary to determine whether or not different organisms can interbreed. For one thing, many species reproduce asexually, so individuals never interbreed — even with members of their own species. When studying extinct species represented by fossils, it is usually impossible to know if different organisms could interbreed. Keep in mind that 99 per cent of all species that have ever existed are now extinct! In practice, many biologists and virtually all paleontologists generally define species on the basis of morphology, rather than breeding behavior. Morphology refers to the form and structure of organisms. For classification purposes, it generally refers to relatively obvious physical traits. Typically, the more similar to one another different organisms appear, the greater the chance that they will be classified in the same species.

Classifying Living Things

People have been trying to classify the tremendous diversity of life on Earth for more than two thousand years. The science of classifying organisms is called taxonomy. Classification is an important step in understanding the present diversity and past evolutionary history of life on Earth. It helps us make sense of the overwhelming diversity of living things.

Linnaean Classification

All modern classification systems have their roots in the Linnaean classification system, which was developed by Swedish botanist Carolus Linnaeus in the 1700s. He tried to classify all living things known in his time by grouping together organisms that s

A diagram of the levels of classification of living things. In order: Life, Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species
Figure 2.4.2 Classification of life.

hared obvious morphological traits, such as number of legs or shape of leaves. For his contribution, Linnaeus is known as the “father of taxonomy.”

The Linnaean system of classification consists of a hierarchy of groupings, called taxa (singular, taxon).  In the original system, taxa ranged from the kingdom to the species. The kingdom (ex. plant kingdom, animal kingdom) is the largest and most inclusive grouping. It consists of organisms that share just a few basic similarities. The species is the smallest and most exclusive grouping. Ideally, it consists of organisms that are similar enough to interbreed, as discussed above. Similar species are classified together in the same genus (plural, genera), then similar genera are classified together in the same family, and so on, all the way up to the kingdom.

A phrase to help you remember the order of the groupings is shown below.  The first letter of each word is the first letter of the level of classification.

Dad Keeps Pots Clean Or Family Gets Sick

 

The hierarchy of taxa in the original Linnaean system of taxonomy included taxa from the species to the kingdom. The domain was added later.

Binomial Nomenclature

Perhaps the single greatest contribution Linnaeus made to science was his method of naming species. This method, called binomial nomenclature, gives each species a unique, two-word Latin name consisting of the genus name followed by a specific species identifier. An example is Homo sapiens, the two-word Latin name for humans. It literally means “wise human.” This is a reference to our big brains.

Why is having two names so important? It is similar to people having a first and a last name. You may know several people with the first name Michael, but adding Michael’s last name usually pins down exactly which Michael you mean. In the same way, having two names for a species helps to uniquely identify it.

Revisions in the Linnaean Classification

Linnaeus published his classification system in the 1700s. Since then, many new species have been discovered. Scientists can also now classify organisms on the basis of their biochemical and genetic similarities and differences, and not just their outward morphology. These changes have led to revisions in the original Linnaean system of classification.

A diagram showing the three domains of life and major groups within each of the domains.
Figure 2.4.3 The three domains of life and major groups within.

A major change to the Linnaean system is the addition of a new taxon called the domain. The domain is a taxon that is larger and more inclusive than the kingdom, as shown in the figure above. Most biologists agree that there are three domains of life on Earth: Bacteria, Archaea, and Eukarya . Both the Bacteria and the Archaea domains consist of single-celled organisms that lack a nucleus. This means that their genetic material is not enclosed within a membrane inside the cell. The Eukarya domain, in contrast, consists of all organisms whose cells do have a nucleus, so that their genetic material is enclosed within a membrane inside the cell. The Eukarya domain is made up of both single-celled and multicellular organisms. This domain includes several kingdoms, including the animal, plant, fungus, and protist kingdoms.

The three domains of life, as well as how they are related to each other and to a common ancestor.  There are several theories about how the three domains are related and which arose first, or from another.

Phylogenetic Classification

Linnaeus classified organisms based on morphology. Basically, organisms were grouped together if they looked alike. After Darwin published his theory of evolution in the 1800s, scientists looked for a way to classify organisms that accounted for phylogeny. Phylogeny is the evolutionary history of a group of related organisms. It is represented by a phylogenetic tree, or some other tree-like diagram, like the one shown above to illustrate the three domains. A phylogenetic tree shows how closely related different groups of organisms are to one another.  Each branching point represents a common ancestor of the branching groups.

2.4 Summary

  • Biodiversity refers to the variety of life that exists on Earth. It includes species diversity, genetic diversity (within species), and ecosystem diversity.
  • The formal biological definition of species is a group of actually or potentially interbreeding organisms. Our own species, Homo sapiens,is an example. In reality, organisms are often classified into species on the basis of morphology.
  • A system for classifying living things was introduced by Linnaeus in the 1700s. It includes taxa from the species (least inclusive) to the kingdom (most inclusive). Linnaeus also introduced a system of naming species, which is called binomial nomenclature.
  • The domain — a taxon higher than the kingdom — was later added to the Linnaean system. Living things are generally grouped into three domains: Bacteria, Archaea, and Eukarya. The human species and other animal species are placed in the Eukarya domain.
  • Modern systems of classification take into account phylogenies, or evolutionary histories of related organisms, rather than just morphological similarities and differences. These relationships are often represented by phylogenetic trees or other tree-like diagrams

2.4 Review Questions

  1. What is biodiversity? Identify three ways that biodiversity may be measured.
  2. Define biological species. Why is this definition often difficult to apply?
  3. Explain why it is important to classify living things, and outline the Linnaean system of classification.
  4. What is binomial nomenclature? Give an example.
  5. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=424

  6. Contrast the Linnaean and phylogenetic systems of classification.
  7. Describe the taxon called the domain, and compare the three widely recognized domains of living things.
  8. Based on the phylogenetic tree for the three domains of life above, explain whether you think Bacteria are more closely related to Archaea or Eukarya.
  9. A scientist discovers a new single-celled organism. Answer the following questions about this discovery.
    1. If this is all you know, can you place the organism into a particular domain? If so, what is the domain? If not, why not?
    2. What is one type of information that could help the scientist classify the organism?
  10. Define morphology. Give an example of a morphological trait in humans.
  11. Which type of biodiversity is represented in the differences between humans?
  12. Why do you think it is important to the definition of a species that members of a species can produce fertile offspring?
  13. Go to the A-Z Animals Animal Classification Page. In the search box, put in your favorite animal and write out it’s classification.

2.4 Explore More

Thumbnail for the embedded element "Classification"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=424

Classification, Amoeba Sisters, 2013.

Attributions

Figure 2.4.1 (6 Kingdoms collage)

Figure 2.4.2

Biological classification, by Pengo [Peter Halasz] on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 2.4.3

The three domains of life and major groups within, by C. Miller, 2019, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2017, March 8). Classification. YouTube. https://www.youtube.com/watch?v=DVouQRAKxYo&feature=youtu.be

A-Z Animals. (2008, December 1). Animal classification. https://a-z-animals.com/reference/animal-classification/

TED-Ed. (2015, April 20). Why is biodiversity so important? – Kim Preshoff. YouTube. https://www.youtube.com/watch?v=GK_vRtHJZu4

Wikipedia contributors. (2020, June 21). Carl Linnaeus. Wikipedia. https://en.wikipedia.org/w/index.php?title=Carl_Linnaeus&oldid=963767022

12

2.5 The Human Animal

Created by CK-12/Adapted by Christine Miller

 

Figure 2.4.1
Figure 2.5.1 Selfie!

Primate Pals

Figure 2.5.1 Humans and monkeys share an evolutionary past.  Humans belong to the animal kingdom, which includes small organisms — like insects — and larger organisms, like humans and monkeys. From genes to morphology to behavior, humans and monkeys are similar in many ways because they share an evolutionary past. Humans and monkeys also both belong to the order Primate, which means they have more traits in common with each other than with insects.

How Humans Are Classified

A diagram showing how humans are classified: Kingdom Animalia, Phylum Chordates, Class mammals, Order primates, Family hominids, Genus homo, Species sapiens
Figure 2.5.2 This taxonomic diagram shows how our Homo sapiens species is classified.

You probably know that modern humans are members of the species Homo sapiens. But what is our place in nature? How is our species classified? A simple classification is represented in the taxonomic diagram below, along with some representative characteristics.

Let’s start at the bottom of the chart, with the kingdom. Among other animal characteristics, humans can move on their own, so they are placed in the animal kingdom. Further, humans belong to the animal phylum known as chordates, because we have a backbone. The human animal has hair and milk glands, so we are placed in the class of mammals. Within the mammal class, humans are placed in the primate order.

Humans as Primates

Living members of the primate order include monkeys, apes, and humans. At some point in the distant past, we shared ape-like ancestors with all of these modern groups of primates. We share between 93 and 99 per cent of our DNA sequences with them, which provides hard evidence that we have relatively recent common ancestors. Besides genes, what traits do we share with other primates?

Primates are considered generalists among the mammals. A generalist is an organism that can thrive in a wide variety of environmental conditions. Generalists also make use of a variety of different resources; for example, they consume many types of food. Although primates exhibit a wide range of characteristics, there are several traits shared by most primates.

Primate Traits

A capuchin monkey sitting on the ground using a stone to break open a large seed pod.
Figure 2.5.3 The capuchin monkey shows manual dexterity common to primates.

Primates have pentadactylism, or five digits (fingers or toes) on each extremity (hand or foot). The fingers and toes have nails instead of claws and are covered with sensitive tactile pads. The thumbs (and in many species, the big toes, as well) are opposable, which means they can be brought into opposition with the other digits, allowing both a power grasp and a precision grip. You can see these features of the primate extremities in the capuchin monkey pictured in Figure 2.15.

The five fingers, opposable thumb, and other primate features of the hand give this capuchin monkey great manual dexterity. This is the primary reason these primates are trained to assist quadraplegic human beings with daily tasks.

The primate body is generally semi-erect or erect, and primates have one of several modes of locomotion, including walking on all four legs (quadrupedalism), vertical clinging and leaping, swinging from branch to branch in trees (brachiation), or walking on two legs (bipedalism, which today only applies to humans). The primate shoulder girdle has a collar bone (clavicle), which is associated with a wide range of motion of the upper limbs.

Relative to other mammals, primates rely less on their sense of smell. They have a reduced snout and relatively small area in the brain for processing olfactory (odor) information. Primates rely more on their sense of vision, which shows several improvements over that of other mammals. Most primates can see in colour. Primates also tend to have large eyes with forward placement in a relatively flat face. This results in an overlap of the visual fields of the two eyes, allowing stereoscopic (or three-dimensional) vision. Other indications of the importance of vision to primates is the protection given the eyes by a complete bony eye socket and the large size of the occipital lobe of the brain, where visual information is processed.

Several pictures of Crab Eating Macaques using stones as tools to help them obtain food by crushing the shells of crabs.
Figure 2.5.4 Crab-eating macaques use a variety of stones as tools to kill and crush crabs in order to get the meat inside the shells.

Primates are noted for their relatively large brains, high degree of intelligence, and complex behaviors. The part of the brain that is especially enlarged in primates is the cerebrum, which analyzes and synthesizes sensory information and transforms it to motor behaviors appropriate to the environment. Primates tend to have longer lifespans than most other mammals. In particular, there is a lengthening of the prenatal period and the postnatal period during which infants depend on adults, providing an extended opportunity for learning among juveniles. Most primates live in social groups. In fact, primates are among the most social of animals. Depending on the species, adult nonhuman primates may live in mated pairs or in groups with hundreds of members. Humans and some nonhuman primates can also make and use tools. The crab-eating macaques pictured below provide examples of tool use in nonhuman primates.

Life in the Trees

Image shows a small monkey perched in the branches of a tree. The monkey has yellow fur on his lower arms, and a dark grey head and snout. His chest, ears and eye area have whitish fur.
Figure 2.5.5 Primates evolved adaptations which suited them to life in the trees.

Scientists think that many primate traits are adaptations to an arboreal (or tree-dwelling) lifestyle. Primates are thought to have evolved in trees, and the majority of primates still live in trees. For life in the trees, the sense of vision trumps the sense of smell, and three-dimensional (3D) vision is especially important for grasping the next branch or limb. Having mobile limbs, a good grip, and manual dexterity are matters of life and death when one lives high above the ground. While some modern primates are mainly terrestrial (ground dwelling) rather than arboreal, all primates possess adaptations for life in the trees.

A map showing the distribution of non human primates. The map shows outlines of the continents, and the areas where non-human primates are highlighted in green. Areas highlighted include the southern portion of the continent of Africa, the northern portion of South America, and the very south east region of Asia, including India.
Figure 2.5.6 This map shows the present worldwide distribution of nonhuman primates.

The map to the left shows the present distribution of nonhuman primates around the world. Tropical forests in Central and South America are home to many species of monkeys, including the squirrel monkey pictured above. Old World tropical forests in Africa and Asia are home to many other species of monkeys, including the crab-eating macaque pictured above, as well as all modern apes.

Humans as Hominids

A mother orangutan holds her baby orangutan in her lap with her arm around the infant.
Figure 2.5.7 Orangutan mother and child.

Who are our closest relatives in the primate order? We are placed in the family called Hominidae. Any member of this family is called a hominid. Hominids include four living genera: chimpanzees, gorillas, orangutans, and humans. Among these four genera are just seven living species: two in each genera, except humans, with our sole living species, Homo sapiens. The orangutan mother pictured cradling her child shows how similar these hominids are to us.

Hominids are relatively large, tailless primates, ranging in size from the bonobo (or pygmy chimpanzee) — which may weigh as little as 30 kg (66 lb) — to the eastern gorilla, which may weigh over 200 kg (440 lb). Most modern humans fall somewhere within that range. In all species of hominids, males are somewhat larger and stronger, on average, than females, but the differences may not be significant. Except for humans, hominids are mainly quadrupedal, although they can get around bipedally if need be to gather food or nesting materials. Humans are the only habitually bipedal species of living hominids.

The Human Genus

Within the hominid family, our species is placed in the genus Homo. Our species, Homo sapiens, is the only living species in this genus. Several earlier species of Homo existed, but have since gone extinct, including the species Homo erectus.

By about 2.8 million years ago, early Homo species such as Homo erectus were probably nearly as efficient at bipedal locomotion as modern humans. Relative to quadrupedal primates, they had a broader pelvis, longer legs, and arched feet. However, from the neck up, they were still quite different from us. They typically had bigger jaws and teeth, a sloping forehead, and a relatively small brain.

Homo sapiens

The skeleton of a homo erectus
Figure 2.5.8 Turkana Boy is the name given to a nearly complete skeleton of a Homo erectus (Homo ergaster) youth who lived at c. 1.5 to 1.6 million years ago. This specimen is the most complete early human skeleton ever found.

During the roughly 2.8 million years of the evolution of the Homo genus, the remaining features of Homo sapiens evolved. These features include:

The increase in brain size occurred very rapidly as far as evolutionary change goes, between about 800 thousand and 100 thousand years ago. During this period, the size of the brain increased from about 600 cm3 to about 1400 cm3 when the earliest Homo sapiens appeared. This was also a period of rapid climate change, and many scientists think that climate change was a major impetus for the evolution of a larger, more complex brain. In this view, as the environment became more unpredictable, bigger and “smarter” brains helped our ancestors survive. Running parallel to the biological evolution of the brain was the development of culture and technology, which were adapted for the purpose of exploiting the environment. These developments, made possible by a big brain, allowed modern humans and their recent ancestors to occupy virtually the entire world and become the dominant land animals.

Our species Homo sapiens is the most recent iteration of the basic primate body plan. Because of our big, complex brain, we clearly have a much greater capacity for abstract thought and technological advances than any other primate — even chimpanzees, who are our closest living relatives. However, it is important to recognize that in other ways, we are not as adept as other living hominids around the world. We are physically weaker than gorillas, far less agile than orangutans, and arguably less well-mannered than bonobos.

Feature: Human Biology in the News

Imagine squeezing through a 7-inch slit in rock to enter a completely dark cave full of lots and lots of old bones. It might sound like a nightmare to most people, but it was a necessary part of a recent exploration of human origins in South Africa as reported in the New York Times in September 2015. The cave and its bones were actually first discovered by spelunkers in 2013, who reported it to paleontologists. An international research project was soon launched to explore the cave. The researchers would eventually conclude that the cave was a hiding place for the dead of a previously unknown early species of Homo, whom they called Homo naledi. Members of this species lived in South Africa around 2.5 to 2.8 million years ago.

This image shows skulls from four different early hominid groups: Homo erectus, Homo habilis, Homo floresiensis, and Homo naldi. Differences in skull thickness, skull shape, brain size and tooth size are shown.
Figure 2.5.9 Comparison of skull features of Homo naledi and other early human species.

Homo naledi individuals were about 5 feet (about 1.5 metres) tall and weighed around 100 pounds (about 45 kilograms), so they probably had no trouble squeezing into the cave. Modern humans are considerably larger on average. In order to retrieve the fossilized bones from the cave, six slender researchers had to be found on social media. They were the only ones who could fit through the crack to access the cave. The work was difficult and dangerous, but also incredibly exciting. The site constitutes one of the largest samples of any extinct early Homo species anywhere in the world, and the fossils represent a completely new species of that genus. The site also suggests that early members of our genus were intentionally depositing their dead in a remote place. This behavior was previously thought to be limited to later humans.

Like other early Homo species, Homo naledi exhibits a mosaic of old and modern traits. From the neck down, these early hominins were well-adapted for upright walking. Their feet were virtually indistinguishable from modern human feet  and their legs were also long like ours. Homo naledi had relatively small front teeth, but also a small brain, no larger than an average orange. Clearly, the spurt in brain growth in Homo did not occur in this species. The image to the left shows the different morphology of early human skulls.

Watch the news for more exciting updates about this early species of our genus. Paleontologists researching the cave site estimate that there are hundreds — if not thousands — of fossilized bones still remaining in the cave. There are sure to be many more discoveries reported in the news media about this extinct Homo species.

2.5 Summary

  • The human species, Homo sapiens, is placed in the primate order of the class of mammals, which are chordates in the animal kingdom.
  • Humans share many traits with other primates. They have five digits with nails and opposable thumbs; an excellent sense of vision, including the ability to see in colour and stereoscopic vision; a large brain, high degree of intelligence, and complex behaviors. Like most other primates, we also live in social groups. Many of our primate traits are adaptations to life in the trees.
  • Within the primate order, our species is placed in the hominid family, which also includes chimpanzees, gorillas, and orangutans. All hominids are relatively large, tailless primates, in which males are generally bigger than females.
  • The genus Homo first evolved about 2.8 million years ago. Early Homo species were fully bipedal, but had small brains. All are now extinct.
  • During the last 800 thousand years, Homo sapiens evolved, with smaller faces, jaws, and front teeth, but much bigger brains than earlier Homo species.

2.5 Review Questions

  1. Outline how humans are classified. Name their taxa, starting with the kingdom and ending with the species.
  2. List several primate traits. Explain how they are related to a life in the trees.
  3. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=427

  4. What are hominids? Describe how living hominids are classified.
  5. Discuss species in the genus Homo.
  6. Relate climatic changes to the evolution of the genus Homo over the last million years.
  7. Why is it significant that we share 93% to 99% of our DNA sequence with other primates?
  8. Which species do you think we are more likely to share a greater amount of DNA sequence with — non-primate mammals (i.e. horses) or non-mammalian chordates (i.e. frogs)? Explain your answer.
  9. What is the relationship between shared DNA and shared traits?
  10. Compared to other mammals, primates have a relatively small area of their brain dedicated to olfactory processing. What does this tell you about the sense of smell in primates compared to other mammals? Why?
  11. Why do you think it is interesting that nonhuman primates can use tools?
  12. Explain why the discovery of Homo naledi was exciting.

2.5 Explore More

Thumbnail for the embedded element "Helping Hands: Matching Capuchins with Those in Need"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=427

Helping Hands: Matching Capuchins with Those in Need, from BUToday, 2009.

 

Thumbnail for the embedded element "New Human Ancestor Discovered: Homo naledi (EXCLUSIVE VIDEO) | National Geographic"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=427

New human ancestor discovered: homo naledi (EXCLUSIVE VIDEO)
by National Geographic, 2015.

Attributions

Figure 2.5.1

Macaca nigra self-portrait on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain). (As the work was created from a non-human animal, it has no human author in whom copyright is vested – See the Wtop.com news article.)

Figure 2.5.2

Classification of the human species, by Christopher Auyeung (based on original image from Peter Halasz on Wikimedia Commons), CK-12 Foundation, is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc-sa/3.0/) license. (Original image in public domain).

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 2.5.3

Stone tool use by a capuchin monkey, by Tiago Falótico , is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/) license.

Figure 2.5.4

Macaca fascicularis aurea stone tools, by Haslam M, Gumert MD, Biro D, Carvalho S, Malaivijitnond S, Figure 2, PLOS One, 2013, is used under a CC BY 2.5 (https://creativecommons.org/licenses/by/2.5/) license.

Figure 2.5.5

Squirrel monkey, on Max Pixel, is used under a CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en) universal public domain dedication license.

Figure 2.5.6

Non-human primate range [map], by Jackhynes, 2008, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 2.5.7

Baby Orangutan 3, by Tony Hisgett, 2012, on Flickr, is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 2.5.8

Homo erectus, by Emőke Dénes, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/) license.

Figure 2.5.9

Comparison of skull features of Homo naledi and other early human species, by Chris Stringer, Natural History Museum, United Kingdom, 2015, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

References

BUToday. (2009, October 5). Helping hands: matching capuchins with those in need. YouTube. https://www.youtube.com/watch?v=vRzp9O9Qc1o&feature=youtu.be

Kelleher, C. (2014, August 22). Monkey ‘selfie’ copyright issue settled [online article]. WTOP.com/News. https://wtop.com/news/2014/08/monkey-selfie-copyright-issue-settled/

National Geographic. (2015, September 5). New human ancestor discovered: homo naledi (EXCLUSIVE VIDEO) | National Geographic. YouTube. https://www.youtube.com/watch?v=oxgnlSbYLSc&feature=youtu.be

13

2.6 Case Study Conclusion: Our Invisible Inhabitants

Created by CK-12/Adapted by Christine Miller

Figure 2.6.1 A photomicrograph of Streptococcus pyogenes bacteria.

As you may recall from the beginning of the chapter, Lanying’s strep throat was caused by Streptococcus pyogenes bacteria, the species shown in the photomicrograph in Figure 2.6.1. She took antibiotics to treat the S. pyogenes infection, but this also affected her “good” bacteria, throwing off the balance of microorganisms living inside her and resulting in diarrhea and a yeast infection.

After reading this chapter, you should know that microorganisms such as bacteria and yeast that live in humans are also similar to us in many ways. They are living organisms, so we share the traits of homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction. Like us, microorganisms contain genes, consist of cells, and have the ability to evolve. Lanying’s beneficial gut bacteria help digest her food as part of their metabolic processes. Lanying got a yeast infection likely because the growth and reproductive rates of the yeast living on her body were not held in check by beneficial bacteria after she took the antibiotics. You can see there are many ways in which an understanding of the basic characteristics of all life can directly apply to your own.

You also learned how living organisms are classified, from bacteria that are in the Bacteria domain, to yeast (fungus kingdom) and humans (animal kingdom) which are both in the Eukarya domain. You probably now recognize that Streptococcus pyogenes is the binomial nomenclature for this species, and the fact that Streptococcus refers to the genus name.

As Lanying’s doctor told her, there are many different species of microorganisms living in the human digestive system. You should recognize this type of biodiversity is called species diversity. This diversity is maintained in a balance, or homeostasis, that can be upset when one type of organism is killed, for instance, by antibiotics.

Lanying’s doctor advised her to complete the entire course of antibiotics because stopping too early would kill or inhibit the bacteria that are most susceptible to the antibiotic, while leaving the bacteria that are more resistant to the antibiotic alive. This difference in susceptibility to antibiotics is an example of genetic diversity. Over time, the surviving antibiotic-resistant bacteria will have increased survival and reproductive rates compared to the more susceptible bacteria, and the trait of antibiotic resistance will become more common in the population. In this way, bacteria can evolve and become better adapted to its environment — at a major cost to our health, because our antibiotics will no longer be effective! Improper use of antibiotics leading to increased antibiotic resistance is an issue of major concern to public health experts.

Figure 2.6.2 Yogurt contains probiotics, including lactobacillus acidophilus, which is important for vaginal health.

Lanying’s doctor suggested that she also take some probiotics — food or supplements that contain “good microorganisms.”  These good bacteria help our bodies digest food and keep out “bad microorganisms.”  Specifically, lactobacillus acidophilus could help combat her yeast infection and help restore normal gut function.

After reading this chapter, you know how humans are classified, and you’ve learned some characteristics of humans and other closely related species. Beyond our more obvious features of big brains, intelligence, and the ability to walk upright, we also serve as a home to many different microorganisms that may be invisible to the naked eye but play a big role in maintaining our health.

Chapter 2 Summary

In this chapter, you learned about the basic principles of biology and how humans are situated among other living organisms. Specifically, you learned:

  • To be classified as a living thing, most scientists agree that an object must exhibit seven characteristics, including:
    • Maintaining a more-or-less constant internal environment, which is called homeostasis.
    • Having multiple levels of organization and consisting of one or more cells.
    • Using energy and being capable of metabolism.
    • Being able to grow and develop.
    • Being capable of evolving adaptations to the environment.
    • Being able to detect and respond to environmental stimuli.
    • Being capable of reproducing, which is the process by which living things give rise to offspring.
  • Four basic principles or theories unify all the fields of biology: cell theory, gene theory, homeostasis, and evolutionary theory.
    • According to cell theory, all living things are made of cells and come from other living cells.
    • Gene theory states that the characteristics of living things are controlled by genes that pass from parents to offspring.
    • All living things — and even the entire biosphere — strive to maintain homeostasis.
    • The characteristics of living things change over time as they evolve, and some acquire adaptations or traits that better suit them to a given environment.
  • Biodiversity refers to the variety of life that exists on Earth. It includes species diversity, genetic diversity within species, and ecosystem diversity.
  • The formal biological definition of “species” is a group of actually or potentially interbreeding organisms. In reality, organisms are often classified into species on the basis of morphology.
  • A system for classifying living things was introduced by Linnaeus in the 1700s. It includes taxa from the species (least inclusive) to the kingdom (most inclusive). Linnaeus also introduced a system of naming species, called binomial nomenclature.
  • The domain, a taxon higher than the kingdom, was later added to the Linnaean system. Living things are generally grouped into three domains: Bacteria, Archaea, and Eukarya. Humans and other animal species are placed in the Eukarya domain.
  • Modern systems of classification take into account phylogenies, or evolutionary histories of related organisms, rather than just morphological similarities and differences. These relationships are often represented by phylogenetic trees or other tree-like diagrams.
  • The human species, Homo sapiens, is placed in the primate order of the class of mammals, which are chordates in the animal kingdom.
  • Traits that humans share with other primates include: five digits with nails and opposable thumbs; an excellent sense of vision, including stereoscopic vision and the ability to see in colour; and a large brain, high degree of intelligence, and complex behaviors. Like most other primates, we also live in social groups. Many of our primate traits are adaptations to life in the trees.
  • Within the primate order, our species is placed in the hominid family, which also includes chimpanzees, gorillas, and orangutans.
  • The genus Homo first evolved about 2.8 million years ago. Early Homo species were fully bipedal but had small brains. All are now extinct.
  • During the last 800 thousand years, Homo sapiens evolved, with smaller faces, jaws, and front teeth, but much bigger brains than earlier Homo species.

Now you understand the basic principles of biology and some of the characteristics of living organisms. In the next chapter, you will learn about the molecules that make up living organisms, as well as the chemistry that allows organisms to exist and function.

Chapter 2 Review 

  1. What are the four basic unifying principles of biology?
  2. A scientist is exploring in a remote area with many unidentified species. He finds an unknown object that does not appear to be living. What is one way he could tell whether it is a dead organism that was once alive or an inanimate object that was never living?
  3. Cows are dependent on bacteria living in their digestive systems to help break down cellulose in the plant material that they eat. Explain what characteristics these bacteria must have to be considered living organisms themselves (and not just part of the cow).
  4. What is the basic unit of structure and function in living things?
  5. Give one example of homeostasis that occurs in humans.
  6. Can a living thing exist without using energy? Why or why not?
  7. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=429

  8. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=429

  9. Give an example of a response to stimuli that occurs in a unicellular organism.
  10. A scientist discovers two types of similar looking insects that have not been previously identified. Answer the following questions about this discovery.
    1. What is one way she can try to determine whether the two types are the same species?
    2. If they are not the same species, what are some ways she can try to determine how closely related they are to each other?
    3. What is the name for a type of diagram she can create to demonstrate their evolutionary relationship to each other and to other insects?
    4. If she determines that the two types are different species but the same genus, create your own names for them using binomial nomenclature. You can be creative and make up the genus and species names, but be sure to put them in the format of binomial nomenclature.
    5. If they are the same species but have different colours, what kind of biodiversity does this most likely reflect?
    6. If they are the same species, but one type of insect has a better sense of smell for their limited food source than the other type, what do you think will happen over time? Assume the insects will experience natural selection.
  1. Amphibians, such as frogs, have a backbone, but no hair. What is the most specific taxon that they share with humans?
  2. What is one characteristic of extinct Homo species that was larger than that of modern humans?
  3. What is one characteristic of modern humans that is larger than that of extinct Homo species?
  4. How does the long period of dependency (of infants on adults) in primates relate to learning?
  5. Name one type of primate in the hominid family, other than humans.
  6. Why do you think that scientists compare the bones of structures (such as the feet) of extinct Homo species to ours?
  7. Some mammals other than primates — such as cats — also have their eyes placed in the front of their face. How do you think the vision of a cat compares to that of a mouse, where the eyes are placed more at the sides?
  8. Living sponges are animals. Are we in the same kingdom as sponges? Explain your answer.

Attributions

Figure 2.6.1

A photomicrograph of Streptococcus pyogenes bacteria, by Centers for Disease Control and Prevention, Public Health Image Library (PHIL) ID#2109, is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 2.6.2

 Yogurt, by Sara Cervera, 2019, is used under the Unsplash License (https://unsplash.com/license).

References

Wikipedia contributors. (2020, April 3). Lactobacillus acidophilus. Wikipedia. https://en.wikipedia.org/w/index.php?title=Lactobacillus_acidophilus&oldid=948947925

III

Chapter 3 - Biological Molecules

14

3.1 Case Study: Chemistry and Your Life

Created by CK-12/Adapted by Christine Miller

Case Study: Diet Dilemma

Image shows equipment related to treatment of diabetes. Blood sugar monitor, insulin, hypodermic needle, and a prescription bottle.
Figure 3.1.1 Diabetes requires careful monitoring and adjustment of blood sugar.

Joseph is a college student who has watched his father suffer from complications of type 2 diabetes for the past few years. For people with type 2 diabetes, the hormone insulin does not transmit its signal sufficiently. Insulin normally removes sugar from the bloodstream and brings it into the body’s cells. Diabetes prevents blood sugar levels from being properly regulated, and this can cause damage to the cells.

Diabetes can be treated with insulin injections, as shown above, as well as with dietary modifications, but complications can still occur. Joseph’s father has some nerve damage (or neuropathy) in his feet which makes his feet numb. He didn’t notice when he developed minor injuries to his feet which caused some serious infections.

A healthy diet and exercise can prevent Type 2 Diabetes. Image shows a man with a backpack on a hike.
Figure 3.1.2 A healthy diet and exercise can prevent Type 2 Diabetes.

Joseph is obese and knows that his weight — along with a family history of diabetes — increases his risk of getting the disease himself. He wants to avoid the health issues that his father suffered, so he begins walking every day for exercise and starts to lose  weight. Joseph also wants to improve his diet in order to lose more weight, lower his risk of diabetes, and improve his general health, but he is overwhelmed with all of the different dietary advice he reads online and hears from his friends and family.

Joseph’s father tells him to limit refined carbohydrates, such as white bread and rice, because that is what he does to help keep his blood sugar at an acceptable level, but Joseph’s friend tells him that eating a diet high in carbohydrates and low in fat is a good way to lose weight. Joseph reads online that “eating clean” by eating whole, unprocessed foods and avoiding food with “chemicals” can help with weight loss. One piece of advice that everyone seems to agree on is that drinking enough water is good for overall health.

All this dietary advice may sound confusing, but you can better understand health conditions, such as diabetes, and the role of diet and nutrition by understanding chemistry. Chemistry is much more than chemical reactions in test tubes in a lab — it is the atoms, molecules, and reactions that make us who we are and keep us alive and functioning properly. Our diets are one of the main ways our bodies take in raw materials that are needed for the important chemical reactions that take place inside of us.

Chapter Overview: Chemistry 

As you read this chapter, you will learn more about how chemistry relates to our lives, health, and the foods we eat. Specifically, you will learn about:

  • The nature of chemical substances, including elements, compounds, and their component atoms and molecules.
  • The structures and functions of biochemical compounds, including carbohydrates, lipids, proteins, and nucleic acids (such as DNA and RNA).
  • What chemical reactions are, how energy is involved in chemical reactions, how enzymes assist in chemical reactions, and some types of biochemical reactions in living organisms.
  • Properties of water and the importance of water for most biochemical processes.
  • What pH is, and why maintaining a proper pH in the body is important for biochemical reactions.

As you read the chapter, think about the following questions regarding Joseph’s situation, as well as how diabetes and diet relate to the chemistry of life:

  1. Why do you think Joseph’s father’s diabetes increases Joseph’s risk of getting diabetes?
  2. What is the difference between refined (simple) carbohydrates and complex carbohydrates? Why are refined carbohydrates particularly problematic for people with diabetes?
  3. Insulin is a peptide hormone. In which class of biochemical compounds would you categorize insulin?
  4. Why is drinking enough water important for overall health? Can you drink too much water?
  5. Sometimes “eating clean” is described as avoiding “chemicals” in food. Think about the definition of “chemicals” and how it relates to what we eat.

Attributions

Figure 3.1.1

Diabetes-equipment by Steve Buissinne [stevepb] on Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).

Figure 3.1.2

Early Morning Hike, by Luke Pamer on Unsplash, is used under the Unsplash license (https://unsplash.com/license).

References

Mayo Clinic Staff. (n.d.). Peripheral neuropathy [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/peripheral-neuropathy/symptoms-causes/syc-20352061

Mayo Clinic Staff. (n.d.). Type 2 diabetes [online article]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/symptoms-causes/syc-20351193

15

3.2 Elements and Compounds

Created by: CK-12/Adapted by Christine Miller

What Are You Made of?

Figure 3.2.1 What are we?

Your entire body is made of cells and cells are made of molecules.If you look at your hand, what do you see? Of course, you see skin, which consists of cells. But what are skin cells made of? Like all living cells, they are made of matter. In fact, all things are made of matter. Matter is anything that takes up space and has mass. Matter, in turn, is made up of chemical substances. A chemical substance is matter that has a definite composition that is consistent throughout. A chemical substance may be either an element or a compound.

Elements and Atoms

An element is a pure substance. It cannot be broken down into other types of substances. Each element is made up of just one type of atom.

Structure of an Atom

Diagram of a lithium atom. Three protons and four neutrons are in the nucleus, and three electrons are orbiting the nucleus.
Figure 3.2.2 An atom consists of three subatomic components: protons, neutrons and electrons.

An atom is the smallest particle of an element that still has the properties of that element. Every substance is composed of atoms. Atoms are extremely small, typically about a ten-billionth of a metre in diametre. However, atoms do not have well-defined boundaries, as suggested by the atomic model shown below.

Every atom is composed of a central area — called the nucleus — and one or more subatomic particles called electrons, which move around the nucleus. The nucleus also consists of subatomic particles. It contains one or more protons and typically a similar number of neutrons. The number of protons in the nucleus determines the type of element an atom represents. An atom of hydrogen, for example, contains just one proton. Atoms of the same element may have different numbers of neutrons in the nucleus. Atoms of the same element with the same number of protons — but different numbers of neutrons — are called isotopes.

Protons have a positive electric charge and neutrons have no electric charge. Virtually all of an atom’s mass is in the protons and neutrons in the nucleus. Electrons surrounding the nucleus have almost no mass, as well as a negative electric charge. If the number of protons and electrons in an atom are equal, then an atom is electrically neutral, because the positive and negative charges cancel each other out. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion.

The negatively-charged electrons of an atom are attracted to the positively-charged protons in the nucleus by a force called electromagnetic force, for which opposite charges attract. Electromagnetic force between protons in the nucleus causes these subatomic particles to repel each other, because they have the same charge. However, the protons and neutrons in the nucleus are attracted to each other by a different force, called nuclear force, which is usually stronger than the electromagnetic force. Nuclear force repels the positively-charged protons from each other.

Periodic Table of the Elements

There are almost 120 known elements. As you can see in the Periodic Table of the Elements shown below, the majority of elements are metals. Examples of metals are iron (Fe) and copper (Cu). Metals are shiny and good conductors of electricity and heat. Nonmetal elements are far fewer in number. They include hydrogen (H) and oxygen (O). They lack the properties of metals.

 The periodic table of the elements arranges elements in groups based on their properties. The element most important to life is carbon (C). Find carbon in the table. What type of element is it: metal or nonmetal?

The Periodic Table of Elements
Figure 3.2.3 The Periodic Table of Elements.

Compounds and Molecules

compound is a unique substance that consists of two or more elements combined in fixed proportions. This means that the composition of a compound is always the same. The smallest particle of most compounds in living things is called a molecule.

Image shows a model of a water molecule. A large central oxygen atom is connected to two adjacent, smaller white hydrogen atoms.
Figure 3.2.4 A molecule of water consists of one atom of oxygen and two atoms of hydrogen connected by covalent bonds.

Consider water as an example. A molecule of water always contains one atom of oxygen and two atoms of hydrogen. The composition of water is expressed by the chemical formula H2O. A model of a water molecule is shown in Figure 3.2.4.

What causes the atoms of a water molecule to “stick” together? The answer is chemical bonds. A chemical bond is a force that holds together the atoms of molecules. Bonds in molecules involve the sharing of electrons among atoms. New chemical bonds form when substances react with one another. A chemical reaction is a process that changes some chemical substances into others. A chemical reaction is needed to form a compound, and another chemical reaction is needed to separate the substances in that compound.

 

3.2 Summary

  • All matter consists of chemical substances. A chemical substance has a definite composition which is consistent throughout. A chemical substance may be either an element or a compound.
  • An element is a pure substance that cannot be broken down into other types of substances.
  • An atom is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative electrons, positive protons, and neutral neutrons. The number of protons in an atom determines the element it represents.
  • Atoms have equal numbers of electrons and protons, so they have no charge. Ions are atoms that have lost or gained electrons, and as a result have either a positive or negative charge. Atoms with the same number of protons — but different numbers of neutrons — are called isotopes.
  • There are almost 120 known elements. The majority of elements are metals. A smaller number are nonmetals. The latter include carbon, hydrogen, and oxygen.
  • A compound is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a molecule. Chemical bonds hold together the atoms of molecules. Compounds can form only in chemical reactions, and they can break down only in other chemical reactions.

3.2 Review Questions

  1. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=436

  2. What is an element? Give three examples.
  3. Define compound. Explain how compounds form.
  4. Compare and contrast atoms and molecules.
  5. The compound called water can be broken down into its constituent elements by applying an electric current to it. What ratio of elements is produced in this process?
  6. Relate ions and isotopes to elements and atoms.
  7. What is the most important element to life?
  8. Iron oxide is often known as rust — the reddish substance you might find on corroded metal. The chemical formula for this type of iron oxide is Fe2O3. Answer the following questions about iron oxide and briefly explain each answer.
    1. Is iron oxide an element or a compound?
    2. Would one particle of iron oxide be considered a molecule or an atom?
    3. Describe the relative proportion of atoms in iron oxide.
    4. What causes the Fe and O to stick together in iron oxide?
    5. Is iron oxide made of metal atoms, metalloid atoms, nonmetal atoms, or a combination of any of these?
  9. 14C is an isotope of carbon used in the radiocarbon dating of organic material. The most common isotope of carbon is 12C. Do you think 14C and 12C have different numbers of neutrons or protons? Explain your answer.
  10. Explain why ions have a positive or negative charge.
  11. Name the three subatomic particles described in this section.

3.2 Explore More

Thumbnail for the embedded element "Just How Small is an Atom?"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=436

Just how small is an atom? TED-Ed, 2012

Attributions

Figure 3.2.1

Man Sitting, by Gregory Culmer, on Unsplash, is used under the Unsplash license (https://unsplash.com/license).

Figure 3.2.2

Lithium Atom diagram, by AG Caesar, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en)

Figure 3.2.3

Periodic Table Armtuk3, by Armtuk, is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/) license.

Figure 3.2.4

Water molecule, by Sakurambo, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

TED-Ed. (2012, April 16). Just how small is an atom. YouTube. https://www.youtube.com/watch?v=yQP4UJhNn0I&feature=youtu.be

 

16

3.3 Biochemical Compounds

Created by: CK-12/Adapted by Christine Miller

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=438

Figure 3.3.1 Carbo-licious!

Carbs Galore

What do all of these foods have in common? All of them consist mainly of large compounds called carbohydrates, often referred to as “carbs.” Contrary to popular belief, carbohydrates are an important part of a healthy diet. They are also one of four major classes of biological macromolecules.

Chemical Compounds in Living Things

Image shows scattered beads and a beaded bracelet.
Figure 3.3.2 The individual beads represent monomers, and when the beads are connected to form the bracelet, it represents a polymer.

The compounds found in living things are known as biochemical compounds or biological molecules. Biochemical compounds make up the cells and other structures of organisms. They also carry out life processes. Carbon is the basis of all biochemical compounds, so carbon is essential to life on Earth. Without carbon, life as we know it could not exist.

Carbon is so basic to life because of its ability to form stable bonds with many elements, including itself. This property allows carbon to create a huge variety of very large and complex molecules. In fact, there are nearly 10 million carbon-based compounds in living things!

Most biochemical compounds are very large molecules called polymers. A polymer is built of repeating units of smaller compounds called monomers. Monomers are like the individual beads on a string of beads, and the whole string is the polymer. The individual beads (monomers) can do some jobs on their own, but sometimes you need a larger molecule, so the monomers can be connected to form polymers.

 

Classes of Biochemical Compounds

Although there are millions of different biochemical compounds in Earth’s living things, all biochemical compounds contain the elements carbon, hydrogen, and oxygen. Some contain only these elements, while others contain additional elements, as well. The vast number of biochemical compounds can be grouped into just four major classes: carbohydrateslipidsproteins, and nucleic acids.

Carbohydrates

Image shows a glucose molecule. The molecule contains 6 carbons fused into a ring with several hydroxide groups.
Figure 3.3.3 Glucose is a common monosaccharide which can form large polymers including starch, glycogen and cellulose.

Carbohydrates include sugars and starches. These compounds contain only the elements carbon, hydrogen, and oxygen. In living things, carbohydrates provide energy to cells, store energy, and form certain structures (such as the cell walls of plants). The monomer that makes up large carbohydrate compounds is called a monosaccharide. The sugar glucose, represented by the chemical model in Figure 3.3.2, is a monosaccharide. It contains six carbon atoms (C), along with several atoms of hydrogen (H) and oxygen (O). Thousands of glucose molecules can join together to form a polysaccharide, such as starch.

 

Lipids

Image shows a bar of butter, two bottles of cooking oil, and a jar of coconut oil.
Figure 3.3.4 Fats and oils are examples of lipids

Lipids include fats and oils. They primarily contain the elements carbon, hydrogen, and oxygen, although some lipids contain additional elements, such as phosphorus. Lipids function in living things to store energy, form cell membranes, and carry messages. Lipids consist of repeating units that join together to form chains called fatty acids. Most naturally occurring fatty acids have an unbranched chain of an even number (generally between 4 and 28) of carbon atoms.

Proteins

Image shows chicken breasts, eggs, nuts and lentils.
Figure 3.3.5 There are many sources of dietary protein.

Proteins include enzymes, antibodies, and many other important compounds in living things. They contain the elements carbon, hydrogen, oxygen, nitrogen, and sulfur. Functions of proteins are very numerous. They help cells keep their shape, compose muscles, speed up chemical reactions, and carry messages and materials. The monomers that make up large protein compounds are called amino acids. There are 20 different amino acids that combine into long chains (called polypeptides) to form the building blocks of a vast array of proteins in living things.

Nucleic Acids

Nucleic acids include the molecules DNA (deoxyribonucleic acid) and RNA(ribonucleic acid). They contain the elements carbon, hydrogen, oxygen, nitrogen, and phosphorus. Their functions in living things are to encode instructions for making proteins, to help make proteins, and to pass instructions between parents and offspring. The monomer that makes up nucleic acids is the nucleotide.  All nucleotides are the same, except for a component called a nitrogen base. There are four different nitrogen bases, and each nucleotide contains one of these four bases. The sequence of nitrogen bases in the chains of nucleotides in DNA and RNA makes up the code for protein synthesis, which is called the genetic code. The animation in Figure 3.3.5 represents the very complex structure of DNA, which consists of two chains of nucleotides.

A rotating model of DNA. It contains long strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The sugar and phosphate groups linking in long chains. Two complementary strands of DNA are bound by hydrogen bonds holding complementary nitrogenous base pairs together.
Figure 3.3.6 DNA is a polymer made of many monomers called nucleotides. DNA carries all the instructions a cell needs to carry out metabolism.

3.3 Summary

  • Biochemical compounds are carbon-based compounds found in living things. They make up cells and other structures of organisms and carry out life processes. Most biochemical compounds are large molecules called polymers that consist of many repeating units of smaller molecules, which are called monomers.
  • There are millions of biochemical compounds, but all of them fall into four major classes: carbohydrates, lipids, proteins, and nucleic acids.
  • Carbohydrates include sugars and starches. They provide cells with energy, store energy, and make up organic structures, such as the cell walls of plants.
  • Lipids include fats and oils. They store energy, form cell membranes, and carry messages.
  • Proteins include enzymes, antibodies, and numerous other important compounds in living things. They have many functions — helping cells keep their shape, making up muscles, speeding up chemical reactions, and carrying messages and materials.
  • Nucleic acids include DNA and RNA. They encode instructions for making proteins, help make proteins, and pass encoded instructions from parents to offspring.

3.3 Review Questions

  1. Why is carbon so important to life on Earth?
  2. What are biochemical compounds?
  3. Describe the diversity of biochemical compounds and explain how they are classified.
  4. Identify two types of carbohydrates. What are the main functions of this class of biochemical compounds?
  5. What roles are played by lipids in living things?
  6. The enzyme amylase is found in saliva. It helps break down starches in foods into simpler sugar molecules. What type of biochemical compound do you think amylase is?
  7. Explain how DNA and RNA contain the genetic code.
  8. What are the three elements present in every class of biochemical compound?
  9. Classify each of the following terms as a monomer or a polymer:
    1. Nucleic acid
    2. Amino acid
    3. Monosaccharide
    4. Protein
    5. Nucleotide
    6. Polysaccharide
  10. Match each  of the above monomers with its correct polymer and identify which class of biochemical compound is represented by each monomer/polymer pair.
  11. Is glucose a monomer or a polymer? Explain your answer.
  12. What is one element contained in proteins and nucleic acids, but not in carbohydrates?
  13. Describe the relationship between proteins and nucleic acids.
  14. Why do you think it is important to eat a diet that contains a balance of carbohydrates, proteins, and fats?
  15. Examine the picture of the meal in Figure 3.3.6.  What types of biochemical compounds can you identify?
Image shows four bowls of food, each containing noodles, a type of meat, green leafy vegetables and green onions in a broth. Each bowl has chopsticks resting on the side, and there are two smaller bowls in the centre holding lime and chilis.
Figure 3.3.7 Which biomolecules do you see represented here?

3.3 Explore More

Thumbnail for the embedded element "Biomolecules (Updated)"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=438

Biomolecules (updated), by the Amoeba Sisters, 2016.

Attributions

Figure 3.3.1

Figure 3.3.2
jewellery_beads_stones_necklace-1200668 on Pxhere, is used under a CC0 1.0 universal public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 3.3.3
Glucose; Structure of beta-D-glucopyranose (Haworth projection), by NEUROtiker on Wikimedia Commons, has been released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.3.4
Lipid Examples; Butter and Oil, by Bill Branson (photographer), on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.3.5
Protein-rich_Foods, by Smastronardo on Wikimedia Commons, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 3.3.6
Bdna_cropped [gif], by Jahobr on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain) (This is a derivative work from Bdna.gif by Spiffistan.)

Figure 3.3.7Dinner by Quốc Trung [@boeing] on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Reference

Amoeba Sisters. (2016, February 11).  Biomolecules (updated). YouTube. https://www.youtube.com/watch?v=YO244P1e9QM&feature=youtu.be

17

3.4 Carbohydrates

Created by: CK-12/Adapted by Christine Miller

The Cellulose of Our Lives

Created by: CK-12/Adapted by Christine Miller

Image shows a pile of jeans of various shades of blue.
Figure 3.4.1 Jeans are made of cotton, and cotton is made of cellulose.

Where would we be without our jeans? They have been the go-to pants for many people for decades, and they are still as popular as ever. Jeans are made of denim, a type of cotton fabric. Cotton is a soft, fluffy fibre that grows in a protective case around the seeds of cotton plants. The fibre is almost pure cellulose. Cellulose is the single most abundant biochemical compound found in Earth’s living things, and it’s one of several types of carbohydrates.

What Are Carbohydrates?

Carbohydrates are the most common class of biochemical compounds. They include sugars and starches. Carbohydrates are used to provide or store energy, among other uses. Like most biochemical compounds, carbohydrates are built of small repeating units, or monomers, which form bonds with each other to make larger molecules, called polymers. In the case of carbohydrates, the small repeating units are known as monosaccharidesEach monosaccharide consists of six carbon atoms, as shown in the model of the monosaccharide glucose shown in Figure 3.4.2.

Figure 3.4.2 A model of the monosaccharide glucose. 

Sugars

Sugars are the general name for sweet, short-chain, soluble carbohydrates, which are found in many foods. Their function in living things is to provide energy. The simplest sugars consist of a single monosaccharide. They include glucose, fructose, and galactose. Glucose is a simple sugar that is used for energy by the cells of living things. Fructose is a simple sugar found in fruits, and galactose is a simple sugar found in milk. Their chemical structures are shown in Figure 3.4.3. All monosaccharides have the formula C6H12O6.

Image shows molecular diagrams of glucose, fructose, galactose, deoxyribose and ribose.
Figure 3.4.3 Five important monosaccharides.

Other sugars contain two monosaccharide molecules and are called disaccharides. These include sucrose (table sugar), maltose, and lactose. Sucrose is composed of one fructose molecule and one glucose molecule, maltose is composed of two glucose molecules, and lactose is composed of one glucose molecule and one galactose molecule. Lactose occurs naturally in milk. Some people are lactose intolerant because they can’t digest lactose. If they drink milk, it causes gas, cramps, and other unpleasant symptoms, unless the milk has been processed to remove the lactose.

Complex Carbohydrates

Some carbohydrates consist of hundreds — or even thousands! — of monosaccharides bonded together in long chains. These carbohydrates are called polysaccharides (“many saccharides”). Polysaccharides are also referred to as complex carbohydrates. Complex carbohydrates that are found in living things include starch, glycogen, cellulose, and chitin. Each type of complex carbohydrate has different functions in living organisms, but they generally either store energy or make up certain structures in living things.

Starch

Image shows potatoes in several colours and sizes.
Figure 3.4.4 Potatoes store glucose made via photosynthesis in the form of starch.

Starch is a complex carbohydrate that is made by plants to store energy. For example, the potatoes pictured in Figure 3.4.4 are packed full of starches that consist mainly of repeating units of glucose and other simple sugars. The leaves of potato plants make sugars by photosynthesis, and the sugars are carried to underground tubers where they are stored as starch. When we eat starchy foods such as potatoes, the starches are broken down by our digestive system into sugars, which provide our cells with energy. Starches are easily and quickly digested with the help of digestive enzymes such as amylase, which is found in the saliva. If you chew a starchy saltine cracker for several minutes, you may start to taste the sugars released as the starch is digested.

Glycogen

Animals do not store energy as starch. Instead, animals store extra energy as the complex carbohydrate glycogen. Glycogen is a polysaccharide of glucose. It serves as a form of energy storage in fungi (as well as animals), and it is the main storage form of glucose in the human body. In humans, glycogen is made and stored primarily in the cells of the liver and muscles. When energy is needed from either storage area, the glycogen is broken down to glucose for use by cells. Muscle glycogen is converted to glucose for use by muscle cells, and liver glycogen is converted to glucose for use throughout the rest of the body. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of lipids, which are the primary form of energy storage in animals.

Glycogen plays a critical part in the homeostasis of glucose levels in the blood. When blood glucose levels rise too high, excess glucose can be stored in the liver by converting it to glycogen. When glucose levels in the blood fall too low, glycogen in the liver can be broken down to glucose and released into the blood.

Diagram shows the way in which the liver controls homeostasis of blood sugar by either storing glucose as glycogen when blood sugar levels are too high, or releasing glucose from glycogen when blood sugar levels are too low.
Figure 3.4.5 Your liver plays an important role in balancing blood sugar levels. Glycogen in your liver can either collect glucose out of your blood stream to lower blood sugar, or release glucose into the bloodstream to increase blood sugar.

Cellulose

Image shows a field of ripe cotton. Waist height dried out brownish plants have white balls of cotton growing from where the flowers once were.
Figure 3.4.6 Cotton fibres represent the purest natural form of cellulose, containing more than 90 per cent of this polysaccharide.

Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands of linked glucose units. Cellulose is an important structural component of the cell walls of plants and many algae. Human uses of cellulose include the production of cardboard and paper, which consist mostly of cellulose from wood and cotton. The cotton fibres pictured are about 90 per cent cellulose.

Certain animals, including termites and ruminants such as cows, can digest cellulose with the help of microorganisms that live in their gut. Humans cannot digest cellulose, but it nonetheless plays an important role in our diet. It acts as a water-attracting bulking agent for feces in the digestive tract and is often referred to as “dietary fibre.”  In simpler terms, it helps you poop.

Chitin

Image shows a ladbug perched on a mushroom.
Figure 3.4.7 Chitin is an important structural component in fungal cell walls and the exoskeletons of insects.

Chitin is a long-chain polymer of a derivative of glucose. It is found in many living things. For example, it is a component of the cell walls of fungi; the exoskeletons of arthropods, such as crustaceans and insects ; and the beaks and internal shells of animals, such as squids and octopuses. The structure of chitin is similar to that of cellulose.

In Figure 3.4.7, both the exoskeleton of the ladybug and the cell walls of the mushroom are made partly of the complex carbohydrate chitin.

The Right Molecule for the Job

Starch, glycogen, cellulose and chitin are all made from the monomer glucose.  So how are they all so different?  Their difference in structure and function is related to how they are linked together.  Starch is linked in long chains with a small amount of branching, glycogen is linked in many branching chains, and chitin and cellulose form long single chains that pack together tightly.  Each of these variations of linking the same monomer, glucose, together creates a different way the molecule can be used.  As shown in the Figure 3.4.8 diagram, starch and glycogen have many exposed “ends” of their chains.  These are areas where a glucose molecule can easily be removed for use as energy, whereas cellulose does not.  For this reason, glycogen and starch are well-suited for energy storage in organisms while cellulose is not.  Conversely, cellulose packs many monomers together in a sort of mesh that is very strong — this is why it is a great option for building strong cell walls.

Image shows molecules of starch, glycogen and cellulose.
Figure 3.4.8 Starch, glycogen and cellulose are all made of many linked monomers of glucose. The shape and bonding of these monomers affects the function of the molecule.

Feature: My Human Biology

You probably know that you should eat plenty of fibre, but do you know how much fibre you need, how fibre contributes to good health, or which foods are good sources of fibre? Dietary fibre consists mainly of cellulose, so it is found primarily in plant-based foods, including fruits, vegetables, whole grains, and legumes. Dietary fibre can’t be broken down and absorbed by your digestive system. Instead, it passes relatively unchanged through your gastrointestinal tract and is excreted in feces (otherwise known as poop). That’s how it helps keep you healthy.

Image shows a bowl of kidney beans.
Figure 3.4.9 Beans are an excellent source of both soluble and insoluble fibre.

Fibre in food is commonly classified as either soluble or insoluble fibre.

How much fibre do you need for good health? That depends on your age and gender. The Institute of Medicine recommends the daily fibre intake for adults shown in Table 3.4.1 below. Most dietitians further recommend a ratio of about three parts of insoluble fibre to one part of soluble fibre each day. Most fibre-rich foods contain both types of fibre, so it usually isn’t necessary to keep track of the two types of fibre as long as your overall fibre intake is adequate.

Table 3.4.1

Recommended Daily Fibre Intake for Males and Females

Recommended Daily Fibre Intake for Males and Females
Gender Age 50 or Younger Age 51 or Older
Male 38 grams 30 grams
Female 25 grams 21 grams

Use food labels like the one shown below in Figure 3.4.10 and online fibre counters to find out how much total fibre you eat in a typical day. Are you consuming enough fibre for good health? If not, consider ways to increase your intake of this important substance. For example, substitute whole grains for refined grains, eat more legumes (such as beans), and try to consume at least five servings of fruits and vegetables each day.

 

Image shows a nutrition label. It lists information about calories, fat, cholesterol, sodium, carbohydrates, protein and vitamins. This example shows that the food contains 4 grams of dietary fibre per serving.
Figure 3.4.10 You can determine how much dietary fibre is in your food by reading the nutrition label.

Table 3.4.2

Carbohydrate Comparison

Name
Class
Function
Location
Glucose Monosaccharide Energy for cells Cells
Starch Polysaccharide Energy storage Plant cells
Glycogen Polysaccharide Energy storage Animal cells
Cellulose Polysaccharide Structural component in cell walls Plant cells
Chitin Polysaccharide Structural component in cell walls and exoskeletons Fungi and arthropods

 

3.4 Summary

  • Carbohydrates are the most common class of biochemical compounds. The basic building block of carbohydrates is the monosaccharide, which consists of six carbon atoms.
  • Sugars are sweet, short-chain, soluble carbohydrates that are found in many foods and supply us with energy. Simple sugars, such as glucose, consist of just one monosaccharide. Some sugars, such as sucrose (or table sugar), consist of two monosaccharides. These are called disaccharides.
  • Complex carbohydrates, or polysaccharides, consist of hundreds — or even thousands — of monosaccharides. They include starch, glycogen, cellulose, and chitin. They generally either store energy or form structures, such as cell walls, in living things.
  • Starch is a complex carbohydrate that is made by plants to store energy. Potatoes are a good food source of dietary starch, which is readily broken down into its component sugars during digestion.
  • Glycogen is a complex carbohydrate that is made by animals and fungi to store energy. Glycogen plays a critical part in the homeostasis of blood glucose levels in humans.
  • Cellulose is the single most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Like most other animals, humans cannot digest cellulose, but it makes up most of the crucial dietary fibre in the human diet.
  • Chitin is a complex carbohydrate, similar to cellulose, that makes up organic structures, such as the cell walls of fungi and the exoskeletons of insects and other arthropods.

3.4 Review Questions

  1. What are carbohydrates? Describe their structure.
  2. Compare and contrast sugars and complex carbohydrates.
  3. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=440

  4. If you chew on a starchy food (such as a saltine cracker) for several minutes, it may start to taste sweet. Explain why.
  5. True or False: Glucose is mainly stored by lipids in the human body.
  6. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=440

  7. Name three carbohydrates that contain glucose as a monomer.
  8. Jeans are made of tough, durable cotton. Based on what you know about the structure of carbohydrates, explain how you think this fabric gets its tough qualities.
  9. Which do you think is faster to digest — simple sugars or complex carbohydrates? Explain your answer.
  10. True or False: Cellulose is broken down in the human digestive system into glucose molecules.
  11. ___________ fibre dissolves in water, __________ fibre does not dissolve in water.
  12. What are the similarities and differences between muscle glycogen and liver glycogen?
  13. Which carbohydrate is used directly by the cells of living things for energy?
  14. Which of the following is not a complex carbohydrate?
    • Chitin
    • Starch
    • Disaccharide
    • None of the above

3.4 Explore More

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How do carbohydrates impact your health? – Richard J. Wood, TED-Ed, 2016

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Why is cotton in everything? – Michael R. Stiff, TED-Ed, 2020

Attributions

Figure 3.4.1

Pile of Jeans by Marco Verch, on Flickr, is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 3.4.2

e-from-xtal-1979-Alpha-D-glucose-from-xtal-1979-3D-balls by Ben Mills [Benjah-bmm27] on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.4.3

Monosasccharides by OpenStax College on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 3.4.4

Potatoes by Jean Beaufort, on Public Domain Pictures.net, is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 3.4.5

Homeostasis_of_blood_sugar by Christine Miller [christinelmiller] Is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 3.4.6

Cotton by David Nance for Agricultural Research Service, the research agency of the United States Department of Agriculture, on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.4.7

Ladybug on a mushroom /Fungi in the Woods by Benjamin Balázs on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.4.8

Carbohydrate structure comparison [Three Important Polysaccharides] by OpenStax College is on Wikimedia Commons, used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 3.4.9

Beans by Milada Vigerova on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.4.10

FDA Nutrition Facts Label 2014, by US Food and Drug Administration, on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Table 3.4.1

Recommended Daily Fibre Intake for Males and Females is from OpenStax, used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license..

Table 3.4.2

Carbohydrate Comparison is from OpenStax. used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

References

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 2.18. Five important monosaccharides [image]. In Anatomy and Physiology.  OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 2.20. Three important polysaccharides [image]. In Anatomy and Physiology.  OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

Mayo Clinic. (n.d.). Lactose intolerance [online article]. Mayo Foundation for Medical Education and Research (MFMER). https://www.mayoclinic.org/diseases-conditions/lactose-intolerance/symptoms-causes/syc-20374232

TED-Ed. (2016, January 11). How do carbohydrates impact your health? – Richard J. Wood. YouTube. https://youtu.be/wxzc_2c6GMg

TED-Ed. (2020, January 23). Why is cotton in everything? – Michael R. Stiff. https://www.youtube.com/watch?v=tKLJ6KQAcjI

18

3.5 Lipids

Created by: CK-12/Adapted by Christine Miller

Yum!

Image shows a cheeseburger and fries in a cardboard lunchbox.
Figure 3.5.1 Lipids can be unhealthy if consumed in large quantities.

It glistens with fat, from the cheese to the fries. Both cheese and fries are typically high-fat foods, so this meal is definitely not recommended if you are following a low-fat diet. We need some fats in our diet for good health, but too much of a good thing can be harmful to our health, no matter how delicious it tastes. What are fats? And why do we have such a love-hate relationship with them? Read on to find out.

Lipids and Fatty Acids

Fats are actually a type of lipid. Lipids are a major class of biochemical compounds that includes oils, as well as fats. Among other things, organisms use lipids to store energy.

Lipid molecules consist mainly of repeating units called fatty acids. There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids. Both types consist mainly of simple chains of carbon atoms bonded to one another and to hydrogen atoms. The two types of fatty acids differ in how many hydrogen atoms they contain and the number of bonds between carbon atoms.

3.5 Cultural Connection: Fats in Tanning

Image shows a plains first nations rifle guncase made from the hide of a buffalo. It has beadwork and fringes.
Figure 3.5.2 The Plains First Nations used buffalo brains to tan their buffalo hides. These tanned hides where soft, flexible, and waterproof.

Ancient civilizations all over the world have used fats in the hide tanning process.  If raw hides (animal skins) aren’t tanned, they get very brittle and can breakdown.  Tanning results in a hide that is soft, flexible, and resists decay.

One method of tanning is called “brain tanning”.  It’s name is quite self-explanatory — a mixture of boiled animal brains is used to tan a hide.  A type of fat in the brain, called lecithin, is a natural tanning agent.  Once the hide has been rubbed with the brain mixture, it is smoked and then it is ready for use!

Brain tanning is preferred in many cultures because it creates hides which are waterproof and it doesn’t create environmentally harmful byproducts.

Saturated Fatty Acids

In saturated fatty acids, carbon atoms are bonded to as many hydrogen atoms as possible. All the carbon-to-carbon atoms share just single bonds between them. This causes the molecules to form straight chains, as shown in the figure below. The straight chains can be packed together very tightly, allowing them to store energy in a compact form. Saturated fatty acids have relatively high melting points, which explains why they are solids at room temperature. Animals use saturated fatty acids to store energy.  Some dietary examples of saturated fats include butter and lard.

Diagram shows examples of the shapes of different types of fatty acids. Saturated fatty acids form long straight chains. Monounsaturated fatty acids have a slight curve and saturated fatty acids can have multiple curves or bends.
Figure 3.5.3 Fatty acids can be saturated, monounsaturated, or unsaturated. This affects their state (solid or liquid) at room temperature.

Unsaturated Fatty Acids

In unsaturated fatty acids, some carbon atoms are not bonded to as many hydrogen atoms as possible. Instead, they form double or even triple bonds with other carbon atoms. This causes the chains to bend (see Figure 3.5.3). The bent chains cannot be packed together very tightly. Unsaturated fatty acids have relatively low melting points, which explains why they are liquids at room temperature. Plants use unsaturated fatty acids to store energy.

Monounsaturated fatty acids contain one less hydrogen atom than the same-length saturated fatty acid chain. Monounsaturated fatty acids are liquids at room temperature, but start to solidify at refrigerator temperatures. Good food sources of monounsaturated fats include olive oils, peanut oils, and avocados.

Polyunsaturated fatty acids contain at least two fewer hydrogen atoms than the same-length saturated fatty acid chain. Polyunsaturated fatty acids are liquids at room temperature and remain in the liquid state in the refrigerator. Good food sources of polyunsaturated fats include safflower oils, soybean oils, and many nuts and seeds.

Types of Lipids

Lipids may consist of fatty acids alone, or they may contain other chemical components, as well. For example, some lipids contain alcohol or phosphate groups. Types of lipids include triglycerides, phospholipids, and steroids. Each type has different functions in living things.

Triglycerides

Triglycerides are formed by combining a molecule of glycerol with three fatty acid molecules, as shown below. Glycerol (also called glycerine) is a simple compound known as a sugar alcohol. It is a colourless, odorless liquid that is sweet tasting and nontoxic. Triglycerides are the main constituent of body fat in humans and other animals. They are also found in fats derived from plants. There are many different types of triglycerides, with the main division being between those that contain saturated fatty acids and those that contain unsaturated fatty acids.

Image shows a model of a triglyceride. The glycerol molecule runs vertically along the left, and three saturated fatty acids run out horizontally from each of the three carbons in the glycerol molecule.
Figure 3.5.4 Triglycerides consist of a glycerol molecule (along the left side) with three attached fatty acids (coming off the right side). This diagram shows a saturated fatty acid, the storage form of fat in animals.

 

In the human bloodstream, triglycerides play an important role in metabolism as energy sources and transporters of dietary fat. They contain more than twice as much energy as carbohydrates, the other major source of energy in the diet. When you eat, your body converts any calories it doesn’t need to use right away into triglycerides, which are stored in your fat cells. When you need energy between meals, hormones trigger the release of some of these stored triglycerides back into the bloodstream.

Phospholipids

Image shows a model of a phospholipid molecule. The phosphate group is at the top of the diagram, it is connected to a glycerol molecule below. The phosphate and glycerol molecule are grouped together and enclosed in a red circle. Two fatty acids are hanging below, attached to two neighbouring carbons on the glycerol molecule. The diagram notes that the glycerol/phosphate portion of the molecule is hydrophilic, and the fatty acids are hydrophobic.
Figure 3.5.5 A phospholipid is made up of a phosphate group connected to glycerol, which is connected to two fatty acids.

Phospholipids are a major component of the cell membranes of all living things. Each phospholipid molecule has a “tail” consisting of two long fatty acids, and a “head” consisting of a phosphate group and glycerol molecule (see Figure 3.5.5). The phosphate group is a small, negatively-charged molecule causing it to be hydrophilic, or attracted to water. The fatty acid tail of the phospholipid is hydrophobic, or repelled by water. These properties allow phospholipids to form a two-layered cell membrane, which is also called a bilayer.

 

As shown in Figure 3.5.6, a phospholipid bilayer forms when many phospholipid molecules line up tail to tail, forming an inner and outer surface of hydrophilic heads. The hydrophyilic heads point toward both the watery extracellular space and the watery inside space (lumen) of the cell.  The hydrophobic fatty acids are nestled in the inner space of the bilayer.

Diagram shows a phospholipid bilayer. It consists of two mats of phospholipids layered on top of one another. The top mat has the hydrophilic heads oriented up, and the bottom layer has the hydrophilic heads oriented down, causing the hydrophobic regions of the two layers to come into contact.
Figure 3.5.6 Cell membranes consist of a double layer of phospholipid molecules.

 

Image shows a ball and stick diagram of the steroid progesterone. Progesterone consists of four fused carbon rings.
Figure 3.5.7 Progesterone is an example of a steroid.

Steroids

Steroids are lipids with a ring structure. Each steroid has a core of 17 carbon atoms, which are arranged in four rings of five or six carbons each (pictured in Figure 3.5.7). Steroids vary by the other components attached to this four-ring core. Hundreds of steroids are found in plants, animals, and fungi, but most steroids have one of just two principal biological functions. Some steroids (such as cholesterol) are important components of cell membranes, while many other steroids are hormones, which are messenger molecules. In humans, steroid hormones include cortisone — a fight-or-flight hormone — and the sex hormones estrogen, progesterone and testosterone.

Feature: My Human Body

During a routine checkup with your family doctor, your blood was collected for a lipid profile. The results are back, and your triglyceride level is 180 mg/dL. Your doctor says this is a little high. A blood triglyceride level of 150 mg/dL or lower is considered normal. Higher levels of triglycerides in the blood have been linked to an increased risk of atherosclerosisheart disease, and stroke.

Image shows a blue plate holding yogurt, soy beans, olives, pimentos, chickpeas, flatbread and various other diced vegetables.
Figure 3.5.8 Changing your diet can help keep blood lipid levels healthy.

If a blood test reveals that you have high triglycerides, the levels can be lowered through healthy lifestyle choices and/or prescription medications. Healthy lifestyle choices to control triglyceride levels include:

If healthy lifestyle changes aren’t enough to bring down high triglyceride levels, drugs prescribed by your doctor are likely to help.

3.5 Summary

  • Lipids are a major class of biochemical compounds that includes oils and fats. Organisms use lipids for storing energy and for making cell membranes and hormones, which are chemical messengers.
  • Lipid molecules consist mainly of repeating units called fatty acids. Depending on the proportion of hydrogen atoms they contain, fatty acids may be saturated or unsaturated. Animals store fat as saturated fatty acids, while plants store fat as unsaturated fatty acids.
  • Types of lipids include triglycerides, phospholipids, and steroids. Each type consists of fatty acids and certain other molecules. Each also has different functions.
  • Triglycerides contain glycerol (an alcohol), in addition to fatty acids. Humans and other animals store fat as triglycerides in fat cells.
  • In addition to fatty acids, phospholipids contain phosphate and glycerol. They are the main component of cell membranes in all living things.
  • Steroids are lipids with a four-ring structure. Some steroids (such as cholesterol) are important components of cell membranes. Many other steroids are hormones. An example of a human hormone is cortisone, which is the fight-or-flight hormone.

3.5 Review Questions

  1. What are lipids?
  2. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=442

  3. Compare and contrast saturated and unsaturated fatty acids.
  4. Identify three major types of lipids. Describe differences in their structures.
  5. How do triglycerides play an important role in human metabolism?
  6. Explain how phospholipids form cell membranes.
  7. What is cholesterol? What is its major function?
  8. Give three examples of steroid hormones in humans.
  9. Which type of fatty acid do you think is predominant in the cheeseburger and fries shown above? Explain your answer.
  10. Which type of fat would be the most likely to stay liquid in colder temperatures: bacon fat, olive oil, or soybean oil? Explain your answer.
  11. Why do you think that the shape of the different types of fatty acid molecules affects how easily they solidify?  Can you think of an analogy for this?
  12. High cholesterol levels in the bloodstream can cause negative health effects. Explain why we wouldn’t want to get rid of all of the cholesterol in our bodies.

3.5 Explore More

Thumbnail for the embedded element "Cortisone and Healing - An overview of the science"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=442

Cortisone and Healing – An overview of the science, by Sportology and OrthoCarolina, 2015

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What is fat? – George Zaidan, TED-Ed, 2013

Attributions

Figure 3.5.1

cheeseburger by kayleigh harrington on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.5.2

Buffalo_Hide_Beaded_Guncase by Unknown onWikimedia Commons, is used under the Missouri History Museum‘s MHS Open Access Policy. Image is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.5.3

Fatty acids by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 3.5.4 

Fat_triglyceride_shorthand_formula by Wolfgang Schaefer on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.5.5

Phospholipid_Structure by OpenStax on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 3.5.6

Phospholipid_Bilayer by OpenStax on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 3.5.7

Progesterone, 5alpha-Dihydroprogesterone 3D ball by Jynto is used under a CC0 1.0 Universal Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).

Figure 3.5.8

Healthy plate by Edgar Castrejon on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.2. Phospholipid structure [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.3. Phospolipid bilayer [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Mayo Clinic. (n.d.). Arteriosclerosis / atherosclerosis [online article]. https://www.mayoclinic.org/diseases-conditions/arteriosclerosis-atherosclerosis/symptoms-causes/syc-20350569

Mayo Clinic. (n.d.). Heart disease [online article]. https://www.mayoclinic.org/diseases-conditions/heart-disease/symptoms-causes/syc-20353118

Mayo Clinic. (n.d.). Stroke [online article]. https://www.mayoclinic.org/diseases-conditions/stroke/symptoms-causes/syc-20350113

Sportology/OrthoCarolina. (2015, February 26). Cortisone and healing – An overview of the science. YouTube. https://www.youtube.com/watch?v=zqSoyaDu4b0&feature=youtu.be

TED-Ed. (2013, May 22). What is fat? – George Zaidan. YouTube. https://www.youtube.com/watch?v=QhUrc4BnPgg&feature=youtu.be

19

3.6 Proteins

Created by: CK-12/Adapted by Christine Miller

Protein Shake

Image shows a glass containing a brown protein shake. Beside the glass are the ingredients used to make the shake: a small container of protein powder and a larger container of milk.
Figure 3.6.1 Protein shakes vary in quality based on which amino acids they contain.

Drinks like this shake contain a lot of protein. Muscle tissue consists mainly of protein, so such drinks are popular with people who want to build muscle. Making up muscles is just one of a plethora of functions of this amazingly diverse class of biochemicals.

What Are Proteins?

Proteins are a major class of biochemical compounds made up of small monomer molecules called amino acidsMore than 20 different amino acids are typically found in the proteins of living things. Small proteins may contain just a few hundred amino acids, while large proteins may contain thousands.

Protein Structure

When amino acids bind together, they may form short chains of two or just a few amino acids. These short chains are called peptides. When amino acids form long chains, the chains are called polypeptides. A protein consists of one or more polypeptides.

Proteins may have up to four levels of structure, from primary to quaternary.  As a result, they can have tremendous diversity. Here are some additional details about the levels of protein structure:

Figure 3.6.2 Four protein structures.

Functions of Proteins

The diversity of protein structures explains why this class of biochemical compounds can play so many important roles in living things. What are the roles of proteins?

The chief characteristic of proteins that allows their diverse set of functions is their ability to bind to other molecules so specifically and tightly. Myoglobin can bind specifically and tightly with oxygen. The region of a protein responsible for binding with another molecule is known as the binding site. This site is often a depression on the molecular surface, determined largely by the tertiary structure of the protein.

Protein Consumption, Digestion, and Synthesis

Proteins are necessary in the diets of humans and other animals. We cannot make all the different amino acids we need, so we must obtain some of them from the foods we consume. In the process of digestion, we break down the proteins in food into free amino acids that can then be used to synthesize our own proteins. Protein synthesis from amino acid monomers takes place in all cells and is controlled by genes. Once new proteins are synthesized, they generally do not last very long before they are degraded and their amino acids are recycled. A protein’s lifespan in mammalian cells is generally just a day or two.

3.6 Summary

  • Proteins are a major class of biochemical compounds. They’re made up of small monomer molecules called amino acids. More than 20 amino acids are commonly found in the proteins of living things. Proteins have tremendous diversity in terms of both structure and function.
  • Long chains of amino acids form polypeptides. The sequence of amino acids in polypeptides makes up the primary structure of proteins. Proteins also have higher levels of structure. Secondary structure refers to configurations — such as helices and sheets — within polypeptide chains. Tertiary structure is a protein’s overall three-dimensional shape, which controls the molecule’s basic function. A quaternary structure forms if multiple protein molecules join together and function as a complex.
  • Proteins help cells keep their shape, make up muscle tissues, act as enzymes or antibodies, and carry messages or materials. The chief characteristic that allows proteins’ diverse functions is their ability to bind specifically and tightly with other molecules.
  • We cannot make all the amino acids we need to synthesize our own proteins, so we must obtain some of them from proteins in the foods we consume.

3.6 Review Questions

  1. What are proteins?
  2. Outline the four levels of protein structure.
  3. Identify four functions of proteins.
  4. Explain why proteins can take on so many different functions in living things.
  5. What is the role of proteins in the human diet?
  6. Can you have a protein with both an alpha helix and a pleated sheet? Why or why not?
  7. If there is a mutation in a gene that causes a different amino acid to be encoded than the one usually encoded in that position within the protein, would that affect:
    • The primary structure of the protein? Explain your answer.
    • The higher structures (secondary, tertiary, quaternary) of the protein? Explain your answer.
    • The function of the protein? Explain your answer.
  8. What is the region of a protein responsible for binding to another molecule? Which level or levels of protein structure creates this region?
  9. What is the region of a protein responsible for binding to another molecule? Which level or levels of protein structure creates this region?
  10. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=444

  11. True or False: You can tell the function of all proteins based on their quaternary structure.
  12. Explain what the reading means when it says that amino acids are “recycled.”

3.6 Explore More

Thumbnail for the embedded element "Protein Structure and Folding"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=444

Protein Structure and Folding by The Amoeba Sisters, 2018.

Attributions

Figure 3.6.1

Protein_shake by Sandstein, on Wikimedia Commons, is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 3.6.2

Structures of Protein by OpenStax, on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

References

Amoeba Sisters. (2018, September 24). Protein structure and folding. YouTube. https://www.youtube.com/watch?v=hok2hyED9go

OpenStax. (2012, Aug 22). Figure 9. The four levels of protein structure can be observed in these illustrations. (credit: modification of work by National Human Genome Research Institute). In Biology. OpenStax CNX. © Rice University. https://cnx.org/contents/GFy_h8cu@10.53:2zzm1QG9@7/Proteins (last revised May 27, 2016).

20

3.7 Nucleic Acids

Created by: CK-12/Adapted by Christine Miller

Who’s Who?

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=446

Figure 3.7.1 Identical twins show clearly the importance of genes in making us who we are. Genes would not be possible without nucleic acids.

What Are Nucleic Acids?

Nucleic acids are the class of biochemical compounds that includes DNA and RNA. These molecules are built of small monomers called nucleotides. Many nucleotides bind together to form a chain called a polynucleotide. The nucleic acid DNA (deoxyribonucleic acid) consists of two polynucleotide chains or strands. Thus, DNA is sometimes called double-stranded. The nucleic acid RNA (ribonucleic acid) consists of just one polynucleotide chain or strand, so RNA is sometimes called single-stranded.

Structure of Nucleic Acids

Each nucleotide consists of three smaller molecules:

  1. A sugar molecule (the sugar deoxyribose in DNA and the sugar ribose in RNA)
  2. A phosphate group
  3. A nitrogen base

The nitrogen bases in a nucleic acid stick out from the backbone. There are four different nitrogen bases: cytosine, adenine, guanine, and either thymine (in DNA) or uracil (in RNA). In DNA, bonds form between bases on the two nucleotide chains and hold the chains together. Each type of base binds with just one other type of base: cytosine always binds with guanine, and adenine always binds with thymine. These pairs of bases are called complementary base  pairs.

A short section of DNA showing complementary base pairing. Shows alternating deoxyribose and phosphate groups forming the two strands of the backbone of the molecule, and the nitrogenous bases pairing in the middle of the polymer- adenine pairing with thymine, and cytosine pairing with guanine.
Figure 3.7.2 A short section of DNA showing complementary base pairing.

As you can see in Figure 3.7.2, sugars and phosphate groups form the backbone of a polynucleotide chain. Hydrogen bonds between complementary bases hold the two polynucleotide chains together.

A rotating model of DNA. It contains long strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The sugar and phosphate groups linking in long chains. Two complementary strands of DNA are bound by hydrogen bonds holding complementary nitrogenous base pairs together.
Figure 3.7.3 DNA is a polymer made of many monomers called nucleotides. DNA carries all the instructions a cell needs to carry out metabolism.

The binding of complementary bases causes DNA molecules automatically to take their well-known double helix shape, which is shown in the animation in Figure 3.7.3. A double helix is like a spiral staircase. It forms naturally and is very strong, making the two polynucleotide chains difficult to break apart.

DNA Molecule. Hydrogen bonds between complementary bases help form the double helix of a DNA molecule. The letters A, T, G, and C stand for the bases adenine, thymine, guanine, and cytosine. The sequence of these four bases in DNA is a code that carries instructions for making proteins. Shown is a representation of how the double helix folds into a chromosome.

 

 

Roles of Nucleic Acids

DNA makes up genes, and the sequence of bases in DNA makes up the genetic code. Between “starts” and “stops,” the code carries instructions for the correct sequence of amino acids in a protein. RNA uses the information in DNA to assemble the correct amino acids and help make the protein. The information in DNA is passed from parent cells to daughter cells whenever cells divide, and it is also passed from parents to offspring when organisms reproduce. This is how inherited characteristics are passed from one generation to the next.

Image shows a diagram of the ATP molecule which consists of adenosine, ribose, and three phosphate groups. When the bond between the second and third phosphate group is broken, energy previously stored in the chemical bonds is released.
Figure 3.7.4 ATP (adenosine TRI phosphate) can be converted to ADP (adenosine DI phosphate) to release the energy stored in the chemical bonds between the second and third phosphate group.

ATP is Energy

There is one type of specialized nucleic acid that exists only as a monomer.  It stands apart from the other nucleic acids because it does not code for, or help create, proteins.   This molecule is ATP, which stands for adenosine triphosphate.  It consists of a sugar, adenosine, and three phosphate groups.  It’s primary role is as the basic energy currency in the cell.  The way ATP works is all based on the phosphates.  As shown in Figure 3.7.4, a large amount of energy is stored in the bond between the second and third phosphate group.  When this bond is broken, it functions as an exothermic reaction and this energy can be used to power other processes taking place in the cell.

 

3.7 Summary

  • Nucleic acids are the class of biochemical compounds that includes DNA and RNA. These molecules are built of small monomers called nucleotides, which bind together in long chains to form polynucleotides. DNA consists of two polynucleotides, and RNA consists of one polynucleotide.
  • Each nucleotide consists of a sugar molecule, phosphate group, and nitrogen base. Sugars and phosphate groups of adjacent nucleotides bind together to form the “backbone” of the polynucleotide. Nitrogen bases jut out to the side of the sugar-phosphate backbone. Bonds between complementary bases hold together the two polynucleotide chains of DNA and cause it to take on its characteristic double helix shape.
  • DNA makes up genes, and the sequence of nitrogen bases in DNA makes up the genetic code for the synthesis of proteins. RNA helps synthesize proteins in cells. The genetic code in DNA is also passed from parents to offspring during reproduction, which explains how inherited characteristics are passed from one generation to the next.

3.7 Review Questions

  1. What are nucleic acids?
  2. How does RNA differ structurally from DNA?  Draw a picture of each.
  3. Describe a nucleotide. Explain how nucleotides bind together to form a polynucleotide.
  4. What role do nitrogen bases in nucleotides play in the structure and function of DNA?
  5. What is a function of RNA?
  6. Using what you learned in this article about nucleic acids, explain why twins look so similar.
  7. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=446

  8. What are the nucleotides on the complementary strand of DNA below?

    An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=446

  9. Arrange the following in order from the smallest to the largest level of organization: DNA, nucleotide, polynucleotide.
  10. As part of the DNA replication process, the two polynucleotide chains are separated from each other, but each individual chain remains intact. What type of bonds are broken in this process?
  11. Adenine, guanine, cytosine, and thymine are _______________.
  12. Some diseases and disorders are caused by genes. Explain why these genetic disorders can be passed down from parents to their children.
  13. Are there any genetic disorders that run in your family?

3.7 Explore More

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DNA: The book of you – Joe Hanson, TED-Ed, 2012.

Attributions

Figure 3.7.1

Figure 3.7.2

DNA-diagram by Christine Miller [Christinelmiller] on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 3.7.3

Bdna_cropped [gif] by Spiffistan, derivative work: Jahobr, on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.7.4

ATP for energy by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Reference

TED-Ed. (2012, November 26). DNA: The book of you – Joe Hanson. YouTube, 2012. https://www.youtube.com/watch?v=aeAL6xThfL8&feature=youtu.be

 

21

3.8 Chemical Reactions

Created by: CK-12/Adapted by Christine Miller

Is It Magic?

Chlorine gas in high concentration in a Florence flask
Figure 3.8.1 Chlorine gas in high concentration.

The harmless-looking bottle in Figure 3.8.1 contains a greenish-yellow, poisonous gas. The gas is chlorine, which is also used as bleach and to keep the water in pools and hot tubs free of germs. Chlorine can kill just about anything. Would you breathe in chlorine gas or drink liquid chlorine? Of course not, but you often eat a compound containing chlorine. You probably eat this chlorine compound just about every day. Can you guess what it is? It’s table salt.

Image shows a salt shaker filled with salt sitting on a wooden counter.
Figure 3.8.2 Table salt contains the elements sodium and chloride.

Table salt is actually sodium chloride (NaCl), which forms when chlorine and sodium (Na) combine in certain proportions. How does the toxic green chemical chlorine change into the harmless white compound we know as table salt? It isn’t magic — it’s chemistry, and it happens in a chemical reaction.

What Is a Chemical Reaction?

chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms as a result of a chemical reaction is called a product. During the reaction, the reactants are used up to create the products.

The burning of methane gas, as shown in the picture below, is a chemical reaction. In this reaction, the reactants are methane (CH4) and oxygen (O2), and the products are carbon dioxide (CO2) and water (H2O). As this example shows, a chemical reaction involves the breaking and forming of chemical bonds, which are forces that hold together the atoms of a molecule. When methane burns, for example, bonds break within the methane and oxygen molecules, and new bonds form in the molecules of carbon dioxide and water.

 

Image shows a lit gas stove burner. The flames are blue and there is a pot on the burner.
Figure 3.8.3 Flames from methane burning.

Chemical Equations

Chemical reactions can be represented by chemical equations. A chemical equation is a symbolic way of showing what happens during a chemical reaction. The burning of methane, for example, can be represented by the chemical equation:

CH4 + 2O2 → CO2 + 2H2O

The arrow in a chemical equation separates the reactants from the products, and shows the direction in which the reaction proceeds. If the reaction could occur in the opposite direction as well, two arrows pointing in opposite directions would be used. The number 2 in front of O2 and H2O, called the coefficient, shows that two oxygen molecules and two water molecules are involved in the reaction. If just one molecule is involved, no number is placed in front of the chemical symbol. Note the subscript of 2 for the oxygen (O) and hydrogen (H) atoms in the oxygen and water molecules, respectively. That tells you that each oxygen molecule is made up of two oxygen atoms. If there is no subscript, then there is a single atom. Thus, one water molecule is made up of two hydrogen atoms and one oxygen atom. In order for this chemical reaction to take place, one methane molecule reacts with two oxygen molecules to form one carbon dioxide molecule and two water molecules.

Shows a black and white caricature of Antoine Lavoisier with a thought bubble above his head containing the words " All the reactants must end up in the product - they can't just disappear".
Figure 3.8.4 Antoine Lavoisier is known as “the father of modern chemistry.”

Conservation of Mass

In a chemical reaction, the quantity of each element does not change. There is the same amount of each element in the products as there was in the reactants. Mass is always conserved. According to the law of conservation of mass — which was first demonstrated convincingly by French chemist Antoine Lavoisier in 1785 — mass is neither created nor destroyed during a chemical reaction. Therefore, during a chemical reaction, the total mass of products is equal to the total mass of reactants. The conservation of mass is reflected in a reaction’s chemical equation. The same number of atoms of each element appears on each side of the arrow. In the chemical equation above, there are four hydrogen atoms on each side of the arrow. Can you find all four of them on each side of the equation?

Chemical vs. Physical Changes

Many processes that happen all around us on a daily basis involve chemical reactions. Not every change, however, is a chemical change. Some changes are simply physical and do not involve chemical reactions. Physical changes include change in size of pieces and change in state.  If you break an eggshell and pour out the egg into a pan, its chemical makeup and properties do not change. This is just a physical change. No chemical reactions have occurred, and no chemical bonds have broken or formed. Other examples of physical changes are cutting paper into smaller pieces and letting an ice cube melt. What if you put the egg in the pan over a hot flame? The egg turns to a rubbery solid and changes colour. The properties of the egg have changed because its chemical makeup has changed. Cooking the egg is a chemical change that involves chemical reactions.

Other common examples of chemical changes include a cake baking, metal rusting, and a candle burning. More practice is below.

An interactive or media element has been excluded from this version of the text. You can view it online here:
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Figure 3.8.5 Chemical changes often involve chemical reactions as well. 

3.8 Summary

  • A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms during a chemical reaction is called a product. During the chemical reaction, bonds break in reactants and new bonds form in products.
  • Chemical reactions can be represented by chemical equations. According to the law of conservation of mass, mass is always conserved in a chemical reaction, so a chemical equation must be balanced, with the same number of atoms of each type of element in the products as in the reactants.
  • Many chemical reactions — such as iron rusting and organic matter rotting — occur all around us each day, but not all changes are chemical processes. Some changes — like ice melting or paper being torn into smaller pieces — are physical processes that do not involve chemical reactions and the formation of new substances.

3.8 Review Questions

  1. What is a chemical reaction?
  2. Define the reactants and products in a chemical reaction.
  3. List three examples of common changes that involve chemical reactions.
  4. Define a chemical bond.
  5. What is a chemical equation? Give an example.
  6. What does it mean for a chemical equation to be balanced? Why must a chemical equation be balanced?
  7. Our cells use glucose (C6H12O6) to obtain energy in a chemical reaction called cellular respiration. In this reaction, six oxygen molecules (O2) react with one glucose molecule. Answer the following questions about this reaction:
    • How many oxygen atoms are in one molecule of glucose?
    • Write out what the reactant side of this equation would look like.
    • In total, how many oxygen atoms are in the reactants? Explain how you calculated your answer.
    • In total, how many oxygen atoms are in the products? Is it possible to answer this question without knowing what the products are? Why or why not?
  8. Answer the following questions about the following equation: CH4+ 2O2 → CO2 + 2H2O
    • Can carbon dioxide (CO2)transform into methane (CH4) and oxygen (O2) in this reaction? Why or why not?
    • How many molecules of carbon dioxide (CO2) are produced in this reaction?
  9. Is the evaporation of liquid water into water vapor a chemical reaction? Why or why not?
  10. Why do bonds break in the reactants during a chemical reaction?

3.8 Explore More

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The law of conservation of mass – Todd Ramsey, TED-Ed, 2015.

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Chemical Changes: Crash Course Kids #19.2, by Crash Course Kids, 2015.

Attributions

Figure 3.8.1

Chlorine_gas_in_high_concentration by Larenmclane on Wikimedia Commons, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 3.8.2

Tags: Salt Salt Shaker Spices Kitchen Spice Component; salt-4160306_1280 by katie175 from Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).

Figure 3.8.3

Tags: Gas Flame Gas Stove Italy Gas Cook Kitchen by moerschy from Pixabay is used under the Pixabay License (https://pixabay.com/de/service/license/).

Figure 3.8.4

Antoine_lavoisier by unknown on Wikimedia Commons has been adapted by Christine Miller. The orginal work, believed to be from http://www.schuster-ingolstadt.de/Chemie.htm has been released into the  public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.8.5

References

Crash Course Kids. (2015, July 16). Chemical changes: Crash Course Kids #19.2. YouTube. https://www.youtube.com/watch?v=37pir0ej_SE

TED-Ed. (2015, February 26 ). The law of conservation of mass – Todd Ramsey. YouTube. https://www.youtube.com/watch?v=2S6e11NBwiw&feature=emb_logo

Wikipedia contributors. (2020, June 15). Antoine Lavoisier. Wikipedia. https://en.wikipedia.org/w/index.php?title=Antoine_Lavoisier&oldid=962631283

22

3.9 Energy in Chemical Reactions

Created by: CK-12/Adapted by Christine Miller

Slow Burn

Image shows a very rusty Ford truck with a surfboard in the back.
Figure 3.9.1 Rusting is a type of combustion reaction.

This old truck gives off a small amount of heat as it rusts. The rusting of iron is a chemical process. It occurs when iron and oxygen go through a chemical reaction similar to burning, or combustion. Obviously, the chemical reaction that occurs when something burns gives off energy. You can feel the heat, and you may be able to see the light of flames. The rusting of iron is a much slower process, but it still gives off energy. It’s just that it releases energy so slowly that you can’t detect a change in temperature.

The Role of Energy in Chemical Reactions

Matter rusting or burning are common examples of chemical changes. Chemical changes involve chemical reactions, in which some substances, called reactants, change at the molecular level to form new substances, which are called products. All chemical reactions involve energy, but not all chemical reactions release energy, like rusting and burning. In some chemical reactions, energy is absorbed rather than released.

Exothermic Reactions

A large pile of compost in a field. The compost has a cloud of steam around it, indicating release of heat into the environment as a result of the decomposition process.
Figure 3.9.2 Exothermic reactions release energy.

A chemical reaction that releases energy is called an exothermic reaction. This type of reaction can be represented with this general chemical equation:

Reactants → Products + Heat

Another example of an exothermic reaction is chlorine combining with sodium to form table salt. The decomposition of organic matter also releases energy because of exothermic reactions. Sometimes on a chilly morning, you can see steam rising from a compost pile because of these chemical reactions (see photo in Figure 3.9.2).

This compost pile is steaming because it is much warmer than the chilly air around it. The heat comes from all the exothermic chemical reactions taking place inside the compost as it decomposes.

A special type of exothermic reaction is an exergonic reaction– not only do exergonic reactions release energy, but in addition, they occur spontaneously.  Many cell processes rely on exergonic reactions: in a chemical process called cellular respiration, which is similar to combustion, the sugar glucose is “burned” to provide cells with energy.

Endothermic Reactions

A chemical reaction that absorbs energy is called an endothermic reaction. This type of reaction can also be represented by a general chemical equation:

Reactants + Energy → Products

Image shows a graphic of an instant cold pack. There are instructions for use on the front of the package. These instructions indicate that to use the cold pack, one must squeeze the package, mix the contents by kneading the bag. Once the cold pack is activated, it can be use to apply cold to minor injuries. The image also lists the two compounds in a cold pack: ammonium nitrate and water. Before use, these two compounds are kept separate, but once the cold pack is activated, these two compounds mix, producing an endothermic reaction, producing "cold".
Figure 3.9.3 This pack gets cold because of an endothermic reaction.

Did you ever use a chemical cold pack like the one pictured? The pack cools down because of an endothermic reaction. When a tube inside the pack is broken, it releases ammonium nitrate, a chemical that reacts with water inside the pack. This reaction absorbs heat energy and quickly cools down the contents of the pack.

Many other chemical processes involve endothermic reactions. Most cooking and baking, for example, involves the use of energy to produce chemical reactions. You can’t bake a cake or cook an egg without adding heat energy.

Arguably, the most important endothermic reactions occur during photosynthesis. When plants produce sugar by photosynthesis, they take in light energy to power the necessary endothermic reactions. The sugar they produce provides plants and virtually all other living things with glucose for cellular respiration.

Activation Energy

All chemical reactions require energy to get started. Even reactions that release energy need a boost of energy in order to begin. The energy needed to start a chemical reaction is called activation energy. Activation energy is like the push a child needs to start going down a playground slide. The push gives the child enough energy to start moving, but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in the graph in Figure 3.9.4.

Image shows a graph of the energy change during a chemical reaction. The reactants have a higher energy level than the products, implying that the reaction is exothermic. However, the reaction cannot occur spontaneously, it requires a small input of energy to get started. This input of energy is the activation energy.
Figure 3.9.4 Even though this reaction is exothermic, it requires “help” to get started. This “help” is the activation energy.

Why do chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must bump into each other, so they must be moving — and movement requires energy. When reactant molecules bump together, they may repel each other because of intermolecular forces pushing them apart. Energy is also required to overcome these forces so the molecules can come together and react.

3.9 Summary

  • All chemical reactions involve energy. Exothermic reactions release energy. Endothermic reactions absorb energy.
  • All chemical reactions need activation energy to begin. Activation energy provides the “push” needed to get the reaction started.

3.9 Review Questions

  1. Compare endothermic and exothermic chemical reactions. Give an example of a process that involves each type of reaction.
  2. Define activation energy.
  3. Explain why chemical reactions require activation energy.
  4. Heat is a form of ____________ .
  5. In which type of reaction is heat added to the reactants?
  6. In which type of reaction is heat produced?
  7. If there was no energy added to an endothermic reaction, would that reaction occur? Why or why not?
  8. If there was no energy added to an exothermic reaction, would that reaction occur? Why or why not?
  9. Explain why a chemical cold pack feels cold when activated.
  10. Explain why cellular respiration and photosynthesis are “opposites” of each other.
  11. Explain how the sun gives our cells energy indirectly.

3.9 Explore More

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Activation energy: Kickstarting chemical reactions – Vance Kite, TED-Ed, 2013.

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The Sci Guys: Science at Home – SE1 – EP7: Hot Ice – Exothermic Reactions and Supercooled solutions, The Sci Guys, 2013

Attributions

Figure 3.9.1

Rusty truck by Ross Sokolovski on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.9.2

CompostGently steaming compost! by John Winfield on Wikimedia Commons, is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.

Figure 3.9.3

Cold Pack by OpenStax /CNX on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 3.9.4 

Activation energy by CK12 is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

References

Brainard, J., Henderson, R. / CK12. (2018, August 22). Figure: Activation Energy [digital image]. In CK-12 College Human Biology. CK12. https://flexbooks.ck12.org/cbook/ck-12-college-human-biology-flexbook-2.0

OpenStax. (2019, Jul 30), Figure 6(b) A cold pack uses an endothermic process to create the sensation of cold. OpenStax Chemistry. OpenStax CNX. http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6@12.2. (Credit: a modification of  work by “Skatebiker”/Wikimedia commons).

TED-Ed. (2013, January 9). Activation energy: Kickstarting chemical reactions – Vance Kite. YouTube. https://www.youtube.com/watch?v=D0ZyjpAin_Y&feature=youtu.be

The Sci Guys. (2013, April 4). The Sci Guys: Science at home – SE1 – EP7: Hot ice – Exothermic reactions and supercooled solutions. YouTube. https://www.youtube.com/watch?v=znsPa1BSaIM&feature=youtu.be

 

23

3.10 Chemical Reactions in Living Things

Image shows a long line of sports cars in a factory. The cars are not yet fully assembled.
Figure 3.10.1. Auto assembly line.

Created by: CK-12/Adapted by Christine Miller

Assembly Line

We stay alive because millions of different chemical reactions are taking place inside our bodies all the time. Each of our cells is like the busy auto assembly line pictured in Figure 3.10.1. Raw materials, half-finished products, and waste materials are constantly being used, produced, transported, and excreted. The “workers” on the cellular assembly line are mainly enzymes. These are the proteins that make biochemical reactions happen.

What Are Biochemical Reactions?

Chemical reactions that take place inside living things are called biochemical reactions. The sum of all the biochemical reactions in an organism is called metabolism. Metabolism includes both exothermic (energy-releasing) chemical reactions and endothermic (energy-absorbing) chemical reactions.

Catabolic Reactions

Exothermic reactions in organisms are called catabolic reactions. These reactions break down molecules into smaller units and release energy. An example of a catabolic reaction is the breakdown of glucose during cellular respiration, which releases energy that cells need to carry out life processes.

Anabolic Reactions

Endothermic reactions in organisms are called anabolic reactions. These reactions build up bigger molecules from smaller ones and absorb energy. An example of an anabolic reaction is the joining of amino acids to form a protein. Which type of reactions — catabolic or anabolic — do you think occur when your body digests food?

Enzymes

Image shows a graph of the energy in a chemical reaction as reactants A and B are converted to product AB. The activation energy for this reaction is shown in two ways: with and without an enzyme. The activation energy with the enzyme is lower than without.
Figure 3.10.2. The activation energy for a reaction is lowered in the presence of an enzyme.

Most of the biochemical reactions that happen inside of living organisms require help. Why is this the case? For one thing, temperatures inside living things are usually too low for biochemical reactions to occur quickly enough to maintain life. The concentrations of reactants may also be too low for them to come together and react. Where do the biochemical reactions get the help they need to proceed? From the enzymes.

An enzyme is a protein that speeds up a biochemical reaction. It is a biological catalyst. An enzyme generally works by reducing the amount of activation energy needed to start the reaction. The graph in Figure 3.10.2 shows the activation energy needed for glucose to combine with oxygen. Less activation energy is needed when the correct enzyme is present than when it is not present.

An enzyme speeds up the reaction by lowering the required activation energy. Compare the activation energy needed with and without the enzyme.

How Well Enzymes Work

Enzymes are involved in most biochemical reactions, and they do their jobs extremely well. A typical biochemical reaction that would take several days or even several centuries to happen without an enzyme is likely to occur in just a split second with the proper enzyme! Without enzymes to speed up biochemical reactions, most organisms could not survive.

Enzymes are substrate-specific. The substrate of an enzyme is the specific substance it affects. Each enzyme works only with a particular substrate, which explains why there are so many different enzymes. In addition, for an enzyme to work, it requires specific conditions, such as the right temperature and pH. Some enzymes work best under acidic conditions, for example, while others work best in neutral environments.

Enzyme-Deficiency Disorders

There are hundreds of known inherited metabolic disorders in humans. In most of them, a single enzyme is either not produced by the body at all, or is otherwise produced in a form that doesn’t work. The missing or defective enzyme is like an absentee worker on the cell’s assembly line. Imagine the auto assembly line from the image at the start of this section.  What if the worker who installed the steering wheel was absent?  How would this impact the overall functioning of the vehicle?  When an enzyme is missing, toxic chemicals build up, or an essential product isn’t made. Generally, the normal enzyme is missing because the individual with the disorder inherited two copies of a gene mutation, which may have originated many generations previously.

Any given inherited metabolic disorder is generally quite rare in the general population. However, there are so many different metabolic disorders that a total of one in 1,000 to 2,500 newborns can be expected to have one.

3.10 Summary

  • Biochemical reactions are chemical reactions that take place inside of living things. The sum of all of the biochemical reactions in an organism is called metabolism.
  • Metabolism includes catabolic reactions, which are energy-releasing (exothermic) reactions, as well as anabolic reactions, which are energy-absorbing (endothermic) reactions.
  • Most biochemical reactions need a biological catalyst called an enzyme to speed up the reaction. Enzymes reduce the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, which is called the enzyme’s substrate.
  • There are many inherited metabolic disorders in humans. Most of them are caused by a single defective or missing enzyme.

3.10 Review Questions

  1. What are biochemical reactions?
  2. Define metabolism.
  3. Compare and contrast catabolic and anabolic reactions.
  4. Explain the role of enzymes in biochemical reactions.
  5. What are enzyme-deficiency disorders?
  6. Explain why the relatively low temperature of living things, along with the low concentration of reactants, would cause biochemical reactions to occur very slowly in the body without enzymes.
  7. Answer the following questions about what happens after you eat a sandwich.
    • Pieces of the sandwich go into your stomach, where there are digestive enzymes that break down the food. Which type of metabolic reaction is this? Explain your answer.
    • During the process of digestion, some of the sandwich is broken down into glucose, which is then further broken down to release energy that your cells can use. Is this an exothermic endothermic reaction? Explain your answer.
    • The proteins in the cheese, meat, and bread in the sandwich are broken down into their component amino acids. Then your body uses those amino acids to build new proteins. Which kind of metabolic reaction is represented by the building of these new proteins? Explain your answer.
  8. Explain why your body doesn’t just use one or two enzymes for all of its biochemical reactions.
  9. A ________ is the specific substance that an enzyme affects in a biochemical reaction.
  10. An enzyme is a biological _____________ .
    • catabolism
    • form of activation energy
    • catalyst
    • reactant

3.10 Explore More

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Enzymes (Updated), by The Amoeba Sisters, 2016.

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What triggers a chemical reaction? – Kareem Jarrah, TED-Ed, 2015.

Figure 3.10.1

Auto Assembly line by Brian Snelson on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0) license.

Figure 3.10.2

Enzyme_activation_energy by G. Andruk [IMeowbot at the English language Wikipedia], is used under a CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Amoeba Sisters. (2016, August 28). Enzymes (updated). YouTube. https://www.youtube.com/watch?v=qgVFkRn8f10&feature=youtu.be

TED-Ed. (2015, January 15). What triggers a chemical reaction? – Kareem Jarrah. YouTube. https://www.youtube.com/watch?v=8m6RtOpqvtU&feature=youtu.be

24

3.11 Water and Life

Created by: CK-12/Adapted by Christine Miller

 

Image shows a photograph of earth taken from space.
Figure 3.11.1. The Blue Marble: 71% of the earth’s surface is covered by water.

The Blue Marble

It’s often called the “water planet,” and it’s been given the nickname “the blue marble.” You probably just call it “home.” Almost three-quarters of our home planet is covered by water, and without it, life as we know it could not exist on Earth. Water, like carbon, has a special role in living things: it is needed by all known forms of life. Although water consists of simple molecules, each containing just three atoms, its structure gives it unique properties that help explain why it is vital to all living organisms.

Image shows a graphic representation of the condition and location of water on earth. 97% of water is saline, and only 3% is freshwater. Of this 3% freshwater, 69% is in icecaps and glaciers, 30% is ground water, and less than 1% is surface water in lakes, streams and rivers.
Figure 3.11.2. Most of the water on Earth consists of saltwater in the oceans. What per cent of Earth’s water is fresh water? Where is most of the fresh water found?

Water, Water Everywhere

If you look at Figure 3.11.2, you will see where Earth’s water is found. The term water generally refers to its liquid state, and water is a liquid over a wide range of temperatures on Earth. Water, however, also occurs on Earth as a solid (ice) and as a gas (water vapor).

Structure and Properties of Water

You are likely already aware of some of the properties of water. For example, you know that water is tasteless and odorless. You also probably know that water is transparent, which means that light can pass through it. This is important for organisms that live in the water, because some of them need sunlight to make food by photosynthesis.

Chemical Structure of Water

Image shows a diagram of water. It is made of a large central oxygen atom attached to two peripheral hydrogen atoms. The oxygen atom has a slight negative charge, and the two hydrogen atoms have a slight positive charge.
Figure 3.11.3. Because of unequal sharing of electrons in the covalent bonds that hold the water molecule together it is considered polar.

To understand some of water’s properties, you need to know more about its chemical structure. Each molecule of water consists of one atom of oxygen and two atoms of hydrogen. The oxygen atom in a water molecule attracts electrons more strongly than the hydrogen atoms do. As a result, the oxygen atom has a slightly negative charge, and the hydrogen atoms have a slightly positive charge. A difference in electrical charge between different parts of the same molecule is called polarity. The diagram in Figure 3.11.3 shows water’s polarity.

 

 

Diagram shows four water molecules. The oxygen in the central water molecule is attracted to the hydrogen atoms in adjacent water molecules due to their opposite charge.
Figure 3.11.4. Hydrogen bonding occurs between adjacent water molecules due to their polarity. A hydrogen bond is a weak intra-molecular force.

When it comes to charged molecules, opposites attract. In the case of water, the positive (hydrogen) end of one water molecule is attracted to the negative (oxygen) end of a nearby water molecule. Because of this attraction, weak bonds form between adjacent water molecules, as shown in Figure 3.11.4. The type of bond that forms between water molecules is called a hydrogen bond. Bonds between molecules are not as strong as bonds within molecules, but in water, they are strong enough to hold together nearby molecules.

How do you think hydrogen bonding affects water’s properties?

Properties of Water

Image shows a close-up photograph of dewdrops on a blade of grass.
Figure 3.11.5. Dew drops cling to blades of grass in this picture. Can you think of other examples of water forming drops? Hint: What happens when it rains on a newly waxed car?

Hydrogen bonds between water molecules explain some of water’s properties — for example, why water molecules tend to “stick” together. Did you ever watch water drip from a leaky faucet or from a melting icicle? If you did, then you know that water always falls in drops, rather than as separate molecules. The dew drops pictured to the left are another example of water molecules sticking together.

Hydrogen bonds cause water to have a relatively high boiling point of 100°C (212°F). Extra energy is needed to break these bonds and separate water molecules so they can escape into the air as water vapor. Because of its high boiling point, most water on Earth is in a liquid state, rather than a gaseous state. Water in its liquid state is needed by all living things. Hydrogen bonds also cause water to expand when it freezes. This, in turn, causes ice to have a lower density (that is, less mass per unit volume) than liquid water. The lower density of ice means that it floats on water. In cold climates, ice floats on top of the water in lakes. This allows lake animals like fish to survive the winter by staying in the liquid water under the ice.

Watch the video below to hear more about hydrogen bonding and it’s effects on the properties of water:

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=455

Why does ice float in water? – George Zaidan and Charles Morton, TED-ED, 2013.

Water and Living Things

The human body is about 70 per cent water (not counting the water in body fat, which varies from person to person). The body needs all this water to function normally. Just why is so much water required by human beings and other organisms? Water can dissolve many substances that organisms need. Water’s polarity helps it dissolve other polar substances. Water is also necessary for many biochemical reactions. The examples below are among the most important biochemical processes that occur in living things, but they are just two of the many ways that water is involved in biochemical reactions.

6CO2 + 6H2O + Energy → C6H12O6 + 6O2

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Water is involved in many other biochemical reactions and almost all life processes depend on water.

Feature: My Human Body

Image shows a woman in a wheelchair taking part in a marathon.
Figure 3.11.6. Endurance athletes are at risk for water intoxication.

Are you a marathon runner or other endurance athlete? Do you live and work in a hot, humid climate? If you answered “yes” to either question, you may be at risk of water intoxication.

Water is considered the least toxic chemical compound, so it may surprise you to learn that drinking too much water can cause serious illness and even death. Water intoxication is a potentially fatal disturbance in brain functions. It results when the normal balance of sodium and other electrolytes in the body is pushed outside safe limits by overhydration, or taking in too much water. The condition is also called hyponatremia, which refers to a lower-than-normal level of sodium in the blood that occurs when more water is entering than leaving the body.

As excessive water is consumed, fluid outside the cells decreases in its concentration of sodium and other electrolytes relative to the concentration inside the cells. This causes fluid to enter the cells by osmosis to balance the electrolyte concentration. The extra fluid in the cells causes them to swell. In the brain, this swelling increases the pressure inside the skull. It is this increase in pressure that leads to the first observable symptoms of water intoxication, which typically include headache, confusion, irritability, and drowsiness. As the condition worsens, additional symptoms may occur, such as difficulty breathing during exertion, muscle weakness and pain, or nausea and vomiting. If the condition persists, the cells in the brain may swell to the point where blood flow is interrupted or pressure is applied to the brain stem. This is extremely dangerous and may lead to seizures, brain damage, coma, or even death.

Under normal circumstances, it is very rare to accidentally consume too much water. However, it is relatively common in athletes who participate in endurance activities, such as marathon running. A study conducted on participants of the 2002 Boston Marathon, for example, found that 13 per cent of the runners finished the race with water intoxication (Almond, et al., 2005). The study also found that water intoxication was just as likely to occur in runners who drank sports drinks containing electrolytes as those who drank plain water. Water intoxication is so common at marathon events that medical personnel who work at such events are trained to suspect water intoxication when runners collapse or show signs of confusion.

Because of the publicity water intoxication has received lately, sports experts have lowered their recommendations for water intake during endurance events. They now advise drinking only when thirsty rather than drinking to “stay ahead of thirst,” which they recommended previously. Keeping water intake in line with water loss is the best way to prevent water intoxication. Mild water intoxication can be treated by restricting fluid intake. In more severe cases, treatment may require the use of diuretic drugs (which increase urination) or other types of drugs to reduce blood volume. Serious water intoxication should be considered a true medical emergency.

3.11 Summary

  • Most water on Earth consists of salt water in the oceans. Only a tiny percentage of the Earth’s water is fresh liquid water.
  • Virtually all living things on Earth require liquid water. Water exists as a liquid over a wide range of temperatures and dissolves many substances. These properties depend on water’s polarity, which causes water molecules to “stick” together.
  • The human body is about 70 per cent water (outside of fat). Organisms need water to dissolve many substances and for most biochemical processes, including photosynthesis and cellular respiration.

3.11 Review Questions

  1. Where is most of Earth’s fresh water found?
  2. Identify properties of water.
  3. What is polarity? Explain why water molecules are polar.
  4. Why do water molecules tend to “stick” together?
  5. What role does water play in photosynthesis and cellular respiration?
  6. Which do you think is stronger: the bonds between the hydrogen and oxygen atoms within a water molecule, or the bonds between the hydrogen and oxygen atoms between water molecules? Explain your answer.
  7. Given what you’ve learned about water intoxication (or hyponatremia), explain why you think drinking salt water would be bad for your cells.
  8. What is the name for the bonds that form between water molecules?
  9. Explain why water can dissolve other polar molecules.
  10. If there is pollution in the ocean that causes the water to become more cloudy or opaque, how do you think the ocean’s photosynthetic organisms will be affected? Explain your answer.
  11. Describe one way in which your body gets rid of excess water.
  12. True or False: Ice floats on top of water because it is denser than water.

3.11 Explore More

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Properties of Water, by The Amoeba Sisters, 2016.

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How polarity makes water behave strangely – Christina Kleinberg,  TED-Ed, 2013.

 

Attributions

Figure 3.11.1

Planet Earth by NASA (photo taken by either Harrison Schmitt  or Ron Evans (of the Apollo 17 crew), on Wikimedia Commons, is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 3.11.2

Total water on earth by LadyofHats at CK12, is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. 

Figure 3.11.3

Polarity of water by Christine Miller is released into the Public Domain (https://creativecommons.org/publicdomain/mark/1.0/).

Figure 3.11.4

Hydrogen bonds, translated by Michal Maňas (User:snek01) is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original uploader was Qwerter at Czech Wikipedia.)

Figure 3.11.5

Dew by Pascal Chanel on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 3.11.6

Woman in a wheelchair marathon by Kevin André on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Almond, C.S., Shin, A.Y., Fortescue, E.B. et al. (2005, April). Hyponatremia among runners in the Boston Marathon. The New England Journal of Medicine, 352 (15), 1550–1624. doi:10.1056/NEJMoa043901. PMID 15829535.

Amoeba Sisters. (2016, July 26). Properties of Water. YouTube. https://www.youtube.com/watch?v=3jwAGWky98c&feature=youtu.be

Ruiz Villarreal, M. (LadyofHats). (2016, August 15). Figure 2. Total water on earth [digital image]. In Brainard, J., Henderson, R., CK-12’s College Human Biology FlexBook® (section 3.11). CK12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/

TED-Ed. (2013, February 4). How polarity makes water behave strangely – Christina Kleinberg. YouTube.  https://www.youtube.com/watch?v=ASLUY2U1M-8&feature=youtu.be

TED-Ed. (2013, October 22). Why does ice float in water? – George Zaidan and Charles Morton. YouTube. https://www.youtube.com/watch?v=UukRgqzk-KE&feature=youtu.be

 

 

 

25

3.12 Acids and Bases

Image shows the end of a battery which has leaked its acidic contents. The leak looks like a thick crust of a whitish substance.
Figure 3.12.1. Batteries contain strong acids which should not come into contact with skin or eyes.

Created by: CK-12/Adapted by Christine Miller

Danger!  Acid!

You probably know that  batteries contain dangerous chemicals, including strong acids. Strong acids can hurt you if they come into contact with your skin or eyes. Therefore, it may surprise you to learn that your life depends on acids. There are many acids inside your body, and some of them are as strong as battery acid. Acids are needed for digestion and some forms of energy production. Genes are made of nucleic acids, proteins of amino acids, and lipids of fatty acids.

Water and Solutions

Acids (such as battery acid) are solutions. A solution is a mixture of two or more substances that has the same composition throughout. Many solutions are a mixture of water and some other substance. Not all solutions are acids. Some are bases and some are neither acids nor bases. To understand acids and bases, you need to know more about pure water.

In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. An ion is an electrically charged atom or molecule. The breakdown of water is represented by the chemical equation:

2 H2O → H3O+ + OH

The products of this reaction are a hydronium ion (H3O+) and a hydroxide ion (OH). The hydroxide ion, which has a negative charge, forms when a water molecule gives up a positively charged hydrogen ion (H+). The hydronium ion, which has a positive charge, forms when another water molecule accepts the hydrogen ion.

Acidity and pH

The concentration of hydronium ions in a solution is known as acidity. In pure water, the concentration of hydronium ions is very low; only about one in ten million water molecules naturally breaks down to form a hydronium ion. As a result, pure water is essentially neutral. Acidity is measured on a scale called pH, as shown in Figure 3.12.2. Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.

Image shows a pH scale. 0-6.9 is acidic, 7 is neutral, and 7.1-14 is basic.
Figure 3.12.2. The pH scale measures acidity. It ranges from 1-14.

This pH scale shows the acidity of many common substances. The lower the pH value, the more acidic a substance is.

Image of the pH scale and examples of substances for each of the numbers on the scale.
Figure 3.12.3. Examples of solutions for various pH levels.

Acids

If a solution has a higher concentration of hydronium ions than pure water, it has a pH lower than 7. A solution with a pH lower than 7 is called an acid. As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is.

Did you ever taste vinegar? Like other acids, it tastes sour. Stronger acids can be harmful to organisms. Even stomach acid would eat through the stomach if it were not lined with a layer of mucus. Strong acids can also damage materials, even hard materials such as glass.

Bases

If a solution has a lower concentration of hydronium ions than pure water, it has a pH higher than 7. A solution with a pH higher than 7 is called a base. Bases, such as baking soda, have a bitter taste. Like strong acids, strong bases can harm organisms and damage materials. For example, lye can burn the skin, and bleach can remove the colour from clothing.

Acids, Bases, and Enzymes

Many acids and bases in living things provide the pH that enzymes need. Enzymes are biological catalysts that must work effectively for biochemical reactions to occur. Most enzymes can do their job only at a certain level of acidity. Cells secrete acids and bases to maintain the proper pH for enzymes to do their work.

Every time you digest food, acids and bases are at work in your digestive system. Consider the enzyme pepsin, which helps break down proteins in the stomach. Pepsin needs an acidic environment to do its job. The stomach secretes a strong acid called hydrochloric acid that allows pepsin to work. When stomach contents enter the small intestine, the acid must be neutralized, because enzymes in the small intestine need a basic environment in order to work. An organ called the pancreas secretes a base named bicarbonate into the small intestine, and this base neutralizes the acid.

Feature: My Human Body

Do you ever have heartburn? The answer is probably “yes.” More than 60 million Americans have heartburn at least once a month, and more than 15 million suffer from it on a daily basis. Knowing more about heartburn may help you prevent it or know when it’s time to seek medical treatment.

Image shows two diagrams of the stomach and esophagus. In the first diagram, the esophageal sphincter is tightly closed, preventing contents of the stomach from re-entering the esophagus. In the second diagram, the esophageal sphincter is relaxed, open, and the stomach contents are able to re-enter the esophagus.
Figure 3.12.4. Acid reflux results when the esophageal sphincter doesn’t close completely.

Heartburn doesn’t have anything to do with the heart, but it does cause a burning sensation in the vicinity of the chest. Normally, the acid secreted into the stomach remains in the stomach where it is needed to allow pepsin to do its job of digesting proteins. A long tube called the esophagus carries food from the mouth to the stomach. A sphincter, or valve, between the esophagus and stomach opens to allow swallowed food to enter the stomach and then closes to prevent stomach contents from backflowing into the esophagus. If this sphincter is weak or relaxes inappropriately, stomach contents flow into the esophagus. Because stomach contents are usually acidic, this causes the burning sensation known as heartburn. People who are prone to heartburn and suffer from it often may be diagnosed with GERD, which stands for gastroesophageal reflux disease.

GERD — as well as occasional heartburn — often can be improved by dietary and other lifestyle changes that decrease the amount and acidity of reflux from the stomach into the esophagus.

If you have frequent heartburn and lifestyle changes don’t help, you may need medication to control the condition. Over-the-counter (OTC) antacids may be all that you need to control the occasional heartburn attack. OTC medications are usually bases that neutralize stomach acids. They may also create bubbles that help block stomach contents from entering the esophagus. For some people, OTC medications are not enough, and prescription medications are instead required for the control of GERD. These prescription medications generally work by inhibiting acid secretion in the stomach.

Be sure to see a doctor if you can’t control your heartburn, or you have it often. Untreated GERD not only interferes with quality of life, it may also lead to more serious complications, ranging from esophageal bleeding to esophageal cancer.

3.12 Summary

  • A solution is a mixture of two or more substances that has the same composition throughout. Many solutions consist of water and one or more dissolved substances.
  • Acidity is a measure of the hydronium ion concentration in a solution. Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the pH scale.
  • Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have a lower hydronium ion concentration than pure water and a pH higher than 7.
  • Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly. Enzymes are the biological catalysts (like pepsin) needed to digest protein in the stomach. Pepsin requires an acidic environment.

3.12 Review Questions

  1. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=457

  2. What is a solution?
  3. Define acidity.
  4. Explain how acidity is measured.
  5. Compare and contrast acids and bases.
  6. Hydrochloric acid is secreted by the stomach to provide an acidic environment for the enzyme pepsin. What is the pH of this acid? How strong of an acid is it compared with other acids?
  7. Define an ion. Identify the ions in the equation below, and explain what makes them ions:
    • 2 H2O → H3O+ + OH
  8. Explain why the pancreas secretes bicarbonate into the small intestine.
  9. Do you think pepsin would work in the small intestine? Why or why not?
  10. You may have mixed vinegar and baking soda and noticed that they bubble and react with each other. Explain why this happens. Explain also what happens to the pH of this solution after you mix the vinegar and baking soda.
  11. Pregnancy hormones can cause the lower esophageal sphincter to relax. What effect do you think this has on pregnant women? Explain your answer.

3.12 Explore More

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=457

pH and Buffers by Bozeman Science, 2014.

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The strengths and weaknesses of acids and bases – George Zaidan and Charles Morton, TED-Ed, 2013.

Attributions

Figure 3.12.1

Leaky battery by Carbon Arc on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license. ​

Figure 3.12.2

PH_Scale by Christinelmiller on Wikimedia Commons is used under a  © CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/) public domain dedication license.


Figure 3.12.3

Ph scale with examples by OpenStax College, on Wikimedia Commons, is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 3.12.4

GERD by BruceBlaus on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

References

Betts, J.G.,  Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.,  Womble, M., DeSaix, P. (2013, April 25). Figure 26.15 The pH Scale [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/26-4-acid-base-balance

Bozeman Science. (2014, February 22). pH and buffers. YouTube. https://www.youtube.com/watch?v=rIvEvwViJGk&feature=youtu.be

TED-Ed. (2013, October 24). The strengths and weaknesses of acids and bases – George Zaidan and Charles Morton. YouTube. https://www.youtube.com/watch?v=DupXDD87oHc&feature=youtu.be

 

26

3.13 Case Study Conclusion: Diet Dilemma

Created by: CK-12/Adapted by Christine Miller

After reading this chapter, you should be able to see numerous connections between chemistry, human life, and health. In Joseph’s situation, chemistry is involved in the reasons why his father has diabetes, why his personal risk of getting diabetes is high, and why the dietary changes he is considering could be effective.

Diagram shows a map of places in the world where diabetes is most prevalent. Northern Africa and the Middle East have high prevalence and South East Africa has low prevalence.
Figure 3.13.1. Prevalence of diabetes by per cent of country population.

Type 2 diabetes affects populations worldwide and is caused primarily by a lack of response in the body to the hormone insulin, which causes problems in the regulation of blood sugar, or glucose. Insulin is a peptide hormone, and as you have learned, peptides are chains of amino acids. Therefore, insulin is in the class of biochemical compounds called proteins. Joseph is at increased risk of diabetes partly because there is a genetic component to the disease. DNA, which is a type of chemical compound called a nucleic acid, is passed down from parents to their offspring, and carries the instructions for the production of proteins in units called genes. If there is a problem in a gene (or genes) that contributes to the development of a disease, such as type 2 diabetes, this can get passed down to the offspring and may raise that child’s risk of getting the disease.

But genetics is only part of the reason why Joseph is at an increased risk of diabetes. Obesity itself is a risk factor, and one that can be shared in families due to shared lifestyle factors (such as poor diet and lack of exercise), as well as genetics. Consumption of too many refined carbohydrates (like white bread and soda) may also contribute to obesity and the development of diabetes. As you probably now know, these simple carbohydrates are more easily and quickly broken down in the digestive system into glucose than larger complex carbohydrate molecules, such as those found in vegetables and whole grains. This can lead to dramatic spikes in blood sugar levels, which is particularly problematic for people with diabetes because they have trouble maintaining their blood sugar at a safe level. You can understand why Joseph’s father limits his consumption of refined carbohydrates, and in fact, some scientific studies have shown that avoiding refined carbohydrates may actually help reduce the risk of getting diabetes in the first place.

Image shows a plate of food containing a salad, fish and broccoli.
Figure 3.13.2. A diet high in vegetables and lean meats can help reduce the risk of Type 2 Diabetes.

Joseph’s friend recommended eating a low fat, high carbohydrate diet to lose weight, but you can see that the type of carbohydrate — simple or complex — is an important consideration. Eating a large amount of white bread and rice may not help Joseph reduce his risk of diabetes, but a healthy diet that helps him lose weight may lower his risk of diabetes, since obesity itself is a factor. Which specific diet will work best to help him lose weight probably depends on a variety of factors, including his biology, lifestyle, and food preferences. Joseph should consult with his doctor about his diet and exercise plan, so that his specific situation can be taken into account and monitored by a medical professional.

Drinking enough water is usually good advice for everyone, especially if it replaces sugary drinks like soda. You now know that water is important for many of the chemical reactions that take place in the body. But you can have too much of a good thing — as in the case of marathon runners who can make themselves sick from drinking too much water! As you can see, proper balance, or homeostasis, is very important to the health of living organisms.

Finally, you probably now realize that “chemicals” do not have to be scary, toxic substances. All matter consists of chemicals, including water, your body, and healthy fresh fruits and vegetables, like the ones pictured in Figure 3.12.2. When people advocate “clean eating” and avoiding “chemicals” in food, they are usually referring to avoiding synthetic — or man-made — chemical additives, such as preservatives. This can be a healthy way to eat because it involves eating a variety of whole, fresh, unprocessed foods. But there is no reason to be scared of chemicals in general — they are simply molecules and how they react depends on what they are, what other molecules are present, and the environmental conditions surrounding them.

Chapter 3 Summary

By now, you should have a good understanding of the basics of the chemistry of life. Specifically, you have learned:

  • All matter consists of chemical substances. A chemical substance has a definite and consistent composition and may be either an element or a compound.
  • An element is a pure substance that cannot be broken down into other types of substances.
    • An atom is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative electrons, positive protons, and neutral neutrons. The number of protons in an atom determines the element it represents.
    • Atoms have equal numbers of electrons and protons, so they have no charge. Ions are atoms that have lost or gained electrons, so they have either a positive or negative charge. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
    • There are almost 120 known elements. The majority of elements are metals. A smaller number are nonmetals, including carbon, hydrogen, and oxygen.
  • A compound is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a molecule. Chemical bonds hold together the atoms of molecules. Compounds can form only in chemical reactions, and they can break down only in other chemical reactions.
    • Biochemical compounds are carbon-based compounds found in living things. They make up cells and other structures of organisms and carry out life processes. Most biochemical compounds are large molecules called polymers that consist of many repeating units of smaller molecules called monomers.
    • There are millions of different biochemical compounds, but all of them fall into four major classes: carbohydrates, lipids, proteins, and nucleic acids.
  • Carbohydrates are the most common class of biochemical compounds. They provide cells with energy, store energy, and make up organic structures, such as the cell walls of plants. The basic building block of carbohydrates is the monosaccharide.
    • Sugars are short-chain carbohydrates that supply us with energy. Simple sugars, such as glucose, consist of just one monosaccharide. Some sugars, such as sucrose (or table sugar) consist of two monosaccharides and are called disaccharides.
    • Complex carbohydrates, or polysaccharides, consist of hundreds or even thousands of monosaccharides. They include starch, glycogen, cellulose, and chitin.
      • Starch is made by plants to store energy and is readily broken down into its component sugars during digestion.
      • Glycogen is made by animals and fungi to store energy and plays a critical part in the homeostasis of blood glucose levels in humans.
      • Cellulose is the most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Humans cannot digest cellulose, but it makes up most of the crucial dietary fibre in the human diet.
      • Chitin makes up organic structures, such as the cell walls of fungi and the exoskeletons of insects and other arthropods.
  • Lipids include fats and oils. They store energy, form cell membranes, and carry messages.
    • Lipid molecules consist mainly of repeating units called fatty acids. Fatty acids may be saturated or unsaturated, depending on the proportion of hydrogen atoms they contain. Animals store fat as saturated fatty acids, while plants store fat as unsaturated fatty acids.
    • Types of lipids include triglycerides, phospholipids, and steroids.
      • Triglycerides contain glycerol (an alcohol) in addition to fatty acids. Humans and other animals store fat as triglycerides in fat cells.
      • Phospholipids contain phosphate and glycerol in addition to fatty acids. They are the main component of cell membranes in all living things.
      • Steroids are lipids with a four-ring structure. Some steroids, such as cholesterol, are important components of cell membranes. Many other steroids are hormones.
  • In living things, proteins include enzymes, antibodies, and numerous other important compounds. They help cells keep their shape, make up muscles, speed up chemical reactions, and carry messages and materials (among other functions).
    • Proteins are made up of small monomer molecules called amino acids.
    • Long chains of amino acids form polypeptides. The sequence of amino acids in polypeptides makes up the primary structure of proteins. Secondary structure refers to configurations such as helices and sheets within polypeptide chains. Tertiary structure is a protein’s overall three-dimensional shape, which controls the molecule’s basic function. A quaternary structure forms if multiple protein molecules join together and function as a complex.
    • The chief characteristic that allows proteins’ diverse functions is their ability to bind specifically and tightly with other molecules.
  • Nucleic acids include DNA and RNA. They encode instructions for making proteins, helping make proteins, and passing the encoded instructions from parents to offspring.
    • Nucleic acids are built of monomers called nucleotides, which bind together in long chains to form polynucleotides. DNA consists of two polynucleotides, and RNA consists of one polynucleotide.
    • Each nucleotide consists of a sugar molecule, phosphate group, and nitrogen base. Sugars and phosphate groups of adjacent nucleotides bind together to form the “backbone” of the polynucleotide. Bonds between complementary bases hold together the two polynucleotide chains of DNA and cause it to take on its characteristic double helix shape.
    • DNA makes up genes, and the sequence of nitrogen bases in DNA makes up the genetic code for the synthesis of proteins. RNA helps synthesize proteins in cells. The genetic code in DNA is also passed from parents to offspring during reproduction, explaining how inherited characteristics are passed from one generation to the next.
  • A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms in a chemical reaction is called a product. During the chemical reaction, bonds break in reactants and new bonds form in products.
  • Chemical reactions can be represented by chemical equations. According to the law of conservation of mass, mass is always conserved in a chemical reaction, so a chemical equation must be balanced, with the same number of atoms of each type of element in the products as in the reactants.
  • Many chemical reactions occur all around us each day, such as iron rusting and organic matter rotting, but not all changes are chemical processes. Some changes, such as ice melting or paper being torn into smaller pieces, are physical processes that do not involve chemical reactions and the formation of new substances.
  • All chemical reactions involve energy, and they require activation energy to begin. Exothermic reactions release energy. Endothermic reactions absorb energy.
  • Biochemical reactions are chemical reactions that take place inside living things. The sum of all the biochemical reactions in an organism is called metabolism. Metabolism includes catabolic reactions (exothermic reactions) and anabolic reactions (endothermic reactions).
  • Most biochemical reactions require a biological catalyst called an enzyme to speed up the reaction by reducing the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, called the enzyme’s substrate.
  • Virtually all living things on Earth require liquid water. Only a tiny per cent of Earth’s water is fresh liquid water. Water exists as a liquid over a wide range of temperatures, and it dissolves many substances. These properties depend on water’s polarity, which causes water molecules to “stick” together through weak bonds called hydrogen bonds.
  • The human body is about 70 per cent water (outside of fat). Organisms need water to dissolve many substances and for most biochemical processes, including photosynthesis and cellular respiration.
  • A solution is a mixture of two or more substances that has the same composition throughout. Many solutions consist of water and one or more dissolved substances.
  • Acidity is a measure of the hydronium ion concentration in a solution. Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the pH scale. Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have a lower hydronium ion concentration than pure water and a pH higher than 7.
  • Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly.

Now you understand the chemistry of the molecules that make up living things. In the next chapter, you will learn how these molecules make up the basic unit of structure and function in living organisms — cells — and you will be able to understand some of the crucial chemical reactions that occur within cells.

Chapter 3 Review

  1. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=459

  2. The chemical formula for the complex carbohydrate glycogen is C24H42O21.
    1. What are the elements in glycogen?
    2. How many atoms are in one molecule of glycogen?
    3. Is glycogen an ion? Why or why not?
    4. Is glycogen a monosaccharide or a polysaccharide? Besides memorizing this fact, how would you know this based on the information in the question?
    5. What is the function of glycogen in the human body?
  3. What is the difference between an ion and a polar molecule? Give an example of each in your explanation.
  4. Define monomer and polymer.
  5. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=459

  6. What is the difference between a protein and a polypeptide?
  7. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=459

  8. People with diabetes have trouble controlling the level of glucose in their bloodstream. Knowing this, why do you think it is often recommended that people with diabetes limit their consumption of carbohydrates?
  9. Identify each of the following reactions as endothermic or exothermic.
    1. cellular respiration
    2. photosynthesis
    3. catabolic reactions
    4. anabolic reactions
  10. Pepsin is an enzyme in the stomach that helps us digest protein. Answer the following questions about pepsin:
    1. What is the substrate for pepsin?
    2. How does pepsin work to speed up protein digestion?
    3. Given what you know about the structure of proteins, what do you think are some of the products of the reaction that pepsin catalyzes?
    4. The stomach is normally acidic. What do you think would happen to the activity of pepsin and protein digestion if the pH is raised significantly?

Attributions

Figure 3.13.1

Prevalence_of_Diabetes_by_Percent_of_Country_Population_(2014)_Gradient_Map by Walter Scott Wilkens [Wwilken2], University of Illinois – Urbana Champaign Department of Geography and GIScience, on Wikimedia Commons, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 3.13.2

Healthy plate by Melinda Young Stuart on Flickr is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/) license.

IV

Chapter 4 Cells

27

4.1 Case Study: The Importance of Cells

Created by: CK-12/Adapted by Christine Miller

Image shows female track and field runners resting after a race. Three women are resting on the ground and two are leaning over with their hands on their knees, catching their breathe.
Figure 4.1.1 Athletes after a difficult competition.

Case Study: More Than Just Tired

We all get tired sometimes, especially if we have been doing a lot of physical activity, like the athletes pictured in Figure 4.1.1. But for Jasmin (Figure 4.1.2), a 34-year-old former high school track star who is now a recreational runner, her tiredness was going far beyond what she thought should be normal for someone in generally good physical shape.

Image shows an Asian woman standing at a bus stop. She is yawning.
Figure 4.1.2 Jasmin was feeling a level of fatigue that was far beyond normal tiredness.

She was experiencing extreme fatigue after her runs, as well as muscle cramping, spasms, and an unusual sense of heaviness in her legs. At first, she just chalked it up to getting older, but her exhaustion and pain worsened to the point where the former athlete could no longer run for more than a few minutes at a time. She began to experience other unusual symptoms, such as blurry vision and vomiting for no apparent reason.

Concerned, Jasmin went to her doctor, who ran many tests and consulted with several specialists. After several months, she was finally diagnosed with a mitochondrial disease. Jasmin is surprised. She has an 8-year-old niece with a mitochondrial disease, but her niece’s symptoms started when she was very young, and they included seizures and learning disabilities. How can Jasmin have the same disease, but different symptoms? Why didn’t she have problems until adulthood, while her niece experienced symptoms at an early age? And what are mitochondria, anyway?

Chapter Overview: The Importance of Cells

As you will learn in this chapter, mitochondria are important structures within our cells. This chapter will describe cells, which are the basic unit of structure and function in all living organisms. Specifically, you will learn:

  • How cells were discovered, their common structures, and the principles of cell theory.
  • The importance of size and shape to the functions of cells.
  • The differences between eukaryotic cells (such as those in humans and other animals) and prokaryotic cells (such as bacteria).
  • The structures and functions of cell parts, including mitochondria, the plasma membrane, cytoplasm, cytoskeleton, nucleus, ribosomes, Golgi apparatus, endoplasmic reticulum, vesicles, and vacuoles.
  • The processes of passive and active transport to move substances into and out of cells and help maintain homeostasis.
  • How organisms obtain the energy needed for life, including how the sugar glucose is broken down to produce ATP through the processes of anaerobic and aerobic cellular respiration.
  • The phases of the cell cycle, how cells divide through mitosis, and how cancer can result from unregulated cell division.

 

As you read this chapter, think about the following questions related to Jasmin’s disease:

  1. What are mitochondria? What is their structure and function, and where did they come from during evolution?
  2. Why are fatigue and “exercise intolerance” (such as Jasmin’s extreme exhaustion after running) common symptoms of mitochondrial diseases?
  3. Why do you think Jasmin has symptoms that affect so many different parts of her body, including her legs, eyes, and digestive system?
  4. Why do you think mitochondrial diseases can run in families like Jasmin’s?

Attributes

Figure 4.1.1

Difficult competition by Massimo Sartirana on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.1.2

Exhausted by Kevin Grieve on Unsplash is used under the Unsplash License (https://unsplash.com/license).

28

4.2 Discovery of Cells and Cell Theory

Created by: CK-12/Adapted by Christine Miller

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=578

Figure 4.2.1 Human cells viewed with a very powerful tool called a scanning electron microscope.

Amazing Cells

What are these incredible objects? Would it surprise you to learn that they are all human cells? Cells are actually too small to see with the unaided eye. It is visible here in such detail because it is being viewed with a very powerful tool called a scanning electron microscope. Cells may be small in size, but they are extremely important to life. Like all other living things, you are made of cells. Cells are the basis of life, and without cells, life as we know it would not exist. You will learn more about these amazing building blocks of life in this section.

What Are Cells?

If you look at living matter with a microscope — even a simple light microscope — you will see that it consists of cells. Cells are the basic units of the structure and function of living things. They are the smallest units that can carry out the processes of life. All organisms are made up of one or more cells, and all cells have many of the same structures and carry out the same basic life processes. Knowing the structure of cells and the processes they carry out is necessary to an understanding of life itself.

Diagram shows sketches from the lab journal of Robert Hooke. It includes a sketch of cork as it appeared under the microscope, a sketch of the cork tree branch his sample came from, and a sketch of the microscope apparatus he used.
Figure 4.2.2 Robert Hooke sketched the cork cells as they appeared under a simple light microscope.

Discovery of Cells

The first time the word cell was used to refer to these tiny units of life was in 1665 by a British scientist named Robert Hooke. Hooke was one of the earliest scientists to study living things under a microscope. The microscopes of his day were not very strong, but Hooke was still able to make an important discovery. When he looked at a thin slice of cork under his microscope, he was surprised to see what looked like a honeycomb. Hooke made the drawing in the figure to the right to show what he saw. As you can see, the cork was made up of many tiny units. Hooke called these units cells because they resembled cells in a monastery.

Soon after Robert Hooke discovered cells in cork, Anton van Leeuwenhoek in Holland made other important discoveries using a microscope. Leeuwenhoek made his own microscope lenses, and he was so good at it that his microscope was more powerful than other microscopes of his day. In fact, Leeuwenhoek’s microscope was almost as strong as modern light microscopes. Using his microscope, Leeuwenhoek was the first person to observe human cells and bacteria.

Cell Theory

By the early 1800s, scientists had observed cells of many different organisms. These observations led two German scientists named Theodor Schwann and Matthias Jakob Schleiden to propose cells as the basic building blocks of all living things. Around 1850, a German doctor named Rudolf Virchow was studying cells under a microscope, when he happened to see them dividing and forming new cells. He realized that living cells produce new cells through division. Based on this realization, Virchow proposed that living cells arise only from other living cells.

The ideas of all three scientists — Schwann, Schleiden, and Virchow — led to cell theory, which is one of the fundamental theories unifying all of biology.

Cell theory states that:

Seeing Inside Cells

Starting with Robert Hooke in the 1600s, the microscope opened up an amazing new world — a world of life at the level of the cell. As microscopes continued to improve, more discoveries were made about the cells of living things, but by the late 1800s, light microscopes had reached their limit. Objects much smaller than cells (including the structures inside cells) were too small to be seen with even the strongest light microscope.

Figure 4.2.3 An electron microscope produced this image of the structures inside of a cell.

Then, in the 1950s, a new type of microscope was invented. Called the electron microscope, it used a beam of electrons instead of light to observe extremely small objects. With an electron microscope, scientists could finally see the tiny structures inside cells. They could even see individual molecules and atoms. The electron microscope had a huge impact on biology. It allowed scientists to study organisms at the level of their molecules, and it led to the emergence of the molecular biology field. With the electron microscope, many more cell discoveries were made.

Structures Shared By All Cells

Although cells are diverse, all cells have certain parts in common. These parts include a plasma membrane, cytoplasm, ribosomes, and DNA.

Image shows a diagram of a cell containing the four basic components of a cell: a plasma membrane, DNA, ribosomes and a cytoplasm.
Figure 4.2.4 Every cell consists of at least a plasma membrane, DNA, ribosomes and a cytoplasm.
  1. The plasma membrane (a type of cell membrane) is a thin coat of lipids that surrounds a cell. It forms the physical boundary between the cell and its environment. You can think of it as the “skin” of the cell.
  2. Cytoplasm refers to all of the cellular material inside of the plasma membrane. Cytoplasm is made up of a watery substance called cytosol, and it contains other cell structures, such as ribosomes.
  3. Ribosomes are the structures in the cytoplasm in which proteins are made.
  4. DNA is a nucleic acid found in cells. It contains the genetic instructions that cells need to make proteins.

These four parts are common to all cells, from organisms as different as bacteria and human beings. How did all known organisms come to have such similar cells? The similarities show that all life on Earth has a common evolutionary history.

4.2 Summary

  • Cells are the basic units of structure and function in living things. They are the smallest units that can carry out the processes of life.
  • In the 1600s, Hooke was the first to observe cells from an organism (cork). Soon after, microscopist van Leeuwenhoek observed many other living cells.
  • In the early 1800s, Schwann and Schleiden theorized that cells are the basic building blocks of all living things. Around 1850, Virchow observed cells dividing. To previous learnings, he added that living cells arise only from other living cells. These ideas led to cell theory, which states that all organisms are made of cells, that all life functions occur in cells, and that all cells come from other cells.
  • It wasn’t until the 1950s that scientists could see what was inside the cell. The invention of the electron microscope allowed them to see organelles and other structures smaller than cells.
  • There is variation in cells, but all cells have a plasma membrane, cytoplasm, ribosomes, and DNA. These similarities show that all life on Earth has a common ancestor in the distant past.

 

4.2 Review Questions

  1. Describe cells.
  2. Explain how cells were discovered.
  3. Outline the development of cell theory.
  4. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=578

  5. Identify the structures shared by all cells.
  6. Proteins are made on _____________ .
  7. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=578

  8. Robert Hooke sketched what looked like honeycombs — or repeated circular or square units — when he observed plant cells under a microscope.
    1. What is each unit?
    2. Of the shared parts of all cells, what makes up the outer surface of each unit?
    3. Of the shared parts of all cells, what makes up the inside of each unit?

4.2 Explore More

Thumbnail for the embedded element "Introduction to Cells: The Grand Cell Tour"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=578

Introduction to Cells: The Grand Cell Tour, by The Amoeba Sisters, 2016.

Attributions

Figure 4.2.1

Figure 4.2.2

Hooke-microscope-cork by Robert Hooke (1635-1702) [uploaded by Alejandro Porto] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.2.3

Electron Microscope image of a cell by Dartmouth Electron Microscope Facility, Dartmouth College on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.2.4

Basic-Components-of-a-cell by Christine Miller is used under a  CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/) license.

References

Amoeba Sisters. (2016, November 1). Introduction to cells: The grand cell tour. YouTube. https://www.youtube.com/watch?v=8IlzKri08kk&feature=youtu.be

National Institute of Allergy and Infectious Diseases (NIAID). (2011). A white blood cell (WBC) known as a neutrophil, as it was in the process of ingesting a number of spheroid shaped, methicillin-resistant, Staphylococcus aureus (MRSA) bacteria [digital image]. CDC/ Public Health Image Library (PHIL) Photo ID# 18129. https://phil.cdc.gov/Details.aspx?pid=18129.

Wikipedia contributors. (2020, June 24). Antonie van Leeuwenhoek. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Antonie_van_Leeuwenhoek&oldid=964339564

Wikipedia contributors. (2020, May 25). Matthias Jakob Schleiden. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Matthias_Jakob_Schleiden&oldid=958819219

Wikipedia contributors. (2020, June 4). Rudolf Virchow. In Wikipedia,.  https://en.wikipedia.org/w/index.php?title=Rudolf_Virchow&oldid=960708716

Wikipedia contributors. (2020, May 16). Theodor Schwann. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Theodor_Schwann&oldid=956919239

 

 

29

4.3 Variation in Cells

Created by: CK-12/Adapted by Christine Miller

Image shows a large red blood cell, with a filamentous green bacterium resting on its surface.
Figure 4.3.1 A bacterium attacks a human erythrocyte. Both are cells.

Bacteria Attack!

The colourful image in Figure 4.3.1 shows a bacterial cell (in green) attacking human red blood cells. The bacterium causes a disease called relapsing fever. The bacterial and human cells look very different in size and shape. Although all living cells have certain things in common — such as a plasma membrane and cytoplasm — different types of cells, even within the same organism, may have their own unique structures and functions. Cells with different functions generally have different shapes that suit them for their particular job. Cells vary not only in shape, but also in size, as this example shows. In most organisms, however, even the largest cells are no bigger than the period at the end of this sentence. Why are cells so small?

Explaining Cell Size

Most organisms, even very large ones, have microscopic cells. Why don’t cells get bigger instead of remaining tiny and multiplying? Why aren’t you one giant cell rolling around school? What limits cell size?

Once you know how a cell functions, the answers to these questions are clear. To carry out life processes, a cell must be able to quickly pass substances in and out of the cell. For example, it must be able to pass nutrients and oxygen into the cell and waste products out of the cell. Anything that enters or leaves a cell must cross its outer surface. The size of a cell is limited by its need to pass substances across that outer surface.

Look at the three cubes in Figure 4.3.2. A larger cube has less surface area relative to its volume than a smaller cube. This relationship also applies to cells — a larger cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs more nutrients and oxygen, and produces more waste. Because all of these substances must pass through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to meet its needs. The larger the cell is, the smaller its ratio of surface area to volume, and the more difficult it will be for the cell to get rid of its waste and take in necessary substances. This is what limits the size of the cell.

 

 

 

Image shows three cubes: a small, a medium and a large. The cube with length of 1 has a surface area to volume ratio of 6:1. The cube with a length of 2 has a surface area to volume ratio of 3:1 and the cube with the length of 3 has a surface area to volume ratio of 2:1.
Figure 4.3.2 Surface area to volume ratio.

Cell Form and Function

Cells with different functions often have varying shapes. The cells pictured below (Figure 4.3.3) are just a few examples of the many different shapes that human cells may have. Each type of cell  has characteristics that help it do its job. The job of the nerve cell, for example, is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail of each tiny sperm cell? Its tail helps a sperm cell “swim” through fluids in the female reproductive tract in order to reach an egg cell. The white blood cell has the job of destroying bacteria and other pathogens. It is a large cell that can engulf foreign invaders.

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=580

Figure 4.3.3 Human cells may have many different shapes that help them to do their jobs.

Cells With and Without a Nucleus

The nucleus is a basic cell structure present in many — but not all — living cells. The nucleus of a cell is a structure in the cytoplasm that is surrounded by a membrane (the nuclear membrane) and contains DNA. Based on whether or not they have a nucleus, there are two basic types of cells: prokaryotic cells and eukaryotic cells.

Prokaryotic Cells

Image shows a diagram of a bacterium. The bacterium is smaller than a typical eukaryotic cell, has fewer organelles and contains no membrane-bound organelles.
Figure 4.3.3 Bacteria are prokaryotes, meaning they do not have a nucleus. Their DNA is contained in a region called the nucleoid.

Prokaryotic cells are cells without a nucleus. The DNA in prokaryotic cells is in the cytoplasm, rather than enclosed within a nuclear membrane.  In addition, these cells are typically smaller than eukaryotic cells and contain fewer organelles.  Prokaryotic cells are found in single-celled organisms, such as the bacterium represented by the model in Figure 4.3.3. Organisms with prokaryotic cells are called prokaryotes. They were the first type of organisms to evolve, and they are still the most common organisms today.

 

Eukaryotic Cells

Image shows a diagram of a eukaryotic cell. The cell has many organelles labelled, including: nucleus, nucleolus, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi body, vesicles, mitochondria and centrioles.
Figure 4.3.4 Eukaryotic cells, like this animal cell, contain a nucleus and many other membrane-bound organelles.

Eukaryotic cells are cells that contain a nucleus. A typical eukaryotic cell is represented by the model in Figure 4.3.4. Eukaryotic cells are usually larger than prokaryotic cells. They are found in some single-celled and all multicellular organisms. Organisms with eukaryotic cells are called eukaryotes, and they range from fungi to humans.

Besides a nucleus, eukaryotic cells also contain other organelles. An organelle is a structure within the cytoplasm that performs a specific job in the cell. Organelles called mitochondria, for example, provide energy to the cell, and organelles called vesicles store substances in the cell. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can.

Interestingly, scientists think that mitochondria were once free-living prokaryotes that infected (or were engulfed by) larger cells. The two organisms developed a symbiotic relationship that was beneficial to both of them, resulting in the smaller prokaryote becoming an organelle within the larger cell. This is called endosymbiotic theory, and it is supported by a lot of evidence, including the fact that mitochondria have their own DNA separate from the DNA in the nucleus of the eukaryotic cell. Endosymbiotic theory will be described in more detail in later sections, and it’s also discussed in the video below.

Thumbnail for the embedded element "Endosymbiotic Theory"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=580

Endosymbiotic Theory, Amoeba Sisters, 2017.

4.3 Summary

  • Cells must be very small so they have a large enough surface area-to-volume ratio to maintain normal cell processes.
  • Cells with different functions often have different shapes.
  • Prokaryotic cells do not have a nucleus. Eukaryotic cells do have a nucleus, along with other organelles.

4.3 Review Questions

  1. Explain why most cells are very small.
  2. Discuss variations in the form and function of cells.
  3. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=580

  4. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=580

  5. Do human cells have organelles? Explain your answer.
  6. Which are usually larger – prokaryotic or eukaryotic cells? What do you think this means for their relative ability to take in needed substances and release wastes? Discuss your answer.
  7. DNA in eukaryotes is enclosed within the _______  ________.
  8. Name three different types of cells in humans.
  9. Which organelle provides energy in eukaryotic cells?
  10. What is a function of a vesicle in a cell?

4.3 Explore More

Thumbnail for the embedded element "How we think complex cells evolved - Adam Jacobson"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=580

How we think complex cells evolved – Adam Jacobson, TED-Ed, 2015.

Thumbnail for the embedded element "Prokaryotic vs. Eukaryotic Cells (Updated)"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=580

Prokaryotic vs. Eukaryotic Cells (updated), Amoeba Sisters, 2018.

Attributions

Figure 4.3.1

Borrelia_hermsii_Bacteria_(13758011613) by NAID on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.3.2

Cell Size by Christine Miller is released into the Public Domain (https://creativecommons.org/publicdomain/mark/1.0/).

Figure 4.3.3


Figure 4.3.4

Model of a prokaryotic cell: bacterium by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.3.5

Animal Cell adapted by Christine Miller is used under a CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en) public domain dedication license. (Original image, Animal Cell Unannotated, is by Kelvin Song on Wikimedia Commons.)

References

Amoeba Sisters. (2017, May 3). Endosymbiotic theory. YouTube. https://www.youtube.com/watch?v=FGnS-Xk0ZqU&feature=youtu.be

Amoeba Sisters. (2018, July 30). Prokaryotic vs. eukaryotic cells (updated). YouTube. https://www.youtube.com/watch?v=Pxujitlv8wc&feature=youtu.be

Brinkmann, V. (November 2005). Neutrophil engulfing Bacillus anthracis. PLoS Pathogens 1 (3): Cover page [digital image]. DOI:10.1371. https://journals.plos.org/plospathogens/issue?id=10.1371/issue.ppat.v01.i03

Lee, W.C.A., Huang, H., Feng, G., Sanes, J.R., Brown, E.N., et al. (2005, December 27) Figure 6f, slightly altered (plus scalebar, minus letter “f”.) [digital image]. Dynamic Remodeling of Dendritic Arbors in GABAergic Interneurons of Adult Visual Cortex. PLoS Biology, 4(2), e29. doi:10.1371/journal.pbio.0040029. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0040029

TED-Ed. (2015, February 17). How we think complex cells evolved – Adam Jacobson. https://www.youtube.com/watch?v=9i7kAt97XYU&feature=youtu.be

 

 

30

4.4 Plasma Membrane

Created by: CK-12/Adapted by Christine Miller

Figure 4.4.1 Simple cut-away model of an animal cell. 
Figure 4.4.2 Jello molds containing fruit.

A Bag Full of Jell-O

The simple cut-away model of an animal cell (Figure 4.4.1) shows that a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit (Figure 4.4.2), the cytoplasm of the cell also contains various structures, including a nucleus and other organelles. Your body is composed of trillions of cells, but all of them perform the same basic life functions. They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out these basic functions and keep themselves — and you — alive? To answer these questions, you need to know more about the structures that make up cells, starting with the plasma membrane.

What is the Plasma Membrane?

The plasma membrane is a structure that forms a barrier between the cytoplasm inside the cell and the environment outside the cell. Without the plasma membrane, there would be no cell. Although it is very thin and flexible, the plasma membrane protects and supports the cell by controlling everything that enters and leaves it. It allows only certain substances to pass through, while keeping others in or out. To understand how the plasma membrane controls what passes into or out of the cell, you need to know its basic structure.

Phospholipid Bilayer

The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown in the simplified diagram in Figure 4.4.3, each individual  phospholipid molecule has a phosphate group head (in red) and two fatty acid tails (in yellow). The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outward, toward either the cytoplasm (intracellular) or the fluid that surrounds the cell (extracellular).

Hydrophobic molecules can easily pass through the plasma membrane if they are small enough, because they are water-hating like the interior of the membrane. Hydrophilic molecules, on the other hand, cannot pass through the plasma membrane — at least not without help — because they are water-loving like the exterior of the membrane.

Image shows a diagram of a phospholipid bilayer. The bilayer is made up of two sheets of phospholipids, with the fatty acid tails facing towards the center, and the phosphate heads on the two external surfaces.
Figure 4.4.3 The phospholipid bilayer is made up of two sheets of phospholipids, with the fatty acid tails facing the centre.

Other Molecules in the Plasma Membrane

The plasma membrane also contains other molecules, primarily other lipids and proteins. The yellow molecules in the diagram here, for example, are the lipid cholesterol. Molecules of the steroid lipid cholesterol help the plasma membrane keep its shape. Proteins in the plasma membrane (shown blue in Figure 4.4.4) include: transport proteins that assist other substances in crossing the cell membrane, receptors that allow the cell to respond to chemical signals in its environment, and cell-identity markers that indicate what type of cell it is and whether it belongs in the body.

Image shows a diagram of a plasma membrane. The lipid bilayer contains embedded molecules including proteins, glycoproteins, glycolipids, and cholesterol.
Figure 4.4.4 The plasma membrane contains many molecules embedded in the lipid bilayer.

Additional Functions of the Plasma Membrane

The plasma membrane may have extensions, such as whip-like flagella (singular flagellum) or brush-like cilia (singular cilium), shown below (Figure 4.4.5), that give it other functions. In single-celled organisms, these membrane extensions may help the organisms move. In multicellular organisms, the extensions have different functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose, while the flagellum on a human sperm cell allows it to swim.

Image shows a scanning electron microscope image of three human sperm on a porous surface.
Figure 4.4.5 Human sperm with their long, whip-like flagella.
Image shows a scanning electron microscope image of the interior surface of bronchi. The cells lining the interior of this tube have clumps of cilia.
Figure 4.4.6 Brush-like cilia on lung epithelial cells.

Feature: My Human Body

If you smoke or use e-cigarettes (vaping) and need another reason to quit, here’s a good one. We usually think of lung cancer as the major disease caused by smoking. But smoking and vaping can have devastating effects on the body’s ability to protect itself from repeated, serious respiratory infections, such as bronchitis and pneumonia.

4.4.7 Adverse Affects of Vaping
Figure 4.4.7 Airways of “healthy” vapors are abnormal – results of vaping.

Cilia are microscopic, hair-like projects on cells that line the respiratory, reproductive, and digestive systems. Cilia in the respiratory system line most of your airways, where they have the job of trapping and removing dust, germs, and other foreign particles before they can make you sick. Cilia secrete mucus that traps particles, and they move in a continuous wave-like motion that sweeps the mucus and particles upward toward the throat, where they can be expelled from the body. When you are sick and cough up phlegm, that’s what you are doing.

Smoking prevents cilia from performing these important functions. Chemicals in tobacco smoke paralyze the cilia so they can’t sweep mucus out of the airways. Those chemicals also inhibit the cilia from producing mucus. Fortunately, these effects start to wear off soon after the most recent exposure to tobacco smoke. If you stop smoking, your cilia will return to normal. Even if prolonged smoking has destroyed cilia, they will regrow and resume functioning in a matter of months after you stop smoking.

4.4 Summary

  • The plasma membrane is a structure that forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell.
  • The plasma membrane is composed mainly of a bilayer of phospholipid molecules. It also contains other molecules, such as the steroid cholesterol, which helps the membrane keep its shape, and transport proteins, which help substances pass through the membrane.
  • The plasma membranes of some cells have extensions that have other functions, like flagella to help sperm move, or cilia to help keep our airways clear.

4.4 Review Questions

  1. What are the general functions of the plasma membrane?
  2. Describe the phospholipid bilayer of the plasma membrane.
  3. Identify other molecules in the plasma membrane. State their functions.
  4. Why do some cells have plasma membrane extensions, like flagella and cilia?
  5. Explain why hydrophilic molecules cannot easily pass through the cell membrane. What type of molecule in the cell membrane might help hydrophilic molecules pass through it?
  6. Which part of a phospholipid molecule in the plasma membrane is made of fatty acid chains? Is this part hydrophobic or hydrophilic?
  7. The two layers of phospholipids in the plasma membrane are called a phospholipid ____________.
  8. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=583

  9. Steroid hormones can pass directly through cell membranes. Why do you think this is the case?
  10. Some antibiotics work by making holes in the plasma membrane of bacterial cells. How do you think this kills the cells?
  11. What is the name of the long, whip-like extensions of the plasma membrane that helps some single-celled organisms move?

4.4 Explore More

Thumbnail for the embedded element "Insights into cell membranes via dish detergent - Ethan Perlstein"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=583

Insights into cell membranes via dish detergent – Ethan Perlstein, TED-Ed, 2013.

Thumbnail for the embedded element "Inside the Cell Membrane"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=583

Inside the cell membrane, by The Amoeba Sisters, 2018.

Attributions

Figure 4.4.1

Animal Cell Unannotated, by Kelvin Song on Wikimedia Commons is used under a CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en) public domain dedication license.

Figure 4.4.2

Jello mold at the mexican bakery photo by Aimée Knight on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 4.4.3

Phospholipid_Bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 4.4.4

Lipid bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 4.4.5

Spermatozoa-human-3140x by No specific author on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.4.6

Cilia/ Bronchiolar epithelium 3 – SEM by Charles Daghlian on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.4.7

Adverse effects of vaping (raster) by Mikael Häggström on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2018, February 27). Inside the cell membrane. YouTube. https://www.youtube.com/watch?v=qBCVVszQQNs&feature=youtu.be

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.3 Phospolipid Bilayer [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.4 Cell Membrane [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Ghosh, A., Coakley, R. C., Mascenik, T., Rowell, T. R., Davis, E. S., Rogers, K., Webster, M. J., Dang, H., Herring, L. E., Sassano, M. F., Livraghi-Butrico, A., Van Buren, S. K., Graves, L. M., Herman, M. A., Randell, S. H., Alexis, N. E., & Tarran, R. (n.d.). Chronic E-Cigarette Exposure Alters the Human Bronchial Epithelial Proteome. American Journal of Respiratory and Critical /Care Medicine198(1), 67–76. https://doi-org.ezproxy.tru.ca/10.1164/rccm.201710-2033OC

TED-Ed. (2013, February 26). Insights into cell membranes via dish detergent – Ethan Perlstein. YouTube. https://www.youtube.com/watch?v=yAXnYcUjn5k&feature=youtu.be

 

31

4.5 Cytoplasm and Cytoskeleton

Created by: CK-12/Adapted by Christine Miller

Image shows a diagram of a cell with many organelles and cell structures labelled, including: nucleus, nuclear envelope, nuclear pore, smooth ER, rough ER, ribosomes, mitochondrion, centrioles, vesicles, golgi body, cell membrane, chromatin.
Figure 4.5.1 The cytoplasm is filled with many organelles, each doing their own specific jobs.

A Peek Inside the Cell

Figure 4.5.1 is an artist’s representation of what you might see if you could take a peek inside one of these basic building blocks of living things.  A cell’s interior is obviously a crowded and busy space. It contains cytoplasm, dissolved substances, and many structures. It’s a hive of countless biochemical activities all going on at once. 

Cytoplasm

Cytoplasm is a thick, usually colourless solution that fills each cell and is enclosed by the cell membrane. Cytoplasm presses against the cell membrane, filling out the cell and giving it its shape. Sometimes, cytoplasm acts like a watery solution, and sometimes, it takes on a more gel-like consistency. In eukaryotic cells, the cytoplasm includes all of the material inside the cell but outside of the nucleus, which contains its own watery substance called the nucleoplasm. All of the organelles in eukaryotic cells (such as the endoplasmic reticulum and mitochondria) are located in the cytoplasm. The cytoplasm helps to keep them in place. It is also the site of most metabolic activities in the cell, and it allows materials to pass easily throughout the cell.

The portion of the cytoplasm surrounding organelles is called cytosol. Cytosol is the liquid part of cytoplasm. It is composed of about 80 per cent water, and it contains dissolved salts, fatty acids, sugars, amino acids, and proteins (such as enzymes). These dissolved substances are needed to keep the cell alive and carry out metabolic processes. Enzymes dissolved in cytosol, for example, break down larger molecules into smaller products that can then be used by organelles of the cell. Waste products are also dissolved in the cytosol before they are taken in by vacuoles or expelled from the cell.

The cytoskeleton gives the cell an internal structure, like the frame of a house. In this photograph, filaments and tubules of the cytoskeleton have been stained green and red, respectively, so that they can be seen clearly. The blue dots are cell nuclei.
Figure 4.5.2 The cytoskeleton gives the cell an internal structure, like the frame of a house. In this photograph, filaments and tubules of the cytoskeleton have been stained green and red, respectively, so that they can be seen clearly. The blue dots are cell nuclei.

Cytoskeleton

Although cytoplasm may appear to have no form or structure, it is actually highly organized. A framework of protein scaffolds called the cytoskeleton provides the cytoplasm and the cell with structure. The cytoskeleton consists of thread-like microfilaments, intermediate filaments, and microtubules that criss-cross the cytoplasm. You can see these filaments and tubules in the cells in Figure 4.5.2. As its name suggests, the cytoskeleton is like a cellular “skeleton.” It helps the cell maintain its shape and also helps to hold cell structures (like organelles) in place within the cytoplasm.

 

Feature: Human Biology in the News

News about an important study of the cytoplasm of eukaryotic cells came out in early 2016. Researchers in Dresden, Germany discovered that when cells are deprived of adequate nutrients, they may essentially shut down and become dormant. Specifically, when cells do not get enough nutrients, they shut down their metabolism, their energy level drops, and the pH of their cytoplasm decreases. Their normally liquid cytoplasm also assumes a solid state. The cells appear dead, as though a kind of rigor mortis has set in. The researchers think that these changes protect the sensitive structures inside the cells and allow the cells to survive difficult conditions. If nutrients are returned to the cells, they can emerge from their dormant state unharmed. They will continue to grow and multiply when conditions improve.

This important basic science research was executed on a nonhuman organism: one-celled fungi called yeasts. Nonetheless, it may have important implications for humans, because yeasts have eukaryotic cells with many of the same structures as human cells. Yeast cells appear to be able to “trick” death by shutting down all life processes in a controlled way. Through continued study, researchers hope to learn whether human cells can be taught this “trick” as well.

4.5 Summary

  • Cytoplasm is a thick solution that fills a cell and is enclosed by the cell membrane. It has many functions. It helps give the cell shape, holds organelles, and provides a site for many of the biochemical reactions inside the cell.
  • The liquid part of the cytoplasm is called cytosol. It is mainly water, and it contains many dissolved substances. The cytoplasm of a eukaryotic cell also contains a membrane-enclosed nucleus and other organelles.
  • The cytoskeleton is a highly organized framework of protein filaments and tubules that criss-cross the cytoplasm of a cell. It gives the cell structure and helps hold cell structures (such as organelles) in place within the cytoplasm.

4.5 Review Questions

  1. Describe the composition of cytoplasm.  Draw a picture of a cell, including the basic components required to be considered a cell, and the organelles you have learned about in this section.
  2. What are some of the functions of cytoplasm?
  3. Outline the structure and functions of the cytoskeleton.
  4. Is the cytoplasm made of cells? Why or why not?
  5. Name two types of cytoskeletal structures.
  6. In the picture of the different cytoskeletal structures above (Figure 4.5.2), what do you notice about these different structures?
  7. Describe one example of a metabolic process that happens in the cytosol.
  8. In eukaryotic cells, all of the material inside of the cell, but outside of the nucleus is called the ___________ .
  9. What is the liquid part of cytoplasm called?
  10. What chemical substance composes most of the cytosol?
  11. When yeast cells deprived of nutrients go dormant, their cytoplasm assumes a solid state. What effect do you think a solid cytoplasm would have on normal cellular processes? Explain your answer.

4.5 Explore More

Thumbnail for the embedded element "Cytoskeleton Structure and Function"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=586

Cytoskeleton Structure and Function, National Center for Case Study
Teaching in Science, 2015.

Attributions

Figure 4.5.1

Cell-organelles-labeled by Koswac on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 4.5.2

Cytoskeleton/ Fluorescent Cells by National Institute of Health (NIH) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

 

References

National Center for Case Study Teaching in Science. (2015, August 3). Cytoskeleton structure and function. YouTube. https://www.youtube.com/watch?v=YTv9ItGd050&feature=youtu.be

32

4.6 Cell Organelles

Created by: CK-12/Adapted by Christine Miller

Image shows a large 3D work of art displayed at the Cold Spring Harbor Laboratory. It is a representation of ribosomes attached to a ribbon of metal meant to represent a strand of messenger RNA.
Figure 4.6.1 “Waltz of the Polypeptides” sculpture by New York City-based artist Mara G. Haseltine, on display at Cold Spring Harbor Laboratory, NY.  This artwork features multiple ribosomes creating polypeptides according to the directions on a piece of messenger RNA.

Ribosome Review

The 25-metre long sculpture shown in Figure 4.6.1 is a recognition of the beauty of one of the metabolic functions that takes place in the cells in your body.  This artwork brings to life an important structure in living cells: the ribosome, the cell structure where proteins are synthesized. The slender silver strand is the messenger RNA(mRNA) bringing the code for a protein out into the cytoplasm.  The purple and green structures are ribosomal subunits (which together form a single ribosome), which can “read” the code on the mRNA and direct the bonding of the correct sequence of amino acids to create a protein.  All living cells — whether they are prokaryotic or eukaryotic — contain ribosomes, but only eukaryotic cells also contain a nucleus and several other types of organelles.

What Are Organelles?

An organelle is a structure within the cytoplasm of a eukaryotic cell that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and vacuoles. Ribosomes are not enclosed within a membrane, but they are still commonly referred to as organelles in eukaryotic cells.

The Nucleus

The nucleus is the largest organelle in a eukaryotic cell, and it’s considered the cell’s control center. It contains most of the cell’s DNA(which makes up chromosomes), and it is encoded with the genetic instructions for making proteins. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called nucleoplasm, which is similar in composition to the cytosol found in the cytoplasm outside the nucleus. Most eukaryotic cells contain just a single nucleus, but some types of cells (such as red blood cells) contain no nucleus and a few other types of cells (such as muscle cells) contain multiple nuclei.

This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.
Figure 4.6.2 This closeup of a cell nucleus shows that it is surrounded by a structure called the nuclear envelope, which contains tiny perforations, or pores. The nucleus also contains a dense center called the nucleolus.

As you can see in the model pictured in Figure 4.6.2, the membrane enclosing the nucleus is called the nuclear envelope. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes called nuclear pores allow large molecules to pass through the nuclear envelope, with the help of special proteins. Large proteins and RNA molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model below is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm, where they are involved in the synthesis of proteins.

Mitochondria

The mitochondrion (plural, mitochondria) is an organelle that makes energy available to the cell. This is why mitochondria are sometimes referred to as the “power plants of the cell.” They use energy from organic compounds (such as glucose) to make molecules of ATP (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy.

Image shows a diagram of a mitochondrion. Labelled are the inner and outer membranes, the intermembrane space, the matrix, DNA and ribosomes.
Figure 4.6.3 Mitochondria contain their own DNA and ribosomes!

Mitochondria (as in the Figure 4.6.3 diagram) have a complex structure including an inner and out membrane.  In addition, mitochondria have their own DNA, ribosomes, and a version of cytoplasm, called matrix.  Does this sound similar to the requirements to be considered a cell?  That’s because they are!

Scientists think that mitochondria were once free-living organisms because they contain their own DNA. They theorize that ancient prokaryotes infected (or were engulfed by) larger prokaryotic cells, and the two organisms evolved a symbiotic relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles inside them. This theory is called endosymbiotic theory, and it is widely accepted by biologists today. (See the video in section 4.3 to learn all about endosymbiotic theory.)

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. There are two types of endoplasmic reticulum: rough endoplasmic reticulum (rER) and smooth endoplasmic reticulum (sER). Both types are shown in Figure 4.6.4.

Image shows a diagram of the organelles included in the endomembrane system, inclduing: nuclear envelope, rough ER, smooth ER, golgi body, cell membrane, and vesicles.
Figure 4.6.4 The rough and smooth ER are part of a larger group of organelles termed “the endomembrane system”. All of the organelles in this system are composed of plasma membrane.

The Figure 4.6.4 drawing includes the nucleus, rER, sER, and Golgi apparatus. From the drawing, you can see how all these organelles work together to make and transport proteins.

Golgi Apparatus

The Golgi apparatus (shown in the Figure 4.6.4 diagram) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. You can see the Golgi apparatus in the figure above. The Golgi apparatus is something like a post office. It receives items (proteins from the ER), then packages and labels them before sending them on to their destinations (to different parts of the cell or to the cell membrane for transport out of the cell). The Golgi apparatus is also involved in the transport of lipids around the cell.

Vesicles and Vacuoles

Both vesicles and vacuoles are sac-like organelles made of phospholipid bilayer that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein and lipid molecules. You can see an example of this type of transport vesicle in the Figure 4.6.4. Some vesicles are used as chambers for biochemical reactions.

There are some vesicles which are specialized to carry out specific functions.  Lysosomes, which use enzymes to break down foreign matter and dead cells, have a double membrane to make sure their contents don’t leak into the rest of the cell.  Peroxisomes are another type of specialized vesicle with the main function of breaking down fatty acids and some toxins. 

Centrioles

Image shows a diagram of a centriole, made up of microtubules. There are nine bundles of microtubules arranged in a circle to form the tube-shaped centriole.
Figure 4.6.5 Centrioles are tiny cylinders near the nucleus, enlarged here to show their tubular structure.

Centrioles are organelles involved in cell division. The function of centrioles is to help organize the chromosomes before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells, and are located near the nucleus. Each centriole is made mainly of a protein named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured in Figure 4.6.5.

Image shows a diagram of a ribosome. It is made up of two sub-units, a smaller sub-unit shown in blue and a larger sub-unit shown in red.
Figure 4.6.6 Ribosomes are made up of two subunits, each consisting of protein and rRNA.

Ribosomes

Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome is formed of two subunits, like the ones pictured at the beginning of this section (Figure 4.6.1) and in  Figure 4.6.6. Both subunits consist of proteins and RNA. mRNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in mRNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm, as well as on the rER.

4.6 Summary

  • An organelle is a structure within the cytoplasm of a eukaryotic cell that is enclosed within a membrane and performs a specific job. Although ribosomes are not enclosed within a membrane, they are still commonly referred to as organelles in eukaryotic cells.
  • The nucleus is the largest organelle in a eukaryotic cell, and it is considered to be the cell’s control center. It controls gene expression, including controlling which proteins the cell makes.
  • The mitochondrion (plural, mitochondria) is an organelle that makes energy available to the cells. It is like the power plant of the cell. According to the widely accepted endosymbiotic theory, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
  • The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum (rER) is studded with ribosomes. Smooth endoplasmic reticulum (sER) has no ribosomes.
  • The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
  • Both vesicles and vacuoles are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are special types of vesicles that break down foreign matter, dead cells, or poisons.
  • Centrioles are organelles located near the nucleus that help organize the chromosomes before cell division so each daughter cell receives the correct number of chromosomes.
  • Ribosomes are small structures where proteins are made. They are found in both prokaryotic and eukaryotic cells. They may be found alone or in groups within the cytoplasm or on the rER.

4.6 Review Questions

  1. What is an organelle?
  2. Describe the structure and function of the nucleus.
  3. Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.
  4. Why are mitochondria referred to as the “power plants of the cell”?
  5. What roles are played by vesicles and vacuoles?
  6. Why do all cells need ribosomes — even prokaryotic cells that lack a nucleus and other cell organelles?
  7. Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?
  8. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=588

4.6 Explore More

Thumbnail for the embedded element "Biology: Cell Structure I Nucleus Medical Media"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=588

Biology: Cell Structure I Nucleus Medical Media, Nucleus Medical Media, 2015.

Thumbnail for the embedded element "David Bolinsky: Visualizing the wonder of a living cell"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=588

David Bolinsky: Visualizing the wonder of a living cell, TED, 2007.

Attributes

Figure 4.6.1 

Ribosomes at Work by Pedrik on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

Figure 4.6.2

Nucleus by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 4.6.3 

Mitochondrion_structure.svg by Kelvinsong; modified by Sowlos on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 4.6.4

Endomembrane_system_diagram_en.svg by Mariana Ruiz [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.6.5

Centrioles by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license. 

Figure 4.6.6

Ribosome_shape by Vossman on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

References

Blausen.com staff. (2014). Nucleus – Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014

Blausen.com staff (2014). Centrioles – Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014

Nucleus Medical Media. (2015, March 18). Biology: Cell structure I Nucleus Medical Media. YouTube. https://www.youtube.com/watch?v=URUJD5NEXC8&feature=youtu.be

TED. (2007, July 24). David Bolinsky: Visualizing the wonder of a living cell. YouTube. https://www.youtube.com/watch?v=Id2rZS59xSE&feature=youtu.be

 

 

33

4.7 Passive Transport

Created by: CK-12/Adapted by Christine Miller

Image shows a photo of a living room with large windows. There is a leather armchair, coffee table, lamp and books. The walls have wood panelling.
Figure 4.7.1 Just as windows in a house let light in, the cell membrane lets certain substances into and out of the cell.

Letting in the Light

Look at the big windows in this house (Figure 4.7.1). Imagine all the light they must let in on a sunny day. Now imagine living in a house that has walls without any windows or doors. Nothing could enter or leave. Or imagine living in a house with holes in the walls instead of windows and doors. Things could enter or leave, but you couldn’t control what came in or went out. Only when a house has walls with windows and doors that can be opened or closed, can you control what enters or leaves. Windows and doors allow you to let in light and the family dog and keep out rain and bugs, for example.

Transport Across Membranes

If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important function of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport — which requires no energy expenditure by the cell — and active transport — which requires energy from the cell.

Transport Without Energy Expenditure By The Cell

Passive transport occurs when substances cross the plasma membrane without any input of energy from the cell. No energy is required because the substances are moving from an area where they have a higher concentration to an area where they have a lower concentration. Concentration refers to the number of particles of a substance per unit of volume. The more particles of a substance in a given volume, the higher the concentration. A substance always moves from an area where it is more concentrated to an area where it is less concentrated.

There are several different types of passive transport, including simple diffusion, osmosis, and facilitated diffusion. Each type is described below.

Simple Diffusion

Diffusion is the movement of a substance due to a difference in concentration. It happens without any help from other molecules. The substance simply moves from the area where it is more concentrated to the area where it is less concentrated. Picture someone spraying perfume in the corner of a room.  Do the perfume molecules stay in the corner?  No, they spread out, or diffuse throughout the room until they are evenly spread out.  Figure 4.7.2 shows how diffusion works across a cell membrane. Substances that can squeeze between the lipid molecules in the plasma membrane by simple diffusion are generally very small, hydrophobic molecules, such as molecules of oxygen and carbon dioxide.

Image shows a diagram of the process of diffusion over time. The diagram shows three stages in time. In the first, all solutes are on one side of the plasma membrane. In the second stage, some of the solute has diffused through the plasma membrane, but there is still more on the first side. In the last stage, the molecules have diffused completely so that there are equal amounts on either side of the plasma membrane.
Figure 4.7.2 Molecules diffuse across a membrane from an area of higher concentration to an area of lower concentration until the concentration is the same on both sides of the membrane.
Diagram shows a time lapse of the contents of a beaker. The beaker's contents are separated into two with a semi-permeable membrane. One the left side of the beaker, there is a solution with low amount of solutes. One the right side of the beaker, there is a solution with a high amount of solutes. The second half of the diagram shows the same beaker after time has passed. Since the solutes could not move through the semi-permeable membrane, the water (the solvent) has moved to the right side, leaving less solution on the left side, but equalizing the concentrations of the two sides.
Figure 4.7.3 Osmosis is a type of diffusion in which only water can cross the plasma membrane.

Osmosis

Osmosis is a special type of diffusion — the diffusion of water molecules across a membrane. Like other molecules, water moves from an area of higher concentration to an area of lower concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane.  In Figure 4.7.3, the dotted red line shows a semi-permeable membrane.  In the first beaker, there is an uneven concentration of solutes on either side of the membrane, but the solute cannot cross — diffusion of the solute can’t occur.  In this case, water will move to even out the concentration as has happened on the beaker on the right side.  The water levels are uneven, but the process of osmosis has evened out the concentration gradient.

Facilitated Diffusion

Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules (such as glucose) all need help with diffusion. This help comes from special proteins in the membrane known as transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. There are several types of transport proteins, including channel proteins and carrier proteins. Both are shown in Figure 4.7.4.

Image shows a diagram of a cell membrane with different types of transport proteins imbedded. There are protein channels which allow small hydrophilic ions or molecules through, and there are carrier proteins which bind with a particular ion of molecule, and then shape in such a way that it moves the ion or molecule across the plasma membrane,
Figure 4.7.4 Facilitated diffusion across a cell membrane. Channel proteins and carrier proteins help substances diffuse across a cell membrane. In this diagram, the channel and carrier proteins are helping substances move into the cell (from the extracellular space to the intracellular space).

Transport and Homeostasis

For a cell to function normally, the inside of it must maintain a stable state. The concentrations of salts, nutrients, and other substances must be kept within certain ranges. The state in which stable conditions are maintained inside a cell (or an entire organism) is called homeostasis. Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell. The transport of substances into and out of cells as described in this section plays an important role in homeostasis. By allowing the movement of substances into and out of cells, transport keeps conditions within normal ranges inside the cells and throughout the organism as a whole.

Watch this video “Cell Transport,” by the Amoeba Sisters:

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=590

Cell Transport with the Amoeba Sisters, 2016.

4.7 Summary

 

  • Controlling the movement of things in and out of the cell is an important function of the plasma membrane. There are two basic ways that substances can cross the plasma membrane: passive transport — which requires no energy expenditure by the cell — and active transport — which requires energy.
  • No energy is needed from the cell for passive transport because it occurs when substances move naturally from an area of higher concentration to an area of lower concentration.
  • Simple diffusion is the movement of a substance due to differences in concentration. It happens without any help from other molecules. This is how very small, hydrophobic molecules (such as oxygen and carbon dioxide) enter and leave the cell.
  • Osmosis is the diffusion of water molecules across a membrane. Water moves in or out of a cell by osmosis until its concentration is the same on both sides of the plasma membrane.
  • Facilitated diffusion is the movement of a substance across a membrane due to differences in concentration, but it only occurs with the help of transport proteins (such as channel proteins or carrier proteins) in the membrane. This is how large or hydrophilic molecules and charged ions enter and leave the cell.
  • Processes of passive transport play important roles in homeostasis. By allowing the movement of substances into and out of the cell, they keep conditions within normal ranges inside the cell and the organism as a whole.

4.7 Review Questions

  1. What is the main difference between passive and active transport?
  2. Summarize three different ways that passive transport can occur. Give an example of a substance that is transported in each way.
  3. Explain how transport across the plasma membrane is related to homeostasis of the cell.
  4. In general, why can only very small, hydrophobic molecules cross the cell membrane by simple diffusion?
  5. Explain how facilitated diffusion assists with osmosis in cells. Define osmosis and facilitated diffusion in your answer.
  6. Imagine a hypothetical cell with a higher concentration of glucose inside the cell than outside. Answer the following questions about this cell, assuming all transport across the membrane is passive, not active.
    • Can the glucose simply diffuse across the cell membrane? Why or why not?
    • Assuming that there are glucose transport proteins in the cell membrane, which way would glucose flow — into or out of the cell? Explain your answer.
    • If the concentration of glucose was equal inside and outside of the cell, do you think there would be a net flow of glucose across the cell membrane in one direction or the other? Explain your answer.
  7. What are the similarities and differences between channel proteins and carrier proteins?
  8. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=590

4.7 Explore More

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=590

Osmosis and Water Potential, Amoeba Sisters, 2018.

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=590

Structure Of The Cell Membrane – Active and Passive Transport, Professor Dave Explains, 2016.

Attributions

Figure 4.7.1

Windows/ The Oyster Suite in Eureka, CA by Drew Coffman on Unsplash is used under the Unsplash License https://unsplash.com/license).

Figure 4.7.2

Diffusion/ Scheme simple diffusion in cell membrane  by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.7.3

Osmosis by OpenStax on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 4.7.4

Scheme facilitated diffusion in cell membrane by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2016, June 24). Cell transport. YouTube. https://www.youtube.com/watch?v=Ptmlvtei8hw&feature=youtu.be

Amoeba Sisters. (2018, June 27). Osmosis and water potential. YouTube.  https://www.youtube.com/watch?v=L-osEc07vMs&feature=youtu.be

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.7 Osmosis [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Professor Dave Explains. (2016, September 5). Structure of the cell membrane – Active and passive transport. https://www.youtube.com/watch?v=AcrqIxt8am8&feature=youtu.be

 

 

34

4.8 Active Transport

Created by: CK-12/Adapted by Christine Miller

Four soldiers pushing a Humvee. Their backs are against the vehicle and their faces show that they are pushing as hard as they can.
Figure 4.8.1 The Humvee challenge – Active transport.

Like Pushing a Humvee Uphill

You can tell by their faces that these airmen (Figure 4.8.1) are expending a lot of energy trying to push this Humvee up a slope. The men are participating in a competition that tests their brute strength against that of other teams. The Humvee weighs about 13 thousand pounds (about 5,897 kilograms), so it takes every ounce of energy they can muster to move it uphill against the force of gravity. Transport of some substances across a plasma membrane is a little like pushing a Humvee uphill — it can’t be done without adding energy.

What Is Active Transport?

Some substances can pass into or out of a cell across the plasma membrane without any energy required because they are moving from an area of higher concentration to an area of lower concentration. This type of transport is called passive transport. Other substances require energy to cross a plasma membrane, often because they are moving from an area of lower concentration to an area of higher concentration, against the concentration gradient. This type of transport is called active transport. The energy for active transport comes from the energy-carrying molecule called ATP (adenosine triphosphate). Active transport may also require proteins called pumps, which are embedded in the plasma membrane. Two types of active transport are membrane pumps (such as the sodium-potassium pump) and vesicle transport.

The Sodium-Potassium Pump

The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of the cell and potassium ions into the cells — in all the trillions of cells in the body! Both ions are moved from areas of lower to higher concentration, so energy is needed for this “uphill” process. The energy is provided by ATP. The sodium-potassium pump also requires carrier proteins. Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Figure 4.8.2 shows in greater detail how the sodium-potassium pump works, as well as the specific roles played by carrier proteins in this process.

Image shows a diagram of a sodium potassium pump. The pump collects three sodium ions, and moves them out of the cell, against the concentration gradient by changing its shape. Then, the pump collects 2 potassium ions and by changing its shape, moves these two ions into the cell, also against the concentration gradient.
Figure 4.8.2 The sodium-potassium pump moves sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. First, three sodium ions bind with a carrier protein in the cell membrane. The carrier protein then changes shape, powered by energy from ATP, and as it does, it pumps the three sodium ions out of the cell. At that point, two potassium ions bind to the carrier protein. The process is reversed, and the potassium ions are pumped into the cell.

To appreciate the importance of the sodium-potassium pump, you need to know more about the roles of sodium and potassium in the body. Both are essential dietary minerals. You need to get them from the foods you eat. Both sodium and potassium are also electrolytes, which means they dissociate into ions (charged particles) in solution, allowing them to conduct electricity. Normal body functions require a very narrow range of concentrations of sodium and potassium ions in body fluids, both inside and outside of cells.

These differences in concentration create an electrical and chemical gradient across the cell membrane, called the membrane potential. Tightly controlling the membrane potential is critical for vital body functions, including the transmission of nerve impulses and contraction of muscles. A large percentage of the body’s energy goes to maintaining this potential across the membranes of its trillions of cells with the sodium-potassium pump.

Vesicle Transport

Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport. Vesicle transport requires energy input from the cell, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis. Both types are shown in Figure 4.8.3.

Image shows a artist's rendition of a cell performing endo and exo cytosis. On the left side of the diagram, the cell is taking in large molecules through the plasma membrane by forming a vesicle around the particle. This is endocytosis. On the right side of the diagram, large molecules are exiting the cell by arriving in vesicles that fuse with the membrane to release their contents. This is exocytosis.
Figure 4.8.3 Large molecules can enter and exit the cell with the help of vesicles. On the left side of the diagram you can see exocytosis, as large molecules exit the cell through the plasma membrane. On the right side of the diagram you can see endocytosis, as large molecules enter the cell through the plasma membrane, via vesicle formation.

Endocytosis

Endocytosis is a type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. When an entire cell or other solid particle is engulfed, the process is called phagocytosis. When fluid is engulfed, the process is called pinocytosis.

Exocytosis

Exocytosis is a type of vesicle transport that moves a substance out of the cell (exo-, like “exit”). A vesicle containing the substance moves through the cytoplasm to the cell membrane. Because the vesicle membrane is a phospholipid bilayer like the plasma membrane, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell.

Image shows a diagram of both endocytosis and exocytosis. On the left side of the diagram, and large particle is being brought into the cell by creating a pocket of plasma membrane around the particle. This pocket deepens and eventually pinches off from the rest of the membrane, forming a vesicle containing the particle. This process is called endocytosis. On the right side of the diagram, a vesicle containing substances for export out of the cell are contained in a vesicle. The vesicle travels to the cell membrane and the vesicular membrane fuses with the cell membrane, releasing the contents of the vesicle outside of the cell.
Figure 4.8.4 Endocytosis brings substances into the cell via vesicle formation. Exocytosis allows substances to exit the cell by merging a transport vesicle with the cell membrane.

Feature: My Human Body

Maintaining the proper balance of sodium and potassium in body fluids by active transport is necessary for life itself, so it’s no surprise that getting the right balance of sodium and potassium in the diet is important for good health. Imbalances may increase the risk of high blood pressureheart diseasediabetes, and other disorders.

If you are like the majority of North Americans, sodium and potassium are out of balance in your diet. You are likely to consume too much sodium and too little potassium. Follow these guidelines to help ensure that these minerals are balanced in the foods you eat:

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=592

Figure 4.8.5 Potassium power! 

4.8 Summary

  • Active transport requires energy to move substances across a plasma membrane, often because the substances are moving from an area of lower concentration to an area of higher concentration, or because of their large size. Two types of active transport are membrane pumps (such as the sodium-potassium pump) and vesicle transport.
  • The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of the cell and potassium ions into the cell against a concentration gradient, in order to maintain the proper concentrations of ions, both inside and outside the cell, and to thereby control membrane potential.
  • Vesicle transport is a type of active transport that uses vesicles to move large molecules into or out of cells.

4.8 Review Questions

  1. Define active transport.
  2. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=592

  3. What is the sodium-potassium pump? Why is it so important?
  4. The drawing below shows the fluid inside and outside of a cell. The dots represent molecules of a substance needed by the cell. Explain which type of transport — active or passive — is needed to move the molecules into the cell.
    Image shows a cell with higher concentrations of a substance on the inside of the cell than on the outside of the cell. The cell is in a hypotonic solution
    Figure 4.8.6 Use this image to answer question #4
  5. What are the similarities and differences between phagocytosis and pinocytosis?
  6. What is the functional significance of the shape change of the carrier protein in the sodium-potassium pump after the sodium ions bind?
  7. A potentially deadly poison derived from plants called ouabain blocks the sodium-potassium pump and prevents it from working. What do you think this does to the sodium and potassium balance in cells? Explain your answer.

4.8 Explore More

Thumbnail for the embedded element "Neutrophil Phagocytosis - White Blood Cell Eats Staphylococcus Aureus Bacteria"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=592

Neutrophil Phagocytosis – White Blood Cell Eats Staphylococcus Aureus Bacteria,
ImmiflexImmuneSystem, 2013.

Thumbnail for the embedded element "Cell Transport"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=592

Cell Transport, The Amoeba Sisters, 2016.

Attributions

Figure 4.8.1

Humvee challenge by Airman 1st Class Collin Schmidt on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.8.2

Sodium Potassium Pump by Christine Miller is used under a CC BY 4.0  (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.8.3

Cytosis by Manu5 on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 4.8.4 

Endocytosis and Exocytosis by Christine Miller is used under a CC BY 4.0  (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.8.5

Figure 4.8.6

Active Transport by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2016, June 24). Cell transport [digital image]. YouTube. https://www.youtube.com/watch?v=Ptmlvtei8hw&feature=youtu.be

ImmiflexImmuneSystem. (2013). Neutrophil phagocytosis – White blood cell eats staphylococcus aureus bacteria. YouTube. https://www.youtube.com/watch?v=Z_mXDvZQ6dU

Mayo Clinic Staff. (n.d.). Diabetes [online]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/diabetes/symptoms-causes/syc-20371444

Mayo Clinic Staff. (n.d.). High blood pressure (hypertension) [online]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/high-blood-pressure/symptoms-causes/syc-20373410

Mayo Clinic Staff. (n.d.). Heart disease [online]. MayoClinic.org.  https://www.mayoclinic.org/diseases-conditions/heart-disease/symptoms-causes/syc-20353118

Wikipedia contributors. (2020, June 19). Ouabain. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ouabain&oldid=963440756

35

4.9 Energy Needs of Living Things

Created by: CK-12/Adapted by Christine Miller

Mush!

Image shows a photo of a sled carrying two men being pulled by 8 huskies.
Figure 4.9.1 All living things require energy to maintain homeostasis. These sled dogs use energy as they pull the sled.

These beautiful sled dogs are a metabolic marvel. While running up to 160 kilometres (about 99 miles) a day, they will each consume and burn about 12 thousand calories — about 240 calories per pound per day, which is the equivalent of about 24 Big Macs! A human endurance athlete, in contrast, typically burns only about 100 calories per pound (0.45 kg) each day. Scientists are intrigued by the amazing metabolism of sled dogs, although they still haven’t determined how they use up so much energy. But one thing is certain: all living things need energy for everything they do, whether it’s running a race or blinking an eye. In fact, every cell of your body constantly needs energy just to carry out basic life processes. You probably know that you get energy from the food you eat, but where does food come from? How does it come to contain energy? And how do your cells get the energy from food?

What Is Energy?

In the scientific world, energy is defined as the ability to do work. You can often see energy at work in living things — a bird flies through the air, a firefly glows in the dark, a dog wags its tail. These are obvious ways that living things use energy, but living things constantly use energy in less obvious ways, as well.

Why Living Things Need Energy

Inside every cell of all living things, energy is needed to carry out life processes. Energy is required to break down and build up molecules, and to transport many molecules across plasma membranes. All of life’s work needs energy. A lot of energy is also simply lost to the environment as heat. The story of life is a story of energy flow — its capture, its change of form, its use for work, and its loss as heat. Energy (unlike matter) cannot be recycled, so organisms require a constant input of energy. Life runs on chemical energy. Where do living organisms get this chemical energy?

How Organisms Get Energy

The chemical energy that organisms need comes from food. Food consists of organic molecules that store energy in their chemical bonds. In terms of obtaining food for energy, there are two types of organisms: autotrophs and heterotrophs.

Autotrophs

Autotrophs are organisms that capture energy from nonliving sources and transfer that energy into the living part of the ecosystem. They are also able to make their own food. Most autotrophs use the energy in sunlight to make food in the process of photosynthesis. Only certain organisms — such as plants, algae, and some bacteria — can make food through photosynthesis. Some photosynthetic organisms are shown in Figure 4.9.2.

Image shows a photo of a leafy plant Image shows a photograph of green algae living on the ocean floor
Figure 4.9.2 Photosynthetic autotrophs, which make food using the energy in sunlight, include plants (left), algae (middle), and certain bacteria (right).

Autotrophs are also called producers. They produce food not only for themselves, but for all other living things (known as consumers), as well. This is why autotrophs form the basis of food chains, such as the food chain shown In Figure 4.9.3.

Diagram shows two food pyramids, each with trophic levels labelled.
Figure 4.9.3 Food chains: Aquatic and terrestrial ecosystems.

A food chain shows how energy and matter flow from producers to consumers. Matter is recycled, but energy must keep flowing into the system. Where does this energy come from?

Watch the video “The simple story of photosynthesis and food – Amanda Ooten” from TED-Ed to learn more about photosynthesis:

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The simple story of photosynthesis and food – Amanda Ooten, TED-Ed, 2013.

Heterotrophs

Heterotrophs are living things that cannot make their own food. Instead, they get their food by consuming other organisms, which is why they are also called consumers. They may consume autotrophs or other heterotrophs. Heterotrophs include all animals and fungi, as well as many single-celled organisms. In Figure 4.9.3, all of the organisms are consumers except for the grasses and phytoplankton. What do you think would happen to consumers if all producers were to vanish from Earth?

Energy Molecules: Glucose and ATP

Organisms mainly use two types of molecules for chemical energy: glucose and ATP. Both molecules are used as fuels throughout the living world. Both molecules are also key players in the process of photosynthesis.

Glucose

Glucose is a simple carbohydrate with the chemical formula C6H12O6. It stores chemical energy in a concentrated, stable form. In your body, glucose is the form of energy that is carried in your blood and taken up by each of your trillions of cells. Glucose is the end product of photosynthesis, and it is the nearly universal food for life.  In Figure 4.9.4, you can see how photosynthesis stores energy from the sun in the glucose molecule and then how cellular respiration breaks the bonds in glucose to retrieve the energy.

Image shows the formula for photosynthesis: Carbon dioxide and water are converted to glucose and oxygen, which is an endothermic reaction drawing its energy from the sun. Cellular respiration carries out the opposite reaction, breaking down glucose in the presence of oxygen to produce carbon dioxide and water, and releasing the energy previously stored in the glucose molecule, which is an exothermic reaction.
Figure 4.9.4 Energy transfer in photosynthesis and cellular respiration.

ATP

If you remember from section 3.7 Nucleic Acids, ATP (adenosine triphosphate) is the energy-carrying molecule that cells use to power most cellular processes (nerve impulse conduction, protein synthesis and active transport are good examples of cell processes that rely on ATP as their energy source).  ATP is made during the first half of photosynthesis and then used for energy during the second half of photosynthesis, when glucose is made. ATP releases energy when it gives up one of its three phosphate groups (Pi) and changes to ADP (adenosine diphosphate, which has two phosphate groups), as shown in Figure 4.9.5. Thus, the breakdown of ATP into ADP + Pi is a catabolic reaction that releases energy (exothermic). ATP is made from the combination of ADP and Pi, an anabolic reaction that takes in energy (endothermic).

Image shows a diagram of the ATP molecule which consists of adenosine, ribose, and three phosphate groups. When the bond between the second and third phosphate group is broken, energy previously stored in the chemical bonds is released.
Figure 4.9.5 ATP (adenosine TRI phosphate) can be converted to ADP (adensosine DI phosphate) to release the energy stored in the chemical bonds between the second and third phosphate group.

Why Organisms Need Both Glucose and ATP

Why do living things need glucose if ATP is the molecule that cells use for energy? Why don’t autotrophs just make ATP and be done with it? The answer is in the “packaging.” A molecule of glucose contains more chemical energy in a smaller “package” than a molecule of ATP. Glucose is also more stable than ATP. Therefore, glucose is better for storing and transporting energy. Glucose, however, is too powerful for cells to use. ATP, on the other hand, contains just the right amount of energy to power life processes within cells. For these reasons, both glucose and ATP are needed by living things.

How Energy Flows Through Living Things

The flow of energy through living organisms begins with photosynthesis. This process stores energy from sunlight in the chemical bonds of glucose. By breaking the chemical bonds in glucose, cells release the stored energy and make the ATP they need. The process in which glucose is broken down and ATP is made is called cellular respiration.

Photosynthesis and cellular respiration are like two sides of the same coin. This is apparent in Figure 4.9.6. The products of one process are the reactants of the other. Together, the two processes store and release energy in living organisms. The two processes also work together to recycle oxygen in the Earth’s atmosphere.

Image shows a diagram of photosynthesis taking place in chloroplasts and converting carbon dioxide and water into glucose and oxygen. The image also shows how the products of photosynthesis can be transferred into the mitochondria to undergo cellular respiration, converting them back into carbon dioxide and water, and in doing so, releasing the stored energy in the glucose molecule.
Figure 4.9.6 This diagram compares and contrasts photosynthesis and cellular respiration. It also shows how the two processes are related.

 

4.9 Summary

  • Energy is the ability to do work. It is needed by all living things and every living cell to carry out life processes, such as breaking down and building up molecules, and transporting many molecules across cell membranes.
  • The form of energy that living things need for these processes is chemical energy, and it comes from food. Food consists of organic molecules that store energy in their chemical bonds.
  • Autotrophs make their own food. Plants, for example, make food by photosynthesis. Autotrophs are also called producers.
  • Heterotrophss obtain food by eating other organisms. Heterotrophs are also known as consumers.
  • Organisms mainly use the molecules glucose and ATP for energy. Glucose is a compact, stable form of energy that is carried in the blood and taken up by cells. ATP contains less energy and is used to power cell processes.
  • The flow of energy through living things begins with photosynthesis, which creates glucose. In a process called cellular respiration, organisms’ cells break down glucose and make the ATP they need.

4.9 Review Questions

  1. Define energy.
  2. Why do living things need energy?
  3. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=596

  4. Compare and contrast the two basic ways that organisms get energy.
  5. Describe the roles and relationships of the energy molecules glucose and ATP.
  6. Summarize how energy flows through living things.
  7. Why does the transformation of ATP to ADP release energy?

4.9 Explore More

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=596

Learn Biology: Autotrophs vs. Heterotrophs, Mahalodotcom, 2011.

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Energy Transfer in Trophic Levels, Teacher’s Pet, 2015.

Attributions

Figure 4.9.1
Three Airmen participate in dog-sled expedition by U.S. Air Force photo by Tech. Sgt. Dan Rea is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.9.2

Figure 4.9.3

Biomass_Pyramid by Swiggity.Swag.YOLO.Bro on Wikipedia is used and adapted by Christine Miller under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.

Figure 4.9.4

Photosynthesis and respiration by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.9.5

Photo synthesis and cellular respiration by Lady of Hats/ CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

 

References

LadyofHats/CK-12 Foundation. (2016, August 15). Figure 5: Photosynthesis and cellular respiration [digital image]. In Brainard, J., and Henderson, R., CK-12’s College Human Biology FlexBook® (Section 4.9). CK-12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/section/4.9/

Mahalodotcom. (2011, January 14). Learn biology: Autotrophs vs. heterotrophs. YouTube. https://www.youtube.com/watch?v=eDalQv7d2cs

Teacher’s Pet. (2015, March 23). Energy transfer in trophic levels. YouTube. https://www.youtube.com/watch?v=0glkXIj1DgE&feature=emb_logo

TED-Ed. (2013, March 5). The simple story of photosynthesis and food – Amanda Ooten. YouTube. https://www.youtube.com/watch?v=eo5XndJaz-Y&feature=youtu.be

36

4.10 Cellular Respiration

Created by: CK-12/Adapted by Christine Miller

Image shows a photo of the ingredients for smores sitting on a table. In the background, a campfire is burning.
Figure 4.10.1 Ready to make s’mores!

Bring on the S’mores!

This inviting camp fire can be used for both heat and light. Heat and light are two forms of energy that are released when a fuel like wood is burned. The cells of living things also get energy by “burning.” They “burn” glucose in a process called cellular respiration.

What Is Cellular Respiration?

Cellular respiration is the process by which living cells break down glucose molecules and release energy. The process is similar to burning, although it doesn’t produce light or intense heat as a campfire does. This is because cellular respiration releases the energy in glucose slowly and in many small steps. It uses the energy released to form molecules of ATP, the energy-carrying molecules that cells use to power biochemical processes. In this way, cellular respiration is an example of energy coupling: glucose is broken down in an exothermic reaction, and then the energy from this reaction powers the endothermic reaction of the formation of ATP.  Cellular respiration involves many chemical reactions, but they can all be summed up with this chemical equation:

C6H12O6  6O2 → 6CO2  6H2O Chemical Energy (in ATP)

In words, the equation shows that glucose (C6H12O6) and oxygen (O2) react to form carbon dioxide (CO2) and water (H2O), releasing energy in the process. Because oxygen is required for cellular respiration, it is an aerobic process.

Cellular respiration occurs in the cells of all living things, both autotrophs and heterotrophs. All of them burn glucose to form ATP. The reactions of cellular respiration can be grouped into three stages: glycolysis, the Krebs cycle (also called the citric acid cycle), and electron transport. Figure 4.10.2 gives an overview of these three stages, which are also described in detail below.

Image shows a diagram of the four stages in cellular respiration: Glycolysis, transition reaction, Kreb's cycle, and the electron transport system.
Figure 4.10.2 Cellular respiration takes place in the stages shown here. The process begins with a molecule of glucose, which has six carbon atoms. What happens to each of these atoms of carbon?

 

Cellular Respiration Stage I: Glycolysis

The first stage of cellular respiration is glycolysis, which happens in the cytosol of the cytoplasm.

Splitting Glucose

The word glycolysis literally means “glucose splitting,” which is exactly what happens in this stage. Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as summarized in the following diagram.

Figure 4.10.3 Glycolysis is a complex ten-step reaction that ultimately converts glucose into two molecules of pyruvate. This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration?

Results of Glycolysis

Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules which go on to stage II of cellular respiration. The energy needed to split glucose is provided by two molecules of ATP; this is called the energy investment phase. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP; this is the energy harvesting phase. As a result, there is a net gain of two ATP molecules during glycolysis. During this stage, high-energy electrons are also transferred to molecules of NAD  to produce two molecules of NADH, another energy-carrying molecule. NADH is used in stage III of cellular respiration to make more ATP.

Transition Reaction

Image shows a diagram of the transition reaction. In this reaction, 2 Pyruvate are converted to two acteyl CoA and 2 Carbon dioxide. In this process, 2 NADH are sent to the ETS carrying high energy electrons. The carbon dioxide leave the cell as metabolic waste and the acetyl CoA enter the Krebs Cycle.
Figure 4.10.4 Transition reaction of 2 pyruvate.

Before pyruvate can enter the next stage of cellular respiration it needs to be modified slightly.  The transition reaction is a very short reaction which converts the two molecules of pyruvate to two molecules of acetyl CoA, carbon dioxide, and two high energy electron pairs convert NAD to NADH.  The carbon dioxide is released, the acetyl CoA moves to the mitochondria to enter the Kreb’s Cycle (stage II), and the NADH carries the high energy electrons to the Electron Transport System (stage III).

Structure of the Mitochondrion

Image shows a diagram of a mitochondria. Several structures are labelled including cristae, matrix, DNA, intermembrane space, inner membrane, outer membrane, and ATP synthase particles.
Figure 4.10.5 Labelled mitochondrion structure.

Before you read about the last two stages of cellular respiration, you need to know more about the mitochondrion, where these two stages take place. A diagram of a mitochondrion is shown in Figure 4.10.5.

The structure of a mitochondrion is defined by an inner and outer membrane. This structure plays an important role in aerobic respiration.

As you can see from the figure, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration (the Krebs cycle) takes place in the matrix. The third stage (electron transport) happens on the inner membrane.

Cellular Respiration Stage II: The Krebs Cycle

Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the Krebs cycle (also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA.  The reactions that occur next are shown in Figure 4.10.6.

Image shows a diagram of the reactants and products of the Krebs Cycle. Two molecules of acetyl CoA are converted to 4 carbon dioxide which are released as cellular waste, 2 ATP which are used in the cell for energy, and 8 NADH and 2 FADH2, both of which travel to the ETS.
Figure 4.10.6 Reactants and products of the Krebs Cycle.

Steps of the Krebs Cycle

The Krebs cycle itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.

After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2, another energy-carrying coenzyme. Carbon dioxide is also released as a waste product of these reactions.

The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvic acid molecules when it splits glucose.

Results of the Glycolysis, Transition Reaction and Krebs Cycle

After glycolysis, transition reaction, and the Krebs cycle, the glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:

The events of cellular respiration up to this point are exergonic reactions– they are releasing energy that had been stored in the bonds of the glucose molecule.  This energy will be transferred to the third and final stage of cellular respiration: the Electron Transport System, which is an endergonic reaction.  Using an exothermic reaction to power an endothermic reaction is known as energy coupling.

Cellular Respiration Stage III: Electron Transport Chain

Image shows the reactants and products of the electron transport chain. In this stage, 32 adenosine diphosphate and 32 inorganic phosphates combine to form 32 ATP. In addition, hydrogen and oxygen combine to form 6 molecules of water.
Figure 4.10.7. Reactants and products of the electron transport chain.

 ETC, the final stage in cellular respiration produces 32 ATP.  The Electron Transport Chain is the final stage of cellular respiration. In this stage, energy being transported by NADH and FADH2 is transferred to ATP.  In addition, oxygen acts as the final proton acceptor for the hydrogens released from all the NADH and FADH2, forming water.  Figure 4.10.8 shows the reactants and products of the ETC.

Transporting Electrons

The Electron transport chain is the third stage of cellular respiration and is illustrated in Figure 4.10.8. During this stage, high-energy electrons are released from NADH and FADH2, and they move along electron-transport chains on the inner membrane of the mitochondrion. An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by chemical reactions. Some of the energy from the electrons is used to pump hydrogen ions (H ) across the inner membrane, from the matrix into the intermembrane space. This ion transfer creates an electrochemical gradient that drives the synthesis of ATP.

 

Figure 4.10.8 Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.

Making ATP

As shown in Figure 4.10.8, the pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate in a process called oxidative phosphorylation. After passing through the electron-transport chain, the “spent” electrons combine with oxygen to form water.

How Much ATP?

You have seen how the three stages of aerobic respiration use the energy in glucose to make ATP. How much ATP is produced in all three stages combined? Glycolysis produces two ATP molecules, and the Krebs cycle produces two more. Electron transport begins with several molecules of NADH and FADH2 from the Krebs cycle and transfers their energy into as many as 34 more ATP molecules. All told, then, up to 38 molecules of ATP can be produced from just one molecule of glucose in the process of cellular respiration.

4.10 Summary

  • Cellular respiration is the aerobic process by which living cells break down glucose molecules, release energy, and form molecules of ATP. Generally speaking, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and water.
  • The first stage of cellular respiration, called glycolysis, takes place in the cytoplasm. In this step, enzymes split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.  Following glycolysis, a short reaction called the transition reaction converts the pyruvate into two molecules of acetyl CoA.
  • The organelle called a mitochondrion is the site of the other two stages of cellular respiration. The mitochondrion has an inner and outer membrane separated by an intermembrane space, and the inner membrane encloses a space called the matrix.
  • The second stage of cellular respiration, called the Krebs cycle, takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their chemical bonds being stored in a total of 16 energy-carrying molecules (including two from glycolysis and two from transition reaction).
  • The third and final stage of cellular respiration, called electron transport, takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions across the membrane, creating an electrochemical gradient that drives the synthesis of many more molecules of ATP.
  • In all three stages of cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.

4.10 Review Questions

  1. What is the purpose of cellular respiration? Provide a concise summary of the process.
  2. State what happens during glycolysis.
  3. Describe the structure of a mitochondrion.
  4. What molecule is present at both the beginning and end of the Krebs cycle?
  5. What happens during the electron transport stage of cellular respiration?
  6. How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?
  7. Do plants undergo cellular respiration? Why or why not?
  8. Explain why the process of cellular respiration described in this section is considered aerobic.
  9. Name three energy-carrying molecules involved in cellular respiration.
  10. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=599

  11. Which stage of aerobic cellular respiration produces the most ATP?
  12. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=599

4.10 Explore More

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ATP & Respiration: Crash Course Biology #7, CrashCourse, 2012.

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Cellular Respiration and the Mighty Mitochondria, The Amoeba Sisters, 2014.

Attributions

Figure 4.10.1

Smores by Jessica Ruscello on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.10.2

Carbohydrate_Metabolism by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 4.10.3

Glycolysis by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.4

Transition Reaction by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.5

Mitochondrion by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.10.6

Krebs cycle by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.7

Electron Transport Chain (ETC) by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.10.8

The_Electron_Transport_Chain by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References

CrashCourse. (2012, March 12). ATP & Respiration: Crash Course Biology #7. YouTube. https://www.youtube.com/watch?time_continue=2&v=00jbG_cfGuQ&feature=emb_logo

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 24.8 Electron Transport Chain [digital image]. In Anatomy & Physiology, Connexions (Section ). OpenStax.  https://openstax.org/books/anatomy-and-physiology/pages/24-2-carbohydrate-metabolism

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 24.9 Carbohydrate Metabolism [digital image]. In Anatomy & Physiology, Connexions (Section 24.2). OpenStax.  https://openstax.org/books/anatomy-and-physiology/pages/24-2-carbohydrate-metabolism

The Amoeba Sisters. (2014, October 22). Cellular Respiration and the Mighty Mitochondria. YouTube. https://www.youtube.com/watch?v=4Eo7JtRA7lg&t=3s

 

37

4.11 Anaerobic Processes

Created by: CK-12/Adapted by Christine Miller

Image shows a photo of women in a short distance running race on a track.
Figure 4.11.1 Sprinters racing on a track. 

Fast and Furious

These sprinters’ muscles will need a lot of energy to complete this short race, because they will be running at top speed. The action won’t last long, but it will be very intense. The energy each sprinter needs can’t be provided quickly enough by aerobic cellular respiration. Instead, their muscle cells must use a different process to power their activity.

Making ATP Without Oxygen

Living things’ cells power their activities with the energy-carrying molecule ATP (adenosine triphosphate). The cells of most living things make ATP from glucose in the process of cellular respiration. This process occurs in three stages: glycolysis, the Krebs cycle, and electron transport. The latter two stages require oxygen, making cellular respiration an aerobic process. When oxygen is not available in cells, the ETS quickly shuts down.  Luckily, there are also ways of making ATP from glucose which are anaerobic, which means that they do not require oxygen. These processes are referred to collectively as anaerobic respiration.

Fermentation

Ome important way of making ATP without oxygen is fermentation. Fermentation starts with glycolysis, which does not require oxygen, but it does not involve the latter two stages of aerobic cellular respiration (the Krebs cycle and electron transport). There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. We make use of both types of fermentation using other organisms, but only lactic acid fermentation actually takes place inside the human body.

Alcoholic Fermentation

Figure 4.11.2 In alcoholic fermentation, pyruvate is converted to ethanol and carbon dioxide.  During this process, NAD+ is formed, which allows glycolysis to continue making ATP.

Alcoholic fermentation is carried out by single-celled fungi (called yeasts), as well as some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine. The biofuel ethanol (a type of alcohol), for example, is produced by alcoholic fermentation of the glucose in corn or other plants. The process by which this happens is summarized in the diagram below. The two pyruvic acid molecules shown in the diagram come from the splitting of glucose in the first stage of the process (glycolysis). ATP is also made during glycolysis. Two molecules of ATP are produced from each molecule of glucose.

Image shows a close up view of a slice of bread. There are holes in the bread created by bubble of carbon dioxide.
Figure 4.11.3 Holes in bread created by carbon dioxide.

Yeasts in bread dough also use alcoholic fermentation for energy. They produce carbon dioxide gas as a waste product. The carbon dioxide released causes bubbles in the dough and explains why the dough rises. Do you see the small holes in the bread pictured to the right? The holes were formed by bubbles of carbon dioxide gas.

As you have probably guessed, yeast is also used in producing alcoholic beverages.  When making beer, brewers will add yeast to a mix of barley and hops.  In the absence of oxygen, yeast will carry out alcoholic fermentation in order to convert the glucose in the barley into energy, producing the alcohol content as well as the carbonation present in beer.

Lactic Acid Fermentation

Lactic acid fermentation is carried out by certain bacteria, including the bacteria in yogurt. It is also carried out by your muscle cells when you work them hard and fast. This is how the muscles of the sprinters pictured above get energy for their short-lived — but intense — activity. When this happens, your muscles are using ATP faster than your cardiovascular system can deliver oxygen!  The process by which this happens is summarized in the diagram below. Again, the two pyruvic acid molecules shown in the diagram come from the splitting of glucose in the first stage of the process (glycolysis). It is also during this stage that two ATP molecules are produced. The rest of the processes produce lactic acid. Note that, unlike in alcoholic fermentation, there is no carbon dioxide waste product in lactic acid fermentation.

Image shows a diagram of the formula of lactic acid fermentation, in which pyruvic acid is converted into lactic acid.
Figure 4.11.4 Lactic acid fermentation formula.

Lactic acid fermentation produces lactic acid and NAD+. The NAD+ cycles back to allow glycolysis to continue so more ATP is made. Each circle represents a carbon atom.

Did you ever run a race, lift heavy weights, or participate in some other intense activity and notice that your muscles start to feel a burning sensation? This may occur when your muscle cells use lactic acid fermentation to provide ATP for energy. The buildup of lactic acid in the muscles causes a burning feeling. This painful sensation is useful if it gets you to stop overworking your muscles and allow them a recovery period, during which cells can eliminate the lactic acid.

Pros and Cons of Anaerobic Respiration

With oxygen, organisms can use aerobic cellular respiration to produce up to 38 molecules of ATP from just one molecule of glucose. Without oxygen, organisms must use anaerobic respiration to produce ATP, and this process produces only two molecules of ATP per molecule of glucose. Although anaerobic respiration produces less ATP, it has the advantage of doing so very quickly. For example, it allows your muscles to get the energy they need for short bursts of intense activity. Aerobic cellular respiration, in contrast, produces ATP more slowly.

Fermentation in Food Production

Anaerobic respiration is also used in the food industry.  You read about yeast’s role in making bread and beer, but did you know that there are many microbes that are used to create the food we eat, including cheese, sour cream, yogurt, soy sauce, olives, pepperoni, and many more.  Watch the video below to learn more about fermentation in the food industry.

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=601

The beneficial bacteria that make delicious food – Erez Garty, TED-Ed, 2016.

4.11 Cultural Connection

Fishing has always been part of the culture and nutrition of Indigenous peoples living on the west coast of Canada.  Fish provides delicious important nutrients such as protein, Omega-3 fatty acids, calcium, iron, and Vitamins A, B, C and D.  Traditionally, no part of the fish was wasted, including head, eyes, internal organs, and eggs.

Eulachon, also known as candle fish or oolichan, (pictured below) have been prized for their oil for thousands of years. The pathways of these fish dictated “grease trails” and are found from Bristol Bay, Alaska, all the way south to the Klamath River, California.  Within BC, the areas of Nass, Knights Inlet, and Bella Coola had large trading centres for this important natural resource.

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=601

Photos by Brodie Guy – www.brodieguy.com CC BY-NC-ND 4.0

 

Euchalon were and are eaten fresh, smoked or dried, and as grease.  The grease remains a highly valued food to Indigenous coastal communities.  The flavour of the grease varies greatly depending not only on where the fish is from and how it is made, but also how long it is left to ferment.  To ferment the eulachon, fish are left in a wood-lined locker dug into the soil for 10 days.  Fermentation uses the action of fungi and bacteria to break down the fish making oil extraction much faster and easier.

To learn more, visit the First Nations Health Authority Traditional Foods Fact Sheet and a feature in the Yukon News, “Eulachon, oolicahn, hooligan: A fish by any other name is just as oily.”

 

 

4.11 Summary

  • The cells of most living things produce ATP from glucose by aerobic cellular respiration, which uses oxygen. Some organisms instead produce ATP from glucose by anaerobic respiration, which does not require oxygen.
  • An important way of making ATP without oxygen is fermentation. There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis, the first (anaerobic) stage of cellular respiration, in which two molecules of ATP are produced from one molecule of glucose.
  • Alcoholic fermentation is carried out by single-celled organisms, including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
  • Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
  • Anaerobic respiration produces far less ATP than does aerobic cellular respiration, but it has the advantage of being much faster. For example, it allows muscles to get the energy they need for short bursts of intense activity.

4.11 Review Questions

  1. Explain the primary difference between aerobic cellular respiration and anaerobic respiration.
  2. What is fermentation?
  3. Compare and contrast alcoholic and lactic acid fermentation.
  4. Identify the major pros and the major cons of anaerobic respiration relative to aerobic cellular respiration.
  5. What process is shared between aerobic cellular respiration and anaerobic respiration? Describe the process briefly. Why can this process happen in anaerobic respiration, as well as aerobic respiration?
  6. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=601

  7. What is the reactant (or starting material)common to aerobic respiration and both types of fermentation?

4.11 Explore More

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Anaerobic Respiration, Bozeman Science, 2013.

Thumbnail for the embedded element "Fermentation"

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Fermentation, The Amoeba Sisters, 2018.

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=601

Science of Beer: Tapping the Power of Brewer’s Yeast, KQED Science, 2014.

 

Attributions

Figure 4.11.1

Sprinters by Jonathan Chng on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.11.2

Alcoholic fermentation by Hana Zavadska/ CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. 

©CK-12 Foundation
Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 4.11.3

Bread [photo] by Orlova Maria on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.11.4

Lactic Acid Fermenation by Hana Zavadska/ CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license. 

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

References

Bozeman Science. (2013, May 2). Anaerobic respiration. YouTube. https://www.youtube.com/watch?v=cDC29iBxb3w&feature=youtu.be

Hana Zavadska/CK-12 Foundation. (2016, August 15). Figure 2: Alcoholic fermentation  [digital image]. In Brainard, J., and Henderson, R., CK-12’s College Human Biology FlexBook® (Section 4.11). CK-12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/section/4.11/

Hana Zavadska/CK-12 Foundation. (2016, August 15). Figure 4: Lactic acid fermentation [digital image]. In Brainard, J., and Henderson, R., CK-12’s College Human Biology FlexBook® (Section 4.11). CK-12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/section/4.11/

First Nations Health Authority. (2019, September 6). First Nations traditional foods facts Sheet [pdf]. https://www.fnha.ca/Documents/Traditional_Food_Fact_Sheets.pdf

Genest, M. (2017, May 24). Eulachon, oolichan, hooligan: A fish by any other name is just as oily [online article]. YukonNews.com. https://www.yukon-news.com/business/eulachon-oolichan-hooligan-a-fish-by-any-other-name-is-just-as-oily/

KQED Science. (2014, February 11). Science of beer: Tapping the power of brewer’s yeast. YouTube. https://www.youtube.com/watch?v=TVtqwWGguFk&feature=youtu.be

TED-Ed. (2016). The beneficial bacteria that make delicious food – Erez Garty. YouTube. https://www.youtube.com/watch?v=eksagPy5tmQ&feature=youtu.be

The Amoeba Sisters. (2018, April 30). Fermentation. YouTube. https://www.youtube.com/watch?v=YbdkbCU20_M&feature=youtu.be

Wikipedia contributors. (2020, June 21). Ethanol fuel. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ethanol_fuel&oldid=963675942

38

4.12 Cell Cycle and Cell Division

Created by: CK-12/Adapted by Christine Miller

Image shows a photo of a mother holding her baby girl.
Figure 4.12.1 Mother and growing baby girl.

So Many Cells!

This baby girl (Figure 4.12.1) has a lot of growing to do before she’s as big as her mom. Most of her growth will be the result of cell division. By the time she is an adult, her body will consist of trillions of cells. Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells, causing them to grow out of control. In fact, this is how cancer cells cause illness. In this concept, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.

The Cell Cycle

Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division. The cell cycle in prokaryotes is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes, the cell cycle is more complicated.

Eukaryotic Cell Cycle

The diagram in Figure 4.12.2 represents the cell cycle of a eukaryotic cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.

Image shows a diagram of the cell cycle, which includes Interphase (made up of three phases called first gap, synthesis and second gap) and the mitotic phase (made up of prophase, metaphase, anaphase, telophase, and cytokinesis).
Figure 4.12.2 Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The First Gap (G1), Synthesis, and Second Gap (G2) phases make up interphase (I). The mitotic phase includes mitosis and cytokinesis. After the mitotic phase, two cells result.

Interphase

The interphase of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.

Control of the Cell Cycle

If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.

Figure 4.12.3 Eukaryotic Cell Cycle – Checkpoints.

Checkpoints in the eukaryotic cell cycle ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.

Cancer and the Cell Cycle

Cancer is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell’s DNA becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a tumor (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.

Image shows a mass of cells in a cluster.
Figure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.

Cell Division

Cell division is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic or eukaryotic. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no nucleus, and few other organelles. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.

Before a eukaryotic cell divides, all of the DNA in the cell’s multiple chromosomes is replicated. Its organelles are also duplicated. Cell division occurs in two major steps, called mitosis and cytokinesis, both of which are described in greater detail in Chapter 5.

Feature: Human Biology in the News

Image shows a black and white photograph of a woman smiling, with her hands on her hips. She is African American, and dressed in the style of the 1940s in a skirt and blazer.
Figure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.

Henrietta Lacks sought treatment for her cancer at Johns Hopkins University Hospital at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks’s doctor, Howard Jones, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing… and dividing and dividing and dividing. Copies of Henrietta’s cells — called HeLa cells, for her name (Henrietta Lacks) — are still alive today. In fact, there are currently  billions of HeLa cells in laboratories around the world!

Why Henrietta’s cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.

You would think that Henrietta’s name would be well known in medical history for her unparalleled contributions to biomedical research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot published a nonfiction book, The Immortal Life of Henrietta Lacks. Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.

Ironically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.

If you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot’s The Immortal Life of Henrietta Lacks (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:

Thumbnail for the embedded element "The immortal cells of Henrietta Lacks - Robin Bulleri"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=603

The immortal cells of Henrietta Lacks – Robin Bulleri, TED-Ed, 2016.

4.12 Summary

  • The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division. The cycle is more complicated in eukaryotic than prokaryotic cells.
  • In a eukaryotic cell, the cell cycle has two major phases: mitotic phase and interphase. During mitotic phase, first the nucleus and then the cytoplasm divide. During interphase, the cell grows, performs routine life processes, and prepares to divide.
  • The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. There are a number of main checkpoints in the regulation of the cell cycle.
  • Cancer is a disease that occurs when the cell cycle is no longer regulated, often because the cell’s DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.
  • The cell division phase of the cell cycle in a eukaryotic cell occurs in two major steps: mitosis — when the nucleus divides — and cytokinesis, when the cytoplasm divides and two daughter cells form.

4.12 Review Questions

  1. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=603

  2. Explain why cell division is more complex in eukaryotic than prokaryotic cells.
  3. Using a technique called flow cytometry, scientists can distinguish between cells with the normal amount of DNA and those that contain twice the normal amount of DNA as they go through the cell cycle. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.
  4. What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal?

4.12 Explore More

Thumbnail for the embedded element "The Cell Cycle (and cancer) [Updated]"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=603

The Cell Cycle (and cancer) [Updated], The Amoeba Sisters, 2018.

Attributions

Figure 4.12.1

Mom and baby by Taiying Lu on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 4.12.2

Cell Cycle by LadyofHats; CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 4.12.3

Cell Cycle Checkpoints by LadyofHats; CK-12 Foundation is used and adapted by Christine Miller under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 4.12.4

Cancer cells forming a tumour by Ed Uthman, MD on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.12.5

Henrietta Lacks by Oregon State University on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.

References

Amoeba Sisters.  (2018, March 20). The cell cycle (and cancer) [Updated]. YouTube. https://www.youtube.com/watch?v=QVCjdNxJreE&feature=youtu.be

TED-Ed. (2016, February 8). The immortal cells of Henrietta Lacks – Robin Bulleri. YouTube. https://www.youtube.com/watch?v=22lGbAVWhro&feature=youtu.be

Wikipedia contributors. (2020, June 23). Henrietta Lacks. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Henrietta_Lacks&oldid=964020268

Wikipedia contributors. (2020, May 11). Howard W. Jones. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Howard_W._Jones&oldid=956033806

Wikipedia contributors. (2020, July 1). George Otto Gey. In Wikipedia. https://en.wikipedia.org/w/index.php?title=George_Otto_Gey&oldid=965394045

Wikipedia contributors. (2020, July 6). Johns Hopkins Hospital. In ,Wikipedia.  https://en.wikipedia.org/w/index.php?title=Johns_Hopkins_Hospital&oldid=966348552

Wikipedia contributors. (2020, June 28). Jonas Salk. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Jonas_Salk&oldid=964883129

Wikipedia contributors. (2020, April 14). Rebecca Skloot. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Rebecca_Skloot&oldid=950837115

Wikipedia contributors. (2020, February 21). The immortal life of Henrietta Lacks. In Wikipedia. https://en.wikipedia.org/w/index.php?title=The_Immortal_Life_of_Henrietta_Lacks&oldid=941942679

 

39

4.13 Mitosis and Cytokinesis

Created by: CK-12/Adapted by Christine Miller

Divide and Split

Image shows a cell in anaphase of mitosis. The image is taken using immunoflourescence microscopy and components of the cell including spindle fibers and genetic material show as vivid blues and greens.
Figure 4.13.1 A cell in anaphase of mitosis.

Can you guess what the colourful image in Figure 4.13.1 represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the cell in a part of cell division called anaphase, where the DNA is being pulled to opposite ends of the cell. Normally, DNA is located in the nucleus of most human cells. The nucleus divides before the cell itself splits in two, and before the nucleus divides, the cell’s DNA is replicated (or copied). There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To answer that question, you first need to know more about DNA and the forms it takes.

The Forms of DNA

Diagram shows the forms that DNA takes, as a double helix, which will coil around itself, which will ultimately form a chromosome.
Figure 4.13.2 Forms of DNA.

Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only once a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown below.

Labelled diagram of a chromosome showing that in a chromosome with the typical "X" shape, it is comprised of two identical pieces of DNA, each called a chromatid.
Figure 4.13.3 Diagram of a chromosome showing that in a chromosome with the typical “X” shape, it is comprised of two identical pieces of DNA, each called a chromatid.

Most cells in the human body have two pairs of 23 different chromosomes, for a total of 46 chromosomes. Cells that have two pairs of chromosomes are called diploid. Because DNA has already replicated when it coils into a chromosome, each chromosome actually consists of two identical structures called sister chromatids. Sister chromatids are joined together at a region called a centromere.

 

 

 

Mitosis

Diagram shows the stages of Mitosis in which DNA replicates, chromosomes align, sister chromatids separate, and then two diploid cell emerge.
Figure 4.13.4 Mitosis is the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells. What happens during mitosis?

The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. This is shown in the figure below.

Mitosis actually occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase.

Prophase

Figure 4.13.5 Mitotic prophase.

The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells. They help ensure that the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The spindle consists of fibres made of microtubules.

Diagram shows a cell in prophase of mitosis. The nuclear envelope is breaking down, chromosomes are condensing, and spindle fibers are forming.
Figure 4.13.6 Diagram of a cell in prophase of mitosis.

 

 

Metaphase

Figure 4.13.7 Metaphase.

During metaphase, spindle fibres attach to the centromere of each pair of sister chromatids. As you can see in Figure 4.13.7, the sister chromatids line up at the equator (or center) of the cell. The spindle fibres ensure that sister chromatids will separate and go to different daughter cells when the cell divides.

Diagram shows metaphase of mitosis, in which the spindle fibers are fully formed and the chromosomes are aligned along the center of the cell.
Figure 4.13.8 Diagram showing the metaphase of mitosis.

Anaphase

Figure 4.13.9 Mitotic anaphase.

During anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibres. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole. At the end of anaphase, each pole of the cell has a complete set of chromosomes.

Image shows a eukaryotic cell in anaphase of the cell cycle, in which sister chromatids have been separated from each other and are being pulled to opposite ends of the cell by spindle fibers.
Figure 4.13.10 Diagram showing eukaryotic cell in anaphase of cell cycle.

Telophase

Figure 4.13.11 Mitotic telophase.

During telophase, the chromosomes begin to uncoil and form chromatin. This prepares the genetic material for directing the metabolic activities of the new cells. The spindle also breaks down, and new nuclear envelopes form.

Telophase is the stage in mitosis in which the nuclear envelope starts to reform, the chromosomes decondense and the cell continues to elongate.
Figure 4.13.12 Diagram showing telophase in mitosis.

Cytokinesis

Figure 4.13.13 Mitotic cytokinesis.

Cytokinesis is the final stage of cell division. During cytokinesis, the cytoplasm splits in two and the cell divides, as shown below. In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. Thus, the goal of mitosis and cytokinesis is now complete, because one parent cell has given rise to two daughter cells. The daughter cells have the same chromosomes as the parent cell.

Cytokinesis is the final step in cell division, in which the cytoplasm of the two new daughter cells completely separates.
Figure 4.13.14 Diagram showing the final step in cell division: cytokinesis.

4.13 Summary

  • Until a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each chromosome actually consists of two sister chromatids, which are joined together at a centromere.
  • Mitosis is the process during which the nucleus of a eukaryotic cell divides. During this process, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases: prophase, metaphase, anaphase, and telophase.
  • Cytokinesis is the final stage of cell division, during which the cytoplasm splits in two and two daughter cells form.

4.13 Review Questions

  1. Describe the different forms that DNA takes before and during cell division in a eukaryotic cell.
  2. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=605

  3. Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase.
  4. Order the diagrams of the stages of mitosis:

    An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=605

  5. Explain what happens during cytokinesis in an animal cell.
  6. What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis?
  7. True or False:

    An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=605

4.13 Explore More

Thumbnail for the embedded element "Mitosis"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=605

Mitosis, NDSU Virtual Cell Animations project (ndsuvirtualcell), 2012.

Thumbnail for the embedded element "Nondisjunction (Trisomy 21) - An Animated Tutorial"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=605

Nondisjunction (Trisomy 21) – An Animated Tutorial, Kristen Koprowski, 2012.

Attributions

Figure 4.13.1

Anaphase_IF by Roy van Heesbeen on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.2

Chromosomes by OpenClipArt-Vectors on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).

Figure 4.13.3

Chromosome/ Chromatid/ Sister Chromatid by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.4

Simple Mitosis by Mariana Ruiz Villarreal [LadyofHats] via CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 4.13.5

Mitotic Prophase [tiny] by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.6

Prophase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.7

Mitotic_Metaphase by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.8

Metaphase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.9

Anaphase [adapted] by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.10

Anaphase_eukaryotic_mitosis.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.11

Mitotic Telophase by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.12

Telophase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.13

Mitotic Cytokinesis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.14

Cytokinesis Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Koprowski, K., Cabey, R. [Kristen Koprowski]. (2012). Nondisjunction (Trisomy 21) – An Animated Tutorial. YouTube. https://www.youtube.com/watch?v=EA0qxhR2oOk&feature=youtu.be

NDSU Virtual Cell Animations project [ndsuvirtualcell]. (2012). Mitosis. YouTube. https://www.youtube.com/watch?v=C6hn3sA0ip0&t=21s

40

4.14 Case Study Conclusion: More Than Just Tired

Created by CK12/Adapted by Christine Miller

Image shows a micrograph of muscle tissue. Two of the cells contain large numbers of small red granules, which are diseased mitochondria.
Figure 4.14.1 When muscle tissue is stained with a particular type of dye, clumps of diseased mitochondria show up in red and are termed “ragged red fibres”.  This is one of the diagnostic tools used to diagnose mitochondrial disease.

Jasmin discovered that her extreme fatigue, muscle pain, vision problems, and vomiting were due to problems in her mitochondria, like the damaged mitochondria shown in red in Figure 4.14.1. Mitochondria are small, membrane-bound organelles found in eukaryotic cells that provide energy for the cells of the body. They do this by carrying out the final two steps of aerobic cellular respiration: the Krebs cycle and electron transport. This is the major way that the human body breaks down the sugar glucose from food into a form of energy cells can use, namely the molecule ATP.

Because mitochondria provide energy for cells, you can understand why Jasmin was experiencing extreme fatigue, particularly after running. Her damaged mitochondria could not keep up with her need for energy, particularly after intense exercise, which requires a lot of additional energy. What is perhaps not so obvious are the reasons for her other symptoms, such as blurry vision, muscle spasms, and vomiting. All of the cells in the body require energy in order to function properly. Mitochondrial diseases can cause problems in mitochondria in any cell of the body, including muscle cells and cells of the nervous system, which includes the brain and nerves. The nervous system and muscles work together to control vision and digestive system functions, such as vomiting, so when they are not functioning properly, a variety of symptoms can emerge. This also explains why Jasmin’s niece, who has a similar mitochondrial disease, has symptoms related to brain function, such as seizures and learning disabilities. Our cells are microscopic, and mitochondria are even tinier — but they are essential for the proper functioning of our bodies. When they are damaged, serious health effects can occur.

 

Image shows an adult and child sitting together.
Figure 4.14.2 Mitochondrial disease can manifest itself very differently in different people, even if they are related.  Jasmin and her niece have the same mitochondrial disease, but with different age of onset, different symptoms and different severity of symptoms.

One seemingly confusing aspect of mitochondrial diseases is that the type of symptoms, severity of symptoms, and age of onset can vary wildly between people — even within the same family! In Jasmin’s case, she did not notice symptoms until adulthood, while her niece had more severe symptoms starting at a much younger age. This makes sense when you know more about how mitochondrial diseases work.

Inherited mitochondrial diseases can be due to damage in either the DNA in the nucleus of cells or in the DNA in the mitochondria themselves. Recall that mitochondria are thought to have evolved from prokaryotic organisms that were once free-living, but were then infected or engulfed by larger cells. One of the pieces of evidence that supports this endosymbiotic theory is that mitochondria have their own, separate DNA. When the mitochondrial DNA is damaged (or mutated) it can result in some types of mitochondrial diseases. However, these mutations do not typically affect all of the mitochondria in a cell. During cell division, organelles such as mitochondria are replicated and passed down to the new daughter cells. If some of the mitochondria are damaged, and others are not, the daughter cells can have different amounts of damaged mitochondria. This helps explain the wide range of symptoms in people with mitochondrial diseases — even ones in the same family — because different cells in their bodies are affected in varying degrees. Jasmin’s niece was affected strongly and her symptoms were noticed early, while Jasmin’s symptoms were more mild and did not become apparent until adulthood.

There is still much more that needs to be discovered about the different types of mitochondrial diseases. But by learning about cells, their organelles, how they obtain energy, and how they divide, you should now have a better understanding of the biology behind these diseases.

Apply your understanding of cells to your own life. Can you think of other diseases that affect cellular structures or functions. Do they affect people you know? Since your entire body is made of cells, when cells are damaged or not functioning properly, it can cause a wide variety of health problems.

Chapter 4 Summary

Type your learning objectives here.
In this chapter you learned many facts about cells. Specifically, you learned that:

  • Cells are the basic units of structure and function of living things.
  • The first cells were observed from cork by Hooke in the 1600s. Soon after, van Leeuwenhoek observed other living cells.
  • In the early 1800s, Schwann and Schleiden theorized that cells are the basic building blocks of all living things. Around 1850, Virchow saw cells dividing, and added his own theory that living cells arise only from other living cells. These ideas led to cell theory, which states that all organisms are made of cells, all life functions occur in cells, and all cells come from other cells.
  • The invention of the electron microscope in the 1950s allowed scientists to see organelles and other structures inside cells for the first time.
  • There is variation in cells, but all cells have a plasma membrane, cytoplasm, ribosomes, and DNA.
    • The plasma membrane is composed mainly of a bilayer of phospholipid molecules and forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell. Some cells have extensions of their plasma membrane with other functions, such as flagella or cilia.
    • Cytoplasm is a thick solution that fills a cell and is enclosed by the plasma membrane. It helps give the cell shape, holds organelles, and provides a site for many of the biochemical reactions inside the cell. The liquid part of the cytoplasm is called cytosol.
    • Ribosomes are small structures where proteins are made.
  • Cells are usually very small, so they have a large enough surface area-to-volume ratio to maintain normal cell processes. Cells with different functions often have different shapes.
  • Prokaryotic cells do not have a nucleus. Eukaryotic cells have a nucleus, as well as other organelles. An organelle is a structure within the cytoplasm of a cell that is enclosed within a membrane and performs a specific job.
  • The cytoskeleton is a highly organized framework of protein filaments and tubules that criss-cross the cytoplasm of a cell. It gives the cell shape and helps to hold cell structures (such as organelles) in place.
  • The nucleus is the largest organelle in a eukaryotic cell. It is considered to be the cell’s control center, and it contains DNA and controls gene expression, including which proteins the cell makes.
  • The mitochondrion is an organelle that makes energy available to cells. According to the widely accepted endosymbiotic theory, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
  • The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum (RER) is studded with ribosomes. Smooth endoplasmic reticulum (SER) has no ribosomes.
  • The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
  • Vesicles and vacuoles are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are vesicles that break down foreign matter, dead cells, or poisons.
  • Centrioles are organelles located near the nucleus that help organize the chromosomes before cell division so each daughter cell receives the correct number of chromosomes.
  • There are two basic ways that substances can cross the cell’s plasma membrane: passive transport (which requires no energy expenditure by the cell) and active transport (which requires energy).
  • No energy is needed from the cell for passive transport because it occurs when substances move naturally from an area of higher concentration to an area of lower concentration. Types of passive transport in cells include:
    • Simple diffusion, which is the movement of a substance due to differences in concentration without any help from other molecules. This is how very small, hydrophobic molecules, such as oxygen and carbon dioxide, enter and leave the cell.
    • Osmosis, which is the diffusion of water molecules across the membrane.
    • Facilitated diffusion, which is the movement of a substance across a membrane due to differences in concentration, but only with the help of transport proteins in the membrane (such as channel proteins or carrier proteins). This is how large or hydrophilic molecules and charged ions enter and leave the cell.
  • Active transport requires energy to move substances across the plasma membrane, often because the substances are moving from an area of lower concentration to an area of higher concentration or because of their large size. Two examples of active transport are the sodium-potassium pump and vesicle transport.
    • The sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, both against a concentration gradient, in order to maintain the proper concentrations of both ions inside and outside the cell and to thereby control membrane potential.
    • Vesicle transport uses vesicles to move large molecules into or out of cells.
  • Energy is the ability to do work. It is needed by every living cell to carry out life processes.
  • The form of energy that living things need is chemical energy, and it comes from food. Food consists of organic molecules that store energy in their chemical bonds.
  • Autotrophs (producers) make their own food. Think of plants that make food by photosynthesis. Heterotrophs (consumers) obtain food by eating other organisms.
  • Organisms mainly use the molecules glucose and ATP for energy. Glucose is the compact, stable form of energy that is carried in the blood and taken up by cells. ATP contains less energy and is used to power cell processes.
  • The flow of energy through living things begins with photosynthesis, which creates glucose. The cells of organisms break down glucose and make ATP.
  • Cellular respiration is the aerobic process by which living cells break down glucose molecules, release energy, and form molecules of ATP. Overall, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and water.
    • Glycolysis, the first stage of cellular respiration, takes place in the cytoplasm. In this step, enzymes split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.
    • Transition Reaction takes place between glycolysis and Krebs Cycle. It is a very short reaction in which the pyruvate molecules from glycolysis are converted into Acetyl CoA in order to enter the Krebs Cycle.
    • Krebs Cycle, the second stage of cellular respiration, takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their chemical bonds being stored in a total of 16 energy-carrying molecules (including four from glycolysis).
    • The Electron Transport System, he third stage of cellular respiration, takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions across the membrane, creating an electrochemical gradient that drives the synthesis of many more molecules of ATP.
    • In all three stages of aerobic cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.
  • Some organisms can produce ATP from glucose by anaerobic respiration, which does not require oxygen. Fermentation is an important type of anaerobic process. There are two types: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis.
    • Alcoholic fermentation is carried out by single-celled organisms, including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
    • Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
    • Anaerobic respiration produces far less ATP (typically produces 2 ATP) than does aerobic cellular respiration, but it has the advantage of being much faster.
  • The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division.
  • In a eukaryotic cell, the cell cycle has two major phases: interphase and mitotic phase. During interphase, the cell grows, performs routine life processes, and prepares to divide. During mitotic phase, first the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis), which produces two daughter cells.
    • Until a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each chromosome consists of two sister chromatids, which are joined together at a centromere.
    • During mitosis, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases: prophase, metaphase, anaphase, and telophase.
  • The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle at key checkpoints.
  • Cancer is a disease that occurs when the cell cycle is no longer regulated, often because the cell’s DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.

In this chapter, you learned about cells and some of their functions, as well as how they pass genetic material in the form of DNA to their daughter cells. In the next chapter, you will learn how DNA is passed down to offspring, which causes traits to be inherited. These traits may be innocuous (such as eye colour) or detrimental (such as mutations that cause disease). The study of how genes are passed down to offspring is called genetics, and as you will learn in the next chapter, this is an interesting topic that is highly relevant to human health.

Chapter 4 Review

  1. Sequence:

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  2. Drag and Drop:

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  3. True or False:

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  4. Multiple Choice: 

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  5. Briefly explain how the energy in the food you eat gets there, and how it provides energy for your neurons in the form necessary to power this process.
  6. Explain why the inside of the plasma membrane — the side that faces the cytoplasm of the cell — must be hydrophilic.
  7. Explain the relationships between interphase, mitosis, and cytokinesis.

Attributions

Figure 4.14.1

Mitochondrial Disease muscle sample by Nephron is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.

Figure 4.14.2

Aunt and Niece by Tatiana Rodriguez on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Reference

Wikipedia contributors. (2020, June 6). Mitochondrial disease. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Mitochondrial_disease&oldid=961126371

V

Chapter 5 Genetics

41

5.1 Case Study: Genes and Inheritance

Created by: CK-12/Adapted by Christine Miller

Case Study: Cancer in the Family

Image shows a family tree with three generations. The tree shows cartoon faces for each person on the tree, not names. The images show a variety of diverse faces.
Figure 5.1.1 Family tree – three generations.

People tend to carry similar traits to their biological parents, as illustrated by the family tree. Beyond just appearance, you can also inherit traits from your parents that you can’t see.

Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle — who have all passed away — had cancer. They all had breast cancer, including her uncle, and her aunt also had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.

Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g., breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.

Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor, because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.

When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer, and how they are related to each other and to Rebecca.

He also asks her ethnicity. Rebecca says that her family on both sides are Ashkenazi Jews (Jews whose ancestors came from central and eastern Europe). “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2, located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About one in 40 Ashkenazi Jewish people have one of these mutations, compared to about one in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer, indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.

In this image, a woman looks thoughtfully out at the countryside.
Figure 5.1.2 Rebecca is not sure if she wants to know if she is at an increased risk of breast and ovarian cancer.

Rebecca says that her 72-year-old mother never had cancer, nor had many other relatives on that side of the family. How could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, while dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65 per cent of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12 per cent of women in the general population who will get breast cancer sometime over the course of their lives.

Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension, but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.

Chapter Overview: Genetics

At the end of this chapter, you will find out Rebecca’s test results. By then, you will have learned how traits are inherited from parents to offspring through genes, and how mutations in genes such as BRCA1 and BRCA2 can be passed down and cause disease. Specifically, you will learn about:

  • The structure of DNA.
  • How DNA replication occurs.
  • How DNA was found to be the inherited genetic material.
  • How genes and their different alleles are located on chromosomes.
  • The 23 pairs of human chromosomes, which include autosomal and sex chromosomes.
  • How genes code for proteins using codons made of the sequence of nitrogen bases within RNA and DNA.
  • The central dogma of molecular biology, which describes how DNA is transcribed into RNA, and then translated into proteins.
  • The structure, functions, and possible evolutionary history of RNA.
  • How proteins are synthesized through the transcription of RNA from DNA and the translation of protein from RNA, including how RNA and proteins can be modified, and the roles of the different types of RNA.
  • What mutations are, what causes them, different specific types of mutations, and the importance of mutations in evolution and to human health.
  • How the expression of genes into proteins is regulated and why problems in this process can cause diseases, such as cancer.
  • How Gregor Mendel discovered the laws of inheritance for certain types of traits.
  • The science of heredity, known as genetics, and the relationship between genes and traits.
  • How gametes, such as eggs and sperm, are produced through meiosis.
  • How sexual reproduction works on the cellular level and how it increases genetic variation.
  • Simple Mendelian and more complex non-Mendelian inheritance of some human traits.
  • Human genetic disorders, such as Down syndrome, hemophilia A, and disorders involving sex chromosomes.
  • How biotechnology — which is the use of technology to alter the genetic makeup of organisms — is used in medicine and agriculture, how it works, and some of the ethical issues it may raise.
  • The human genome, how it was sequenced, and how it is contributing to discoveries in science and medicine.

As you read this chapter, keep Rebecca’s situation in mind and think about the following questions:

  1. BCRA1 and BCRA2 are also called Breast cancer type 1 and 2 susceptibility proteins.  What do the BRCA1 and BRCA2 genes normally do? How can they cause cancer?
  2. Are BRCA1 and BRCA2 linked genes? Are they on autosomal or sex chromosomes?
  3. After learning more about pedigrees, draw the pedigree for cancer in Rebecca’s family. Use the pedigree to help you think about why it is possible that her mother does not have one of the BRCA gene mutations, even if her grandmother, aunt, and uncle did have it.
  4. Why do you think certain gene mutations are prevalent in certain ethnic groups?

Attributions

Figure 5.1.1

Family Tree [all individual face images] from Clker.com used and adapted by Christine Miller under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 5.1.2

Rebecca by Kyle Broad on Unsplash is used under the Unsplash License (https://unsplash.com/license).

References

Wikipedia contributors. (2020, June 27). Ashkenazi Jews. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ashkenazi_Jews&oldid=964691647

Wikipedia contributors. (2020, June 22). BRCA1. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA1&oldid=963868423

Wikipedia contributors. (2020, May 25). BRCA2. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=BRCA2&oldid=958722957

42

5.2 Chromosomes and Genes

Created by: CK-12/Adapted by Christine Miller

Identical Twins, Identical Genes

Figure 5.2.1 Identical twins share the same DNA since they came from a single zygote.

You probably can tell by their close resemblance that these two young ladies are identical twins (Figure 5.2.1). Identical twins develop from the same fertilized egg, so they inherit copies of the same chromosomes and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same genes as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are? Let’s find out!

Introducing Chromosomes and Genes

Figure 5.2.2 Human male karyotype. There are 23 pairs of chromosomes per cell. The chromosomes in a pair are known as [pb_glossary id="2104"]homologous chromosomes[/pb_glossary].

Chromosomes are coiled structures made of DNA and proteins. They are encoded with genetic instructions for making RNA and proteins. These instructions are organized into units called genes. There may be hundreds (or even thousands!) of genes on a single chromosome. Genes are segments of DNA that code for particular pieces of RNA. Once formed, some RNA molecules go on to act as blueprints for building proteins, while other RNA molecules help regulate various processes inside the cell. Some regions of DNA do not code for RNA and serve a regulatory function, or have no known function.

Human Chromosomes

Each species is characterized by a set number of chromosomes. Humans cells normally have two sets of chromosomes in each of their cells, one set inherited from each parent. Because chromosomes occur in pairs, these cells are called diploid or 2N. There are 23 chromosomes in each set, for a total of 46 chromosomes per diploid cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape, and they also contain the same genes. The chromosomes in a pair are known as homologous chromosomes.

All human cells (except gametes, which are sperm and egg cells) have the 23 pairs of chromosomes as shown in Figure 5.2.2.

 

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Secrets of the X chromosome – Robin Ball, TED-Ed, 2019.

Autosomes

Of the 23 pairs of human chromosomes, 22 pairs are called autosomes (pairs 1-22 in the Figure 5.2.2), or autosomal chromosomes. Autosomes are chromosomes that contain genes for characteristics that are unrelated to biological sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes.

Sex Chromosomes

Image shows a artists rendition of the comparative sizes of the X and Y chromosome. The X chromosome is much larger than the Y chromsosome.
Figure 5.2.3 The X and Y chromosomes, also known as the sex chromosomes, determine the biological sex of an individual.

The remaining pair of human chromosomes consists of the sex chromosomes, X and Y (Pair 23 in Figure 5.2.2 and in Figure 5.2.3). Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.

As you can see from Figure 5.2.3, the X chromosome is much larger than the Y chromosome. The X chromosome has about two thousand genes, whereas the Y chromosome has fewer than 100, none of which is essential to survival. Virtually all of the X chromosome genes are unrelated to sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of female as the default sex of the human species.

Human Genes

Humans have an estimated 20 thousand to 22 thousand genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Only about 25 per cent of the nitrogen base pairs of DNA in human chromosomes make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear, but with more time and research their roles may become understood.

The majority of human genes have two or more possible versions, called alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA base pairs within alleles.

Linkage

Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair colour and eye colour are linked, so certain hair and eye colours tend to be inherited together, such as dark hair with dark eyes and blonde hair with blue eyes. Can you think of other human traits that seem to occur together? Do you think they might be controlled by linked genes?

Genes located on the sex chromosomes are called sex-linked genes. Most sex-linked genes are on the X chromosome, because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are X-linked genes, but the term sex-linked is often used to refer to them. The diagram below is called a linkage map: a linkage map shows the locations of specific genes on a chromosome. The linkage map below (Figure 5.2.4) shows the locations of a few of the genes on the human X chromosome.

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Figure 5.2.4 Linkage Map for the Human X Chromosome. This linkage map shows the locations of several genes on the X chromosome. Some of the genes code for normal proteins. Others code for abnormal proteins that lead to genetic disorders.

5.2 Summary

  • Chromosomes are coiled structures made of DNA and proteins that are encoded with genetic instructions for making RNA and proteins. The instructions are organized into units called genes, which are segments of DNA that code for particular pieces of RNA. The RNA molecules can then act as a blueprint for proteins, or directly help regulate various cellular processes.
  • Each species is characterized by a set number of chromosomes. The normal chromosome complement of a human cell is 23 pairs of chromosomes. Of these, 22 pairs are autosomes, which contain genes for characteristics unrelated to sex. The other pair consists of sex chromosomes (XX in females, XY in males). Only the Y chromosome contains genes that determine sex.
  • Humans have an estimated 20 thousand to 22 thousand genes. The majority of human genes have two or more possible versions, which are called alleles.
  • Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together. A linkage map shows the locations of specific genes on a chromosome.

5.2 Review Questions

  1. What are chromosomes and genes? How are the two related?
  2. Describe human chromosomes and genes.
  3. Explain the difference between autosomes and sex chromosomes.
  4. What are linked genes, and what does a linkage map show?
  5. Explain why females are considered the default sex in humans.
  6. Explain the relationship between genes and alleles.
  7. Most males and females have two sex chromosomes. Why do only females have Barr bodies?
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5.2 Explore More

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WACE Biology: Coding and Non-Coding DNA, Atomi, 2019.

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How Sex Genes Are More Complicated Than You Thought, Seeker, 2015.

Attributions

Figure 5.2.1

Twins5 [photo] by Bùi Thanh Tâm on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.2.2

Human_male_karyotype by National Human Genome Research Institute/ NIH  on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original from the Talking Glossary of Genetics.)

Figure 5.2.3

Comparison between X and Y chromosomes byJonathan Bailey, National Human Genome Research Institute, National Institutes of Health [NIH] Image Gallery, on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.

Figure 5.2.4

Linkage Map of Human X Chromosome by Christine Miller is used under a
CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

 

References

Atomi. (2019, October 27). WACE Biology: Coding and Non-Coding DNA. YouTube. https://www.youtube.com/watch?v=M4ut72kfUJM&feature=youtu.be

Seeker. (2015, July 26). How Sex Genes Are More Complicated Than You Thought. YouTube. https://www.youtube.com/watch?v=jhHGCvMlrb0&feature=youtu.be

TED-Ed. (2017, April 18). Secrets of the X chromosome – Robin Ball. YouTube. https://www.youtube.com/watch?v=veB31XmUQm8&feature=youtu.be

43

5.3 DNA

Created by: CK-12/Adapted by Christine Miller

Figure 5.3.1 Woman with natural red hair.

What Makes You…You?

This young woman has naturally red hair (Figure 5.3.1). Why is her hair red instead of some other colour? In general, what gives her the specific traits she has? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.

Introducing DNA

Today, it is commonly known that DNA is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed — that is, they were aware that genetic information is contained within biochemical molecules. What they didn’t know was which specific molecules play this role. In fact, for many decades, scientists thought that proteins were the molecules that contain genetic information.

Discovery that DNA is the Genetic Material

Determining that DNA is the genetic material was an important milestone in biology. It took many scientists undertaking creative experiments over several decades to show with certainty that DNA is the molecule that determines the traits of organisms. This research began in the early part of the 20th century.

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Griffith’s Experiments with Mice

Diagram showing the results of Griffith's experiments with mice exposed to R-strain and S-strain viruses.
Figure 5.3.2 Griffith’s Experimental Results. Griffith showed that a substance could be transferred to harmless bacteria and make them deadly.

One of the first important discoveries was made in the 1920s by an American scientist named Frederick Griffith.  Griffith was studying mice and two different strains of a bacterium, called R (rough)-strain and S (smooth)-strain. He injected the two bacterial strains into mice. The S-strain was virulent and killed the mice, whereas the R-strain was not virulent and did not kill the mice. You can see these details in Figure 5.3.2. Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the dead bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died.

Based on his observations, Griffith deduced that something in the dead S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. What was this “something?” What type of substance could change the characteristics of the organism that received it?

Avery and His Colleagues Make a Major Contribution

In the early 1940s, a team of scientists led by Canadian-American Oswald Avery tried to answer the question raised by Griffith’s research results. First, they inactivated various substances in the S-strain bacteria. Then they killed the S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind that the R-strain bacteria normally did not harm the mice.) When they inactivated proteins, the R-strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R-strain was changed (or transformed) into the deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA — and not protein — is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.

Hershey and Chase Confirm the Results

The conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. Instead, they are basically DNA (or RNA) inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then, it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.

Chargaff Focuses on DNA Bases

Other important discoveries about DNA were made in the mid-1900s by Erwin Chargaff. He studied DNA from many different species and was especially interested in the four different nitrogen bases of DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). Chargaff found that concentrations of the four bases differed between species. Within any given species, however, the concentration of adenine was always the same as the concentration of thymine, and the concentration of guanine was always the same as the concentration of cytosine. These observations came to be known as Chargaff’s rules. The significance of the rules would not be revealed until the double-helix structure of DNA was discovered.

Discovery of the Double Helix

Image shows a diagram of DNA. It is in the form of an alpha helix, each double strand is 2 nanometers wide, and a full turn of the helix is 10 base pairs and measures approximately 3.4 nanometers.
Figure 5.3.3 Watson and Crick developed a model of DNA showing its helical shape.

After DNA was shown to be the genetic material, scientists wanted to learn more about its structure and function. James Watson and Francis Crick are usually given credit for discovering that DNA has a double helix shape, as shown in Figure 5.3.3. In fact, Watson and Crick’s discovery of the double helix depended heavily on the prior work of Rosalind Franklin and other scientists, who had used X-rays to learn more about DNA’s structure. Unfortunately, Franklin and these others have not always been given credit for their important contributions to the discovery of the double helix.

The DNA molecule has a double helix shape — the same basic shape as a spiral staircase. Do you see the resemblance? Which parts of the DNA molecule are like the steps of the spiral staircase?

The double helix shape of DNA, along with Chargaff’s rules, led to a better understanding of DNA. As a nucleic acid, DNA is made from nucleotide monomers. Long chains of nucleotides form polynucleotides, and the DNA double helix consists of two polynucleotide chains. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of the four bases (adenine, cytosine, guanine, or thymine). The sugar and phosphate molecules in adjacent nucleotides bond together and form the “backbone” of each polynucleotide chain.

Scientists concluded that bonds between the bases hold together the two polynucleotide chains of DNA. Moreover, adenine always bonds with thymine, and cytosine always bonds with guanine. That’s why these pairs of bases are called complementary base pairs.  Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If adenine were to bond with guanine, as well as thymine, for example, the distance between the two DNA chains would vary. When a one-ring molecule (like thymine) always bonds with a two-ring molecule (like adenine), however, the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix. The bonded base pairs (A-T and G-C) stick into the middle of the double helix, forming the “steps” of the spiral staircase.

 

 

5.3 Summary

  • Determining that DNA is the genetic material was an important milestone in biology. One of the first important discoveries was made in the 1920s, when Griffith showed that something in virulent bacteria could be transferred to nonvirulent bacteria, making them virulent, as well.
  • In the early 1940s, Avery and colleagues showed that the “something” Griffith found in his research was DNA and not protein. This result was confirmed by Hershey and Chase, who demonstrated that viruses insert DNA into bacterial cells so the cells will make copies of the viruses.
  • In the mid-1950s, Chargaff showed that, within the DNA of any given species, the concentration of adenine is always the same as the concentration of thymine, and that the concentration of guanine is always the same as the concentration of cytosine. These observations came to be known as Chargaff's rules.
  • Around the same time, James Watson and Francis Crick, building on the prior X-ray research of Rosalind Franklin and others, discovered the double-helix structure of the DNA molecule. Along with Chargaff’s rules, this led to a better understanding of DNA’s structure and function.
  • Knowledge of DNA’s structure helped scientists understand how DNA replicates, which must occur before cell division occurs so each daughter cell will have a complete set of chromosomes.

5.3 Review Questions

  1. Outline the discoveries that led to the determination that DNA (not protein) is the biochemical molecule that contains genetic information.
  2. State Chargaff’s rules. Explain how the rules are related to the structure of the DNA molecule.
  3. Explain how the structure of a DNA molecule is like a spiral staircase. Which parts of the staircase represent the various parts of the molecule?
  4. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=617

  5. Why do you think dead S-strain bacteria injected into mice did not harm the mice, but killed them when mixed with living (and normally harmless) R-strain bacteria?
  6. In Griffith’s experiment, do you think the heat treatment that killed the bacteria also inactivated the bacterial DNA? Why or why not?
  7. Give one example of the specific evidence that helped rule out proteins as genetic material.

5.3 Explore More

Thumbnail for the embedded element "The Discovery of the Structure of DNA"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=617

The Discovery of the Structure of DNA, OpenMind, 2017.

Thumbnail for the embedded element "Rosalind Franklin: Great Minds"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=617

Rosalind Franklin: Great Minds, SciShow, 2013.

 

Attributions

Figure 5.3.1

Redhead [photo] by Hichem Dahmani on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 5.3.2

Griffith’s mice by Mariana Ruiz Villarreal [LadyofHats] for CK-12 Foundation is used under a
CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 5.3.3

DNA_Overview by Michael Ströck [mstroeck] on Wikimedia Commons is used under a CC BY SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.

References

Brainard, J/ CK-12. (2012). Concentration. In Physical Science [website]. CK12.org. https://www.ck12.org/c/physical-science/concentration/?referrer=crossref

OpenMind. (2017, September 11). The discovery of the structure of DNA. YouTube. https://www.youtube.com/watch?v=V6bKn34nSbk&feature=youtu.be

SciShow. (2013, July 9). Rosalind Franklin: Great minds. YouTube. https://www.youtube.com/watch?v=JiME-W58KpU&feature=youtu.be

Wikipedia contributors. (2020, June 27). Alfred Hershey. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Alfred_Hershey&oldid=964789559

Wikipedia contributors. (2020, June 5). Erwin Chargaff. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Erwin_Chargaff&oldid=960942873

Wikipedia contributors. (2020, June 29). Francis Crick. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Francis_Crick&oldid=965135362

Wikipedia contributors. (2020, July 6). Frederick Griffith. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Frederick_Griffith&oldid=966352134

Wikipedia contributors. (2020, July 5). James Watson. In Wikipedia. https://en.wikipedia.org/w/index.php?title=James_Watson&oldid=966111944

Wikipedia contributors. (2020, March 31). Martha Chase. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Martha_Chase&oldid=948408219

Wikipedia contributors. (2020, July 2). Oswald Avery. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Oswald_Avery&oldid=965632585

Wikipedia contributors. (2020, June 30). Rosalind Franklin. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Rosalind_Franklin&oldid=965334881

 

44

5.4 DNA Replication

Image shows a diagram of DNA replication taking place. A single strand of DNA is partly unwound and new sections of complementary DNA are being added on each of the separated strands.
Figure 5.4.1 DNA replication takes place before a cell starts the process of cell division.

By Christine Miller

DNA Replication: Overview

DNA replication is required for the growth or replication of an organism.  You started as one single cell and are now made up of approximately 37 trillion cells!  Each and every one of these cells contains the exact same copy of DNA, which originated from the first cell that was you.  How did you get from one set of DNA, to 37 million sets, one for each cell?  Through DNA replication.

Knowledge of DNA’s structure helped scientists understand  DNA replication, the process by which DNA is copied. It occurs during the synthesis (S) phase of the eukaryotic cell cycle. DNA must be copied so that each new daughter cell will have a complete set of chromosomes after cell division occurs.

DNA replication is referred to as “semi-conservative”.  What this means is when a strand of DNA is replicated, each of the two original strands acts as a template for a new complementary strand.  When the replication process is complete, there are two identical sets of DNA, each containing one of the original strands of DNA, and one newly synthesized strand.

DNA replication involves a certain sequence of events.  For each event, there is a specific enzyme which facilitates the process.  There are four main enzymes that facilitate DNA replication: helicase, primase, DNA polymerase, and ligase.

DNA Replication: The Process

DNA replication begins when an enzyme called helicase unwinds, and unzips the DNA molecule.  If you recall the structure of DNA, you may remember that it consists of two long strands of nucleotides held together by hydrogen bonds between complementary nitrogenous bases. This forms a ladder-like structure which is in a coiled shape.  In order to start DNA replication, helicase needs to unwind the molecule and break apart the hydrogen bonds holding together complementary nitrogenous bases.  This causes the two strands of DNA to separate.

Small molecules called single-stranded binding proteins (SSB) attach to the loose strands of DNA to keep them from re-forming the hydrogen bonds that helicase just broke apart.

Image shows a diagram of helicase unwinding and unzipping a double stranded section of DNA. Single stranded binding proteins bind to the newly separated strands to prevent them from re-forming the hydrogen bonds.
Figure 5.4.2 Helicase unwinds and unzips the DNA molecule. SSB keep the two strands from re-attaching to one another.

Once the nitrogenous bases from the inside of the DNA molecule are exposed, the creation of a new, complementary strand can begin.  DNA polymerase creates the new strand, but it needs some help in finding the correct place to begin, so primase lays down a short section of RNA primer (shown in green in Figure 5.4.3).  Once this short section of primer is laid, DNA polymerase can bind to the DNA molecule and start connecting nucleotides in the correct order to match the sequence of nitrogenous bases on the template (original) strand.

Image shows a diagram of DNA replication. Helicase is separating the two strands of DNA, single stranded binding proteins are holding open the strand of DNA. Primase is laying down primer sequences to cue DNA polymerase where to begin synthesizing the new strand of DNA
Figure 5.4.3 DNA Replication. DNA replication is a semi-conservative process. Half of the parent DNA molecule is conserved in each of the two daughter DNA molecules.
Image shows a diagram of DNA in which the two strands run antiparallel to one another. This means that the nucleotides in the left-hand strand are oriented with the phosphate group in the "up" position, but in the right-hand strand the phosphate group is oriented in the "down" position.
Figure 5.4.4 The two strands of nucleotides that make up DNA run antiparallel to one another. Note in the left-hand strand the phosphate group is in the “up” position, and in the right-hand strand, the phosphate group is in the “down” position.

If we think about the DNA molecule, we may remember that the two strands of DNA run antiparallel to one another.  This means that in the sugar-phosphate backbone, one strand of the DNA has the sugar oriented in the “up” position, and the other strand has the phosphate oriented in the “up” position (see Figure 5.4.4).  DNA polymerase is an enzyme which can only work in one direction on the DNA molecule.  This means that one strand of DNA can be replicated in one long string, as DNA polymerase follows helicase as it unzips the DNA molecule.  This strand is termed the “leading strand”.  The other strand, however, can only be replicated in small chunks since the DNA polymerase replicates in the opposite direction that helicase is unzipping.  This strand is termed the “lagging strand”.  These small chunks of replicated DNA on the lagging strand are called Okazaki fragments.

Take a look at Figure 5.4.5 and find the Okazaki fragments, the leading strand and the lagging strand.

Image shows a diagram of DNADNA polymerase can only synthesize new DNA in one direction on the template strand. This results in one set of DNA being replicated in one long strand (the leading strand) and one replicated in small chunks called Okazaki fragments (the lagging strand).
Figure 5.4.5 DNA polymerase can only synthesize new DNA in one direction on the template strand. This results in one set of DNA being replicated in one long strand (the leading strand) and one replicated in small chunks called Okazaki fragments (the lagging strand).

Once DNA polymerase has replicated the DNA, a third enzyme called ligase completes the final stage of DNA replication, which is repairing the sugar-phosphate backbone.  This connects the gaps in the backbone between Okazaki fragments.  Once this is complete, the DNA coils back into its classic double helix structure.

Semi-Conservative Replication

When DNA replication is complete, there are two identical sets of double stranded DNA, each with one strand from the original, template, DNA molecule, and one strand that was newly synthesized during the DNA replication process.  Because each new set of DNA contains one old and one new strand, we describe DNA as being semi-conservative.

 

Watch this video for a great overview of DNA replication:

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A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=2141

DNA Replication (Updated), Amoeba Sisters, 2019.

 

5.4 Summary

  • DNA replication requires the action of three main enzymes each with their own specific role:
    • Helicase unzips and unwinds the DNA molecule.
    • DNA polymerase creates a new complementary strand of DNA on each of the originals halves that were separated by helicase.  New nucleotides are added through complementary base pairing: A pairs with T, and C with G.
    • Ligase repairs gaps in the sugar-phosphate backbone between Okazaki fragments.
  • DNA replication is semi-conservative because each daughter molecule contains one strand from the parent molecule and one new complementary strand.

5.4 Review Questions

  1. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=2141

2. Why are Okazaki fragments formed?

  1. Because helicase only unzips DNA in one direction.
  2. Because DNA is in a double helix.
  3. Because DNA polymerase only replicates DNA in one direction.
  4. Because DNA replication is semi-conservative.

3. Drag and drop to label the diagram.

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=2141

 

5.4 Explore More

Thumbnail for the embedded element "DNA replication - 3D"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=2141

DNA replication – 3D, yourgenome, 2015.

Attributions

Figure 5.4.1

DNA_replication_split.svg by Madprime on Wikimedia Commons is used under a CC0 1.0
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).

Figure 5.4.2

Helicase and single stranded binding proteins (1) by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 5.4.3

DNA polymerase and primase by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 5.4.4

DNA strands run antiparallel by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 5.4.5

Leading and lagging strand/ DNA Replication/  by yourgenome on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.

References

Amoeba Sisters. (2019, June 28). DNA replication (Updated). YouTube. https://www.youtube.com/watch?v=Qqe4thU-os8&feature=youtu.be

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.24 DNA Replication [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-3-the-nucleus-and-dna-replication CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

yourgenome. (2015, June 26). DNA replication – 3D. YouTube. https://www.youtube.com/watch?v=TNKWgcFPHqw&feature=youtu.be

45

5.5 RNA

Created by: CK-12/Adapted by Christine Miller

 

Image shows a diagram of a basic overview of protein
Figure 5.5.1 Diagram of a basic overview of protein.

A Deceptively Simple Model

This simple model sums up one of the most important ideas in biology, which is called the central dogma of molecular biology (you’ll read more about it below). You probably recognize the spiral-shaped structure in the nucleus. It represents a molecule of DNA, the biochemical molecule that stores genetic information in most living cells. The yellow chain represents a newly formed polypeptide — the beginning stage of creating a protein. Proteins are the class of biochemical molecules that carry out virtually all life processes. What is the structure in the center of the model? It appears to resemble DNA, but it is smaller and simpler. This molecule is the key to the central dogma, and it may have been the first type of biochemical molecule to evolve.

Central Dogma of Molecular Biology

DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain in the nucleus, but proteins are made at ribosomes in the cytoplasm. How do the instructions in DNA get to the site of protein synthesis outside the nucleus?

Another type of nucleic acid is responsible. This nucleic acid is RNA, or ribonucleic acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in the nucleus to a ribosome in the cytoplasm and then helps assemble the protein. In short:

DNA RNA  Protein

This expresses in words what the diagram in Figure 5.5.1 shows. The genetic instructions encoded in DNA in the nucleus are transcribed to RNA. Then, RNA carries the instructions to a ribosome in the cytoplasm, where they are translated into a protein. Discovering this sequence of events was a major milestone in molecular biology. It’s called the central dogma of molecular biology.

Introducing RNA

A strand of RNA
Figure 5.5.2 RNA is a single strand of nucleotides, each containing the sugar ribose, a phosphate group, and one of four bases, A, C, G, or U.

DNA alone cannot “tell” your cells how to make proteins. It needs the help of RNA, the other main player in the central dogma of molecular biology. Like DNA, RNA is a nucleic acid, so it consists of repeating nucleotides bonded together to form a polynucleotide chain. RNA differs from DNA in several ways: it exists as a single stranded molecule, contains the sugar ribose (as opposed to deoxyribose) and uses the base uracil instead of thymine.

 

Functions of RNA

The main function of RNA is to help make proteins. There are three main types of RNA involved in protein synthesis:

  1. Image shows a diagram of the three types of RNA: Messenger RNA, which is a single strand of RNA, Ribosomal RNA, which is an RNA-protein complex with two subunits, and transfer RNA, which is a single strand of RNA enfolded on itself with an anticodon region and a region which can carry a single amino acid.
    Figure 5.5.3 The three types of RNA take very different forms.

    Messenger RNA (mRNA) copies (or transcribes) the genetic instructions from DNA in the nucleus and carries them to the cytoplasm.

  2. Ribosomal RNA (rRNA) helps form ribosomes, where proteins are assembled. Ribosomes also contain proteins.
  3. Transfer RNA (tRNA) brings amino acids to ribosomes, where rRNA catalyzes the formation of chemical bonds between them to form a protein.

In section 5.7 Protein Synthesis, you can read in detail about how these three types of RNA build primary structure of proteins.

RNA is a very versatile molecule which plays multiple roles in living things. In addition to helping to make proteins, for example, there are RNA molecules that regulate the expression of genes, and RNA molecules that catalyze other biochemical reactions needed to sustain life. Because of the diversity of roles that RNA molecules play, they have been called the Swiss Army knives of the cellular world.

It’s an RNA World

The function of DNA is to store genetic information inside cells. It does this job well, but that’s about all it can do. DNA can’t act as an enzyme, for example, to catalyze biochemical reactions that are needed to keep us alive. Proteins are needed for this and many other life functions. Proteins work exceptionally well to keep us alive, but they are unable to store genetic information. Proteins need DNA for that. Without DNA, proteins could not exist. On the other hand, without proteins, DNA could not survive. This poses a chicken-and-egg sort of problem: Which evolved first? DNA or proteins?

Some scientists think that the answer is neither. They speculate instead that RNA was the first biochemical to evolve. The reason? RNA can do more than one job. It can store information as DNA does, but it can also perform various jobs (such as catalysis) to keep cells alive, as proteins do. The idea that RNA was the first biochemical to evolve, predating both DNA and proteins, is called the RNA world hypothesis. According to this hypothesis, billions of years ago, RNA molecules evolved that could both survive and make copies of themselves. According to the hypothesis, early RNA molecules eventually evolved the ability to make proteins, and at some point RNA mutated to form DNA.

Feature: Reliable Sources

The RNA world hypothesis has not gained enough support in the scientific community to be accepted as a scientific theory. In fact, there are probably as many detractors as supporters of the hypothesis. Do a web search to learn more about the RNA world hypothesis and the evidence and arguments for and against it. When weighing the information you gather, consider the likely reliability of the different websites you visit. Based on what you determine are the most reliable sources and the most convincing arguments, form your own opinion about the hypothesis. You may decide to accept or reject the hypothesis. Alternatively, you may decide to reserve judgement until — or if — more evidence or arguments are forthcoming.

5.5 Summary

  • The central dogma of molecular biology can be summed up as: DNA → RNA → Protein. This means that the genetic instructions encoded in DNA are first transcribed to RNA, and then from RNA they are translated into a protein.
  • Like DNA, RNA is a nucleic acid. Unlike DNA, RNA consists of just one polynucleotide chain instead of two, contains the base uracil instead of thymine, and contains the sugar ribose instead of deoxyribose.
  • The main function of RNA is helping to make proteins. There are three main types of RNA involved in protein synthesis: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). RNA has additional functions, including regulating gene expression and catalyzing other biochemical reactions.
  • According to the RNA world hypothesis, RNA was the first type of biochemical molecule to evolve, predating both DNA and proteins. The hypothesis is based mainly on the multiple functions of RNA, which can store genetic information like DNA and carry out life processes (like proteins).

5.5 Review Questions

  1. State the central dogma of molecular biology.
  2. Drag and drop to compare the structure and function of DNA and RNA:

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=619

3.

An interactive or media element has been excluded from this version of the text. You can view it online here:
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4.

An interactive or media element has been excluded from this version of the text. You can view it online here:
http://humanbiology.pressbooks.tru.ca/?p=619

5.5 Explore More

Thumbnail for the embedded element "The RNA Origin of Life"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=619

The RNA Origin of Life, NOVA PBS Official, 2014.

Thumbnail for the embedded element "DNA vs RNA (Updated)"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=619

DNA vs RNA (Updated), Amoeba Sisters, 2019.

 

Attributions

Figure 5.5.1

From DNA to Protein: Transcription through Translation by OpenStax College on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 5.5.2

Molbio-Header by Squidonius  on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.5.2

ARNm-Rasmol by Corentin Le Reun on Wikimedia Commons is is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).ublic domain.

References

Amoeba Sisters. (2019, August 29). DNA vs RNA (Updated). YouTube. https://www.youtube.com/watch?v=JQByjprj_mA&feature=youtu.be

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.29 From DNA to Protein: Transcription through Translation [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-4-protein-synthesis#fig-ch03_04_05

NOVA PBS Official. (2014, April 23). The RNA origin of life. YouTube. https://www.youtube.com/watch?v=VYQQD0KNOis&feature=youtu.be

Wikipedia contributors. (2020, June 28). RNA world. In Wikipedia. https://en.wikipedia.org/w/index.php?title=RNA_world&oldid=964998696

 

46

5.6 Genetic Code

Created by: CK-12/Adapted by Christine Miller

Figure 5.6.1 DNA stores information in the sequence of nitrogenous bases. This works similarly to computer coding.

Can You Code?

If someone asks you whether you can code, you probably assume they are referring to computer code. The image in Figure 5.6.1 represents an important code that you use all the time — but not with a computer! It’s the genetic code, and it is used by your cells to store information, as well as to make RNA and proteins.

What Is the Genetic Code?

The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The bases are adenine (A), cytosine (C), guanine (G), and thymine (T) (or uracil, U, in RNA). The four bases make up the “letters” of the genetic code. The letters are combined in groups of three to form code “words,” called codons. Each codon stands for (encodes) one amino acid, unless it codes for a start or stop signal. There are 20 common amino acids in proteins. With four bases forming three-base codons, there are 64 possible codons. This is more than enough to code for the 20 amino acids. The genetic code is shown in Figure 5.6.2.

Figure 5.6.2 The Genetic Code (decoder).

To find the amino acid for a particular codon, find the first base in the codon in the centre of the circle in Figure 5.6.2, then the second base in the middle row out from the center, and finally the third base in the outer ring  For example, CUG codes for leucine, AAG codes for lysine, and GGG codes for glycine. Try it out: Can you figure out what the codon AGC codes for?

Reading the Genetic Code

If you find the codon AUG in Figure 5.6.2, you will see that it codes for the amino acid methionine. This codon is also the start codon that establishes the reading frame of the code.  The start codon is a necessary tool in translation, since a single chromosome contains many genes.  In order to transcribe and translate a gene for a specific protein, we need to know where in the DNA code to start “reading” the instructions.  AUG signals the start of a reading frame.  After the AUG start codon, the next three bases are read as the second codon. The next three bases after that are read as the third codon, and so on. The sequence of bases is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons. They do not code for any amino acids.

The importance of the reading frame is illustrated in the hypothetical situation  below:

The section of mRNA in Figure 5.6.3 is designed to create a chain of five specific amino acids.

Illustrates the importance of the reading frame and the start codon in decoding mRNA
Figure 5.6.3 This segment of mRNA could be read a few ways, but only one of them produces the desired protein. The start codon, AUG, indicates where to begin the reading frame.

Characteristics of the Genetic Code

The genetic code has a number of important characteristics:

Cracking the Code

The double helix structure of DNA was discovered in 1953. It took just eight more years to crack the genetic code. The scientist primarily responsible for deciphering the code was American biochemist Marshall Nirenberg, who worked at the National Institutes of Health in the United States. When Nirenberg began the research in 1959, the manner in which proteins are synthesized in cells was not well understood, and messenger RNA had not yet been discovered. At that time, scientists didn’t even know whether DNA or RNA was the molecule used as a template for protein synthesis. Nirenberg, along with a collaborator named Heinrich Matthaei, devised an ingenious experiment to determine which molecule — DNA or RNA — has this important role. They also began deciphering the genetic code.

Nirenberg and Matthaei added the contents of bacterial cells to each of 20 test tubes. The cell contents provided the necessary “machinery” for the synthesis of a polypeptide molecule. The researchers also added all 20 amino acids to the test tubes, with a different amino acid “tagged” by a radioactive element in each test tube. That way, if a polypeptide formed in a test tube, they would be able to tell which amino acid it contained. Then, they added synthetic RNA containing just one nitrogen base to all 20 test tubes. They used the base uracil in their first experiment. They discovered that an RNA chain consisting only of uracil bases produces a polypeptide chain of the amino acid phenylalanine. This experiment showed that RNA (rather than DNA) is the template for protein synthesis, but it also showed that a sequence of uracil bases codes for the amino acid phenylalanine. The year was 1961, and it was a momentous occasion. When Nirenberg presented the discovery at a scientific conference later that year, he received a standing ovation. As Nirenberg puts it, “…for the next five years I became like a scientific rock star.”

After Nirenberg and Matthaei cracked the first word of the genetic code, they used similar experiments to show that each codon consists of three bases. Before long, they had discovered the codons for all 20 amino acids. In 1968, in recognition of this important achievement, Nirenberg was named a co-winner of the Nobel Prize in Physiology or Medicine.

5.6 Summary

  • The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The four bases make up the “letters” of the code. The letters are combined in groups of three to form code “words” known as codons, each of which encodes for one amino acid or a start or stop signal.
  • AUG is the start codon, and it establishes the reading frame of the code. After the start codon, the next three bases are read as the second codon, the three bases after that as the third codon, and so on until a stop codon is reached.
  • The genetic code is universal, unambiguous, and redundant.
  • The genetic code was cracked in the 1960s, mainly by a series of ingenious experiments carried out by Marshall Nirenberg, who won a Nobel Prize for this achievement.

5.6 Review Questions

  1. Describe the genetic code and explain how it is “read”
  2. Identify three important characteristics of the genetic code.
  3. Summarize how the genetic code was deciphered.
  1. Use the decoder above to answer the following questions:
    • Is the code from DNA or RNA? How do you know?
    • Which amino acid does the codon CAA code for?
    • What does UGA code for?
    • Look at the codons that code for the amino acid glycine. How many of them are there and how are they similar and different?
  2. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=621

5.6 Explore More

Thumbnail for the embedded element "Comparing DNA Sequences"

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Comparing DNA Sequences, Bozeman Science, 2012.

Thumbnail for the embedded element "How to Read a Codon Chart"

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How to Read a Codon Chart, Amoeba Sisters, 2019.

Attributions

Figure 5.6.1

AMY1gene by unknown author from National Science Foundation on Wikimedia Commons  is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.6.2

Aminoacids table (Adapted) by Mouagip on Wikimedia Commons  is released into the public domain. (Original: Codons sun (“codesonne” in German) by Onie~commonswiki])

Figure 5.6.3

Reading Frame (3 Options) by Christine Miller is used under a CC BY-NC-SA 4.0  (https://creativecommons.org/licenses/by-nc-sa/4.0/) license.

References

Amoeba Sisters. (2019, September 17). How to read a codon chart. YouTube. https://www.youtube.com/watch?v=LsEYgwuP6ko&feature=youtu.be

Bozeman Science. (2012, September 15). Comparing DNA sequences. YouTube. https://www.youtube.com/watch?v=OSKwuOccAak&feature=youtu.be

Wikipedia contributors. (2020, July 2). Marshall Warren Nirenberg.  In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Marshall_Warren_Nirenberg&oldid=965562106

47

5.7 Protein Synthesis

Created by: CK-12/Adapted by Christine Miller

Figure 5.7.1 How proteins are made.

The Art of Protein Synthesis

This amazing artwork (Figure 5.7.1) shows a process that takes place in the cells of all living things: the production of proteins. This process is called protein synthesis, and it actually consists of two processes — transcription and translation. In eukaryotic cells, transcription takes place in the nucleus. During transcription, DNA is used as a template to make a molecule of messenger RNA (mRNA). The molecule of mRNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. During translation, the genetic code in mRNA is read and used to make a polypeptide. These two processes are summed up by the central dogma of molecular biology: DNA  RNA  Protein.

Transcription

Transcription is the first part of the central dogma of molecular biology: DNA  RNA. It is the transfer of genetic instructions in DNA to mRNA. During transcription, a strand of mRNA is made to complement a strand of DNA. You can see how this happens in Figure 5.7.2.

Figure 5.7.2 Transcription uses the sequence of bases in a strand of DNA to make a complementary strand of mRNA. Triplets are groups of three successive nucleotide bases in DNA. Codons are complementary groups of bases in mRNA.

Transcription begins when the enzyme RNA polymerase binds to a region of a gene called the promoter sequence. This signals the DNA to unwind so the enzyme can “read” the bases of DNA.  The two strands of DNA are named based on whether they will be used as a template for RNA or not.  The strand that is used as a template is called the template strand, or can also be called the antisense strand.  The sequence of bases on the opposite strand of DNA is called the non-coding or sense strand.  Once the DNA has opened, and RNA polymerase has attached, the RNA polymerase moves along the DNA, adding RNA nucleotides to the growing mRNA strand.  The template strand of DNA is used as to create mRNA through complementary base pairing. Once the mRNA strand is complete, and it detaches from DNA. The result is  a strand of mRNA that is nearly identical to the coding strand DNA – the only difference being that DNA uses the base thymine, and the mRNA uses uracil in the place of thymine

Processing mRNA

In eukaryotes, the new mRNA is not yet ready for translation. At this stage, it is called pre-mRNA, and it must go through more processing before it leaves the nucleus as mature mRNA. The processing may include splicing, editing, and polyadenylation. These processes modify the mRNA in various ways. Such modifications allow a single gene to be used to make more than one protein.

mRNA requires processing before it leaves the nucleus
Figure 5.7.3 Pre mRNA processing. mRNA requires processing before it leaves the nucleus.

Translation

Translation is the second part of the central dogma of molecular biology: RNA Protein. It is the process in which the genetic code in mRNA is read to make a protein. Translation is illustrated in Figure 5.7.4. After mRNA leaves the nucleus, it moves to a ribosome, which consists of rRNA and proteins. The ribosome reads the sequence of codons in mRNA, and molecules of tRNA bring amino acids to the ribosome in the correct sequence.

Translation occurs in three stages: Initiation, Elongation and Termination.

Initiation:

After transcription in the nucleus, the mRNA exits through a nuclear pore and enters the cytoplasm.  At the region on the mRNA containing the methylated cap and the start codon, the small and large subunits of the ribosome  bind to the mRNA.  These are then joined by a tRNA which contains the anticodons matching the start codon on the mRNA.  This group of molecues (mRNA, ribosome, tRNA) is called an initiation complex.

Elongation:

tRNA keep bringing amino acids to the growing polypeptide according to complementary base pairing between the codons on the mRNA and the anticodons on the tRNA.  As a tRNA moves into the ribosome, its amino acid is transferred to the growing polypeptide.  Once this transfer is complete, the tRNA leaves the ribosome, the ribosome moves one codon length down the mRNA, and a new tRNA enters with its corresponding amino acid.  This process repeats and the polypeptide grows.

Termination:

At the end of the mRNA coding is a stop codon which will end the elongation stage.  The stop codon doesn’t call for a tRNA, but instead for a type of protein called a release factor, which will cause the entire complex (mRNA, ribosome, tRNA, and polypeptide) to break apart, releasing all of the components.

 

 

Figure 5.7.4 Translation takes place in three stages: Initiation, Elongation and Termination.

Watch this video “Protein Synthesis (Updated) with the Amoeba Sisters” to see this process in action:

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Protein Synthesis (Updated), Amoeba Sisters, 2018.

What Happens Next?

After a polypeptide chain is synthesized, it may undergo additional processes. For example, it may assume a folded shape due to interactions between its amino acids. It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates. Many proteins travel to the Golgi apparatus within the cytoplasm to be modified for the specific job they will do.7 Summary

5.7 Summary

  • Protein synthesis is the process in which cells make proteins. It occurs in two stages: transcription and translation.
  • Transcription is the transfer of genetic instructions in DNA to mRNA in the nucleus. It includes three steps: initiation, elongation, and termination. After the mRNA is processed, it carries the instructions to a ribosome in the cytoplasm.
  • Translation occurs at the ribosome, which consists of rRNA and proteins. In translation, the instructions in mRNA are read, and tRNA brings the correct sequence of amino acids to the ribosome. Then, rRNA helps bonds form between the amino acids, producing a polypeptide chain.
  • After a polypeptide chain is synthesized, it may undergo additional processing to form the finished protein.

5.7 Review Questions

  1. Relate protein synthesis and its two major phases to the central dogma of molecular biology.
  2. Explain how mRNA is processed before it leaves the nucleus.
  3. What additional processes might a polypeptide chain undergo after it is synthesized?
  4. Where does transcription take place in eukaryotes?
  5. Where does translation take place?
  6. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=623

5.7 Explore More

Thumbnail for the embedded element "Protein Synthesis"

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Protein Synthesis, Teacher’s Pet, 2014.

 

Attributions

Figure 5.7.1

How proteins are made by Nicolle Rager, National Science Foundation on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.7.2

Transcription by National Human Genome Research Institute, (reworked and vectorized by Sulai) on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 5.7.3

Pre mRNA processing by Christine Miller is used under a CC BY-NC-SA 4.0  (https://creativecommons.org/licenses/by-nc-sa/4.0/) license.

Figure 5.7.4

Translation by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

References

Amoeba Sisters. (2018, January 18) Protein synthesis (Updated). YouTube. https://www.youtube.com/watch?v=oefAI2x2CQM&feature=youtu.be

Parker, N., Schneegurt, M., Thi Tu, A-H., Lister, P., Forster, B.M. (2016, November 1). Microbiology [online]. Figure 11.15 Translation in bacteria begins with the formation of the initiation complex. In Microbiology (Section 11-4). OpenStax. https://openstax.org/books/microbiology/pages/11-4-protein-synthesis-translation

Teacher’s Pet. (2014, December 7). Protein synthesis. YouTube. https://www.youtube.com/watch?v=2zAGAmTkZNY&feature=youtu.be

48

5.8 Mutations

Created by: CK-12/Adapted by Christine Miller

Figure 5.8.1 Teenage Mutant Ninja Turtles Cosplay: Raphael and Michelangelo.

Mutant Cosplay

You probably recognize these costumed comic fans in Figure 5.8.1 as two of the four Teenage Mutant Ninja Turtles. Can a mutation really turn a reptile into an anthropomorphic superhero? Of course not — but mutations can often result in other drastic (but more realistic) changes in living things.

What Are Mutations?

Mutations are random changes in the sequence of bases in DNA or RNA. The word mutation may make you think of the Ninja Turtles, but that’s a misrepresentation of how most mutations work. First of all, everyone has mutations. In fact, most people have dozens (or even hundreds!) of mutations in their DNA. Secondly, from an evolutionary perspective, mutations are essential. They are needed for evolution to occur because they are the ultimate source of all new genetic variation in any species.

Causes of Mutations

Mutations have many possible causes. Some mutations seem to happen spontaneously, without any outside influence. They occur when errors are made during DNA replication or during the transcription phase of protein synthesis. Other mutations are caused by environmental factors. Anything in the environment that can cause a mutation is known as a mutagen. Examples of mutagens are shown in the figure below.

Examples of Radiation, chemicals and infectious agents: An mage of a sun icon and hand x-ray for UV and x-ray radiation; a picture of hands holding a cigarrette and a vape, 3 smokies on a grill (nitrates/ nitrites and mutagenic BBQ chemicals) and a stylized image of a woman in a green acne face mask with benzoyl peroxide to represent chemicals. To represent infectious agents: an orange spherical virus as human papillomavirus (HPV) and a purple spirilla bacterium with flagella for Helicobacter Pylori - a bacteria spread through contaminated food.
Figure 5.8.2 Examples of Mutagens. Types of mutagens include radiation, chemicals, and infectious agents. Do you know of other examples of each type of mutagen shown here?

 

Types of Mutations

Mutations come in a variety of types. Two major categories of mutations are germline mutations and somatic mutations.

Mutations also differ in the way that the genetic material is changed. Mutations may change an entire chromosome, or they may alter just one or a few nucleotides.

Chromosomal Alterations

Chromosomal alterations are mutations that change chromosome structure. They occur when a section of a chromosome breaks off and rejoins incorrectly, or otherwise does not rejoin at all. Possible ways in which these mutations can occur are illustrated in the figure below. Chromosomal alterations are very serious. They often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. An example of a human disease caused by a chromosomal duplication is Charcot-Marie-Tooth disease type 1 (CMT1). It is characterized by muscle weakness, as well as loss of muscle tissue and sensation. The most common cause of CMT1 is a duplication of part of chromosome 17.

 

Figure 5.8.3 Chromosomal alterations are major changes in the genetic material.

Point Mutations

point mutation is a change in a single nucleotide in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be silent, missense, or nonsense mutations, as described in Table 5.8.1. The effects of point mutations depend on how they change the genetic code.

Table 5.8.1: The Effects of Point Mutations
Type Description Example Effect
Silent mutated codon codes for the same amino acid CAA (glutamine) → CAG (glutamine) none
Missense mutated codon codes for a different amino acid CAA (glutamine) → CCA (proline) variable
Nonsense mutated codon is a premature stop codon CAA (glutamine) → UAA (stop) usually serious

Frameshift Mutations

frameshift mutation is a deletion or insertion of one or more nucleotides, changing the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:

AUG-AAU-ACG-GCU = start-asparagine-threonine-alanine

Now, assume that an insertion occurs in this sequence. Let’s say an A nucleotide is inserted after the start codon AUG. The sequence of bases becomes:

AUG-AAA-UAC-GGC-U = start-lysine-tyrosine-glycine

Even though the rest of the sequence is unchanged, this insertion changes the reading frame and, therefore, all of the codons that follow it. As this example shows, a frameshift mutation can dramatically change how the codons in mRNA are read. This can have a drastic effect on the protein product.

Effects of Mutations

The majority of mutations have neither negative nor positive effects on the organism in which they occur. These mutations are called neutral mutations. Examples include silent point mutations, which are neutral because they do not change the amino acids in the proteins they encode.

Many other mutations have no effects on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix mutations in DNA.

Beneficial Mutations

Some mutations — known as beneficial mutations — have a positive effect on the organism in which they occur. They generally code for new versions of proteins that help organisms adapt to their environment. If they increase an organism’s chances of surviving or reproducing, the mutations are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are two such examples:

  1. Mutations have occurred in bacteria that allow the bacteria to survive in the presence of antibiotic drugs, leading to the evolution of antibiotic-resistant strains of bacteria.
  2. A unique mutation is found in people in Limone,  a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels despite a high-fat diet. The individual in which this mutation first appeared has even been identified and many of his descendants carry this gene.

Harmful Mutations

Imagine making a random change in a complicated machine, such as a car engine. There is a chance that the random change would result in a car that does not run well — or perhaps does not run at all. By the same token, a random change in a gene’s DNA may result in the production of a protein that does not function normally… or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer.

Feature: My Human Body

Inherited mutations are thought to play a role in roughly five to ten per cent of all cancers. Specific mutations that cause many of the known hereditary cancers have been identified. Most of the mutations occur in genes that control the growth of cells or the repair of damaged DNA.

Genetic testing can be done to determine whether individuals have inherited specific cancer-causing mutations. Some of the most common inherited cancers for which genetic testing is available include hereditary breast and ovarian cancer, caused by mutations in genes called BRCA1 and BRCA2. Besides breast and ovarian cancers, mutations in these genes may also cause pancreatic and prostate cancers. Genetic testing is generally done on a small sample of body fluid or tissue, such as blood, saliva, or skin cells. The sample is analyzed by a lab that specializes in genetic testing, and it usually takes at least a few weeks to get the test results.

Should you get genetic testing to find out whether you have inherited a cancer-causing mutation? Such testing is not done routinely just to screen patients for risk of cancer. Instead, the tests are generally done only when the following three criteria are met:

  1. The test can determine definitively whether a specific gene mutation is present. This is the case with the BRCA1 and BRCA2 gene mutations, for example.
  2. The test results would be useful to help guide future medical care. For example, if you found out you had a mutation in the BRCA1 or BRCA2 gene, you might get more frequent breast and ovarian cancer screenings than are generally recommended.
  3. You have a personal or family history that suggests you are at risk of an inherited cancer.

Criterion number 3 is based, in turn, on such factors as:

If you meet the criteria for genetic testing and are advised to undergo it, genetic counseling is highly recommended. A genetic counselor can help you understand what the results mean and how to make use of them to reduce your risk of developing cancer. For example, a positive test result that shows the presence of a mutation may not necessarily mean that you will develop cancer. It may depend on whether the gene is located on an autosome or sex chromosome, and whether the mutation is dominant or recessive. Lifestyle factors may also play a role in cancer risk even for hereditary cancers. Early detection can often be life saving if cancer does develop. Genetic counseling can also help you assess the chances that any children you may have will inherit the mutation.

5.8 Summary

  • Mutations are random changes in the sequence of bases in DNA or RNA. Most people have multiple mutations in their DNA without ill effects. Mutations are the ultimate source of all new genetic variation in any species.
  • Mutations may happen spontaneously during DNA replication or transcription. Other mutations are caused by environmental factors called mutagens. Mutagens include radiation, certain chemicals, and some infectious agents.
  • Germline mutations occur in gametes and may be passed onto offspring. Every cell in the offspring will then have the mutation. Somatic mutations occur in cells other than gametes and are confined to just one cell and its daughter cells. These mutations cannot be passed on to offspring.
  • Chromosomal alterations are mutations that change chromosome structure and usually affect the organism in multiple ways. Charcot-Marie-Tooth disease type 1 is an example of a chromosomal alteration in humans.
  • Point mutations are changes in a single nucleotide. The effects of point mutations depend on how they change the genetic code and may range from no effects to very serious effects.
  • Frameshift mutations change the reading frame of the genetic code and are likely to have a drastic effect on the encoded protein.
  • Many mutations are neutral and have no effect on the organism in which they occur. Some mutations are beneficial and improve fitness. An example is a mutation that confers antibiotic resistance in bacteria. Other mutations are harmful and decrease fitness, such as the mutations that cause genetic disorders or cancers.

5.8 Review Question

  1. Define mutation.
  2. Identify causes of mutation.
  3. Compare and contrast germline and somatic mutations.
  4. Describe chromosomal alterations, point mutations, and frameshift mutations. Identify the potential effects of each type of mutation.
  5. Why do many mutations have neutral effects?
  6. Give one example of a beneficial mutation and one example of a harmful mutation.
  7. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=625

  8. Why do you think that exposure to mutagens (such as cigarette smoke) can cause cancer?
  9. Explain why the insertion or deletion of a single nucleotide can cause a frameshift mutation.
  10. Compare and contrast missense and nonsense mutations.
  11. Explain why mutations are important to evolution.

 

5.8 Explore More

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How Radiation Changes Your DNA, Seeker, 2016.

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Where do genes come from? – Carl Zimmer, TED-Ed, 2014.

Thumbnail for the embedded element "What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin"

A YouTube element has been excluded from this version of the text. You can view it online here: http://humanbiology.pressbooks.tru.ca/?p=625

What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin,
TED, 2019.

Attributions

Figure 5.8.1

Ninja Turtles by Pat Loika on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 5.8.2

Examples of Mutagens by Christine MIller is used under a CC BY SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/) license.
Separate images are all in public domain or CC licensed:

Figure 5.8.3

Scheme of possible chromosome mutations/ Chromosomenmutationen by unknown on Wikimedia Commons is adapted from NIH‘s Talking Glossary of Genetics. [Changes as described by de:user:Dietzel65]. Further use and adapation (text translated to English) by Christine Miller as image is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

 

References

Seeker. (2016, April 23). How radiation changes your DNA. YouTube. https://www.youtube.com/watch?v=PQjL4ZDuq2o&feature=youtu.be

TED. (2019, June 5). What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin. YouTube. https://www.youtube.com/watch?v=a63t8r70QN0&feature=youtu.be

TED-Ed. (2014, September 22). Where do genes come from? – Carl Zimmer. YouTube. https://www.youtube.com/watch?v=z9HIYjRRaDE&feature=youtu.be

Wikipedia contributors. (2020, July 6). Breast cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Breast_cancer&oldid=966366739

Wikipedia contributors. (2020, July 9). Charcot–Marie–Tooth disease. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Charcot%E2%80%93Marie%E2%80%93Tooth_disease&oldid=966912915

Wikipedia contributors. (2020, July 7). Cystic fibrosis. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Cystic_fibrosis&oldid=966566921

Wikipedia contributors. (2020, June 4). Limone sul Garda. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Limone_sul_Garda&oldid=960771991

Wikipedia contributors. (2020, June 23). Ovarian cancer. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ovarian_cancer&oldid=964157192

Wikipedia contributors. (2020, May 7). BRCA mutation. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA_mutation&oldid=955463902

Wikipedia contributors. (2020, July 10). Teenage Mutant Ninja Turtles. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Teenage_Mutant_Ninja_Turtles&oldid=967030468

 

49

5.9 Regulation of Gene Expression

Created by: CK-12/Adapted by Christine Miller

Shows differentiation pathways a stem cell can take, based on gene regulation: Sex cell, muscle cell, fat cell, bone cell, blood cell, nervous cell, epithelial cell or immune cell. .
Figure 5.9.1 Differentiation pathways for a stem cell based on gene regulation.

Express Yourself

This sketch illustrates some of the variability in human cells. The shape and other characteristics that make each type of cell unique depend mainly on the specific proteins that particular cell type makes. Proteins are encoded in genes. All the cells in an organism have the same genes, so they all have genetic instructions for the same proteins. Obviously, different types of cells must use (or express) different genes to make different proteins.

What Is Gene Expression?

Using a gene to make a protein is called gene expression. It includes the synthesis of the protein by the processes of transcription of DNA into mRNA,  and translation of mRNA into a protein. It may also include further processing of the protein after synthesis.

Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of the transcription phase of protein synthesis to the processing of a protein after synthesis occurs. The regulation of transcription is one of the most complicated parts of gene regulation in eukaryotic cells, and it is the focus of this concept.

Regulation of Transcription

Figure 5.9.2 Regulation of Transcription. Regulatory proteins bind to their corresponding regulatory elements in order to control transcription.

As shown in Figure 5.9.2, transcription is controlled by regulatory proteins. These proteins bind to regions of DNA, called regulatory elements, which are located near promoters. The promoter is the region of a gene where RNA polymerase binds to initiate transcription of the DNA to mRNA. After regulatory proteins bind to regulatory elements, the proteins can interact with RNA polymerase. Regulatory proteins are typically either activators or repressors. Activators are regulatory proteins that promote transcription by enhancing the interaction of RNA polymerase with the promoter. Repressors are regulatory proteins that prevent transcription by impeding the progress of RNA polymerase along the DNA strand, so the DNA cannot be transcribed to mRNA.

 

Enhancers

Although regulatory proteins and elements are typically the key players in the regulation of transcription, other factors may also be involved. Regulation of transcription may also involve enhancers. Enhancers are distant regions of DNA that can loop back to interact with a gene’s promoter. They can also increase the likelihood that transcription of the gene will occur.Enhancers

The TATA Box

Different types of cells have unique patterns of regulatory elements that result in only the necessary genes being transcribed. That’s why a blood cell and nerve cell, for example, are so different from each other. Some regulatory elements, however, are common to virtually all genes, regardless of the cells in which they occur. An example is the TATA box, which is a regulatory element that is part of the promoter of almost every eukaryotic gene. A number of regulatory proteins bind to the TATA box, forming a multi-protein complex. It is only when all of the appropriate proteins are bound to the TATA box that RNA polymerase recognizes the complex and binds to the promoter so transcription can begin.

Components of DNA regulating transcription: upstream enhancer, promoter sequences, TATA box: TATAWAW, Exons and Introns.
Figure 5.9.3 Components of DNA Regulating Transcription. W in the TATA box sequence can be either A or T.

Regulation During Development

The regulation of gene expression is extremely important in an organism’s early development. Regulatory proteins must “turn on” certain genes in particular cells at just the right time, so the individual develops normal organs and organ systems. Homeobox genes are important genes that regulate development.

Homeobox genes are a large group of similar genes that direct the formation of many body structures during the embryonic stage. In humans, there are an estimated 235 functional homeobox genes. They are present on every chromosome and generally grouped in clusters. Homeobox genes contain instructions for making chains of 60 amino acids, called homeodomains. Proteins containing homeodomains are transcription factors that bind to and control the activities of other genes. The homeodomain is the part of the protein that binds to the target gene and controls its expression.

Gene Expression and Cancer

This flow chart shows how a series of mutations in tumor-suppressor genes and proto-oncogenes leads to cancer.
Figure 5.9.4 This flow chart shows how a series of mutations in tumor-suppressor genes and proto-oncogenes leads to cancer.

Some types of cancer occur because of mutations in the genes that control the cell cycle. Cancer-causing mutations most often occur in two types of regulatory genes: proto-oncogenes and tumor-suppressor genes. Both are shown in Figure 5.9.4.

 

 

5.9 Summary

  • Using a gene to make a protein is called gene expression. Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any stage of protein synthesis or processing.
  • The regulation of transcription is controlled by regulatory proteins that bind to regions of DNA called regulatory elements, which are usually located near promoters. Most regulatory proteins are either activators that promote transcription, or repressors that impede transcription.
  • A regulatory element common to almost all eukaryotic genes is the TATA box. A number of regulatory proteins must bind to the TATA box in the promoter before transcription can proceed.
  • Regulation of gene expression is extremely important during an organism’s early development. Homeobox genes — which encode for chains of amino acids called homeodomains — are important genes that regulate development.
  • Some types of cancer occur because of mutations in the genes that control the cell cycle. Cancer-causing mutations most often occur in two types of regulatory genes: tumor-suppressor genes and proto-oncogenes.

5.9 Review Questions

  1. Define gene expression.
  2. Why must gene expression be regulated?
  3. Explain how regulatory proteins may activate or repress transcription.
  4. An interactive or media element has been excluded from this version of the text. You can view it online here:
    http://humanbiology.pressbooks.tru.ca/?p=627

  5. What is the TATA box, and how does it work?
  6. Describe homeobox genes and their role in an organism’s development.
  7. Discuss the role of regulatory gene mutations in cancer.
  8. Explain the relationship between proto-oncogenes and oncogenes.
  9. If a newly fertilized egg contained a mutation in a homeobox gene, how do you think this would affect the developing embryo? Explain your answer.
  10. Compare and contrast enhancers and activators.

 

5.9 Explore More

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Regulated Transcription, ndsuvirtualcell, 2008.

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How do cancer cells behave differently from healthy ones? – George Zaidan,
TED-Ed, 2012.

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What is leukemia? – Danilo Allegra and Dania Puggioni, 2015.

Attributions

Figure 5.9.1

Stem_cell_differentiation.svg by Haileyfournier on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 5.9.2

Activators and Repressors by Christine Miller is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 5.9.3

TATA_box_description by Luttysar on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 5.9.4

Pathways to cancer by CK-12 Foundation is used under a CC BY-NC 3.0 (http://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

 

References

Brainard, J/ CK-12 Foundation. (2012). Figure 3 Flow chart (series of mutations leading to cancer) [digital image]. In CK-12 College Human Biology (Section 5.8) [online Flexbook]. CK12.org. https://www.ck12.org/c/physical-science/concentration/?referrer=crossref

ndsuvirtualcell.(2008). Regulated transcription. YouTube. https://www.youtube.com/watch?v=vi-zWoobt_Q&feature=youtu.be

TED-Ed. (2012, December 5). How do cancer cells behave differently from healthy ones? – George Zaidan. YouTube. https://www.youtube.com/watch?v=BmFEoCFDi-w&feature=youtu.be

TED-Ed. (2015, April 15). What is leukemia? – Danilo Allegra and Dania Puggioni. YouTube. https://www.youtube.com/watch?v=Z3B-AaqjyjE&feature=youtu.be

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5.10 Mendel's Experiments and Laws of Inheritance

Created by: CK-12/Adapted by Christine Miller

Of Peas and People

5.10.1
Figure 5.10.1 Mendel conducted his research in genetics using pea plants.

These purple-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. They’re common garden peas, and they were studied in the mid-1800s by an Austrian monk named Gregor Mendel. Through careful experimentation, Mendel uncovered the secrets of heredity, or how parents pass characteristics to their offspring. You may not care much about heredity in pea plants, but you probably care about your own heredity. Mendel’s discoveries apply to people, as well as to peas — and to all other living things that reproduce sexually. In this concept, you will read about Mendel’s experiments and the secrets of heredity that he discovered.

Mendel and His Pea Plants

Image shows a photograph of Gregor Mendel
Figure 5.10.2 Gregor Mendel. The Austrian monk Gregor Mendel experimented with pea plants. He did all of his research in the garden of the monastery where he lived.

Gregor Mendel (Figure 5.10.2) was born in 1822. He grew up on his parents’ farm in Austria. He did well in school and became a friar (and later an abbot) at St. Thomas’ Abbey. Through sponsorship from the monastery, he went on to the University of Vienna, where he studied science and math. His professors encouraged him to learn science through experimentation, and to use math to make sense of his results. Mendel is best known for his experiments with pea plants (like the purple flower pictured in Figure 5.10.1).

 

 

 

Blending Theory of Inheritance

Figure 5.10.3 Gregor carried out much of his research at St. Thomas’ Abbey.

During Mendel’s time, the blending theory of inheritance was popular. According to this theory, offspring have a blend (or mix) of their parents’ characteristics. Mendel, however, noticed plants in his own garden that weren’t a blend of the parents. For example, a tall plant and a short plant had offspring that were either tall or short — not medium in height. Observations such as these led Mendel to question the blending theory. He wondered if there was a different underlying principle that could explain how characteristics are inherited. He decided to experiment with pea plants to find out. In fact, Mendel experimented with almost 30 thousand pea plants over the next several years!

Why Study Pea Plants?

Why did Mendel choose common, garden-variety pea plants for his experiments? Pea plants are a