CHAPTER 6
Cellular Respiration: Obtaining Energy from Food
Why This Chapter Matters
1.Photosynthesis powers chemical processes that make organic molecules. We can trace the energy in all of our food to the sun.
2.The mechanisms of cellular respiration are fundamental to the lives of most organisms on Earth and reveal their basic evolutionary relationships.
3.Fermentation is used by our cells in times of temporary oxygen shortage and by humans to produce foods and beverages.
Chapter Objectives
Biology and Society: Marathoners versus Sprinters
1.Explain how differences in muscle fibers relate to athletic performance.
Energy Flow and Chemical Cycling in the Biosphere
2.Explain why photosynthesis is important to most ecosystems.
3.Define and compare autotrophs and heterotrophs, producers and consumers.
4.Explain how the processes of photosynthesis and cellular respiration are complementary to each other.
Cellular Respiration: Aerobic Harvest of Food Energy
5.Compare the processes of cellular respiration and breathing.
6.Write and explain the overall equation for cellular respiration.
7.Explain how the processes of oxidation and reduction are used to transfer electrons from food molecules to NADH, the electron transport chain, and oxygen.
8.Compare the reactants, products, location in the cell, and energy yield of the three stages of cellular respiration (glycolysis, citric acid cycle, and electron transport chain).
Fermentation: Anaerobic Harvest of Food Energy
9.Compare the reactants, products, and energy yield of aerobic metabolism versus fermentation in human cells. Indicate where this process occurs in each type of cell.
Evolution Connection: Life before and after Oxygen
10.Describe the lines of evidence that suggest that glycolysis is a primitive form of metabolism.
Key Terms
aerobic
anaerobic
ATP synthase
autotroph
cellular respiration
citric acid cycle
consumer
electron transport
electron transport chain
fermentation
glycolysis
heterotroph
NADH
oxidation
photosynthesis
producer
redox reaction
reduction
Word Roots
aero = air (aerobic: chemical reaction using oxygen)
auto = self; troph = food (autotroph: organism that makes its own organic matter)
glyco = sweet; lysis = split (glycolysis: process that splits glucose into two molecules)
hetero = other (heterotroph: Greek word that means “other feeder”)
photo = light (photosynthesis: process using light energy to make organic molecules)
Student Media
Activities
Build a Chemical Cycling System
Overview of Cellular Respiration
Glycolysis
The Citric Acid Cycle
Electron Transport
Fermentation
BioFlix
Cellular Respiration
Biology Labs On-Line
Mitochondria Lab
BLAST Animations
Harvesting Energy: Krebs Cycle
LabBench
Cell Respiration
MP3 Tutors
Cellular Respiration Part 1—Glycolysis
Cellular Respiration Part 2—Citric Acid Cycle and Electron Transport
Process of Science
How Is the Rate of Cellular Respiration Measured?
Videos
Discovery Channel Video: Space Plants
Discovery Channel Video: Tasty Bacteria
Relevant Songs to Play in Class
“Breathe,” Pink Floyd
“Amber (is the color of your energy),” 311
“Stayin’ Alive,” Bee Gees
“The Energy,” Audiovent
“The Energy Blues” (Schoolhouse Rock! Rocks), Biz Markie
Chapter Guide to Teaching Resources
Energy Flow and Chemical Cycling in the Biosphere
Student Misconceptions and Concerns
1.Students should be cautioned against making statements that “energy is created” when it is converted from one form to another. This might be a good time to review the principle of conservation of energy (the first law of thermodynamics).
2.Students frequently think that plants have chloroplasts instead of mitochondria. Care should be taken to point out the need for mitochondria in plants when photosynthesis is not efficient or possible (such as during the night).
Teaching Tips
1.You might wish to elaborate on the amount of solar energy striking the earth. Every day the earth is bombarded with solar radiation equal to the energy of 100 million atomic bombs. Of the tiny fraction of light that reaches photosynthetic organisms, only about 1% is converted to chemical energy by photosynthesis.
2.You might share with your students that it takes about 10 million ATP molecules per second to power one active muscle cell.
3.Energy coupling at the cellular level may be new to many students, but it is a familiar concept when related to the use of money in our society. Students might be discouraged if the only benefit of work was the ability to make purchases from the employer. (We all might soon tire of a fast-food job that pays its employees only in food!) Money permits the coupling of a generation of value (a paycheck, analogous to an energy-releasing reaction) to an energy-consuming reaction (money, which enables us to make purchases in distant locations). This idea of “earn and spend” is a common concept we all know well.
Cellular Respiration: Aerobic Harvest of Food Energy
Student Misconceptions and Concerns
1.Perhaps more than anywhere else in general biology, students studying aerobic metabolism fail to see “the forest for the trees.” Students often focus on the details of each stage of aerobic metabolism and devote little attention to the overall process and products. Consider emphasizing the products, locations, and energy yields associated with glycolysis, the citric acid cycle, and electron transport before detailing the specifics of each reaction.
2.Students often fail to realize that aerobic metabolism is a process generally similar to the burning of wood in a fireplace or campfire. Pointing out the general similarities can help students comprehend the overall reaction and heat generation associated with both processes.
3.The advantage of the gradual degradation of glucose may not be obvious to some students. Many analogies exist that reveal the advantages of short and steady steps. Fuel in an automobile is burned slowly to best utilize the energy released from the fuel. A few fireplace logs release gradual heat to keep a room temperature steady. In both situations, excessive use of fuel becomes wasteful, reducing the efficiencies of the systems.
Teaching Tips
1.During cellular respiration, our cells convert about 40% of our food energy to useful work. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37°C (98–99°F). This is about the same amount of heat generated by a 100-watt incandescent lightbulb. If you choose to include a discussion of heat generated by aerobic metabolism, consider the following:
a.Ask your students why they feel warm when it is 30°C (86°F) outside if their core body temperature is 37°C (98.6°F). Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that help us get rid of the extra heat from cellular respiration.
b.Share this calculation with your students. Depending on the size and activity of a person, a human might burn 2,000 dietary Calories (kilocalories) a day. This is enough energy to raise the temperature of 20 L of liquid water from 0 to 100°C. This is something to think about the next time you heat water on the stove! (Note: Consider bringing a 2-L bottle as a visual aid, or ten 2-L bottles to make the point above; 100 Calories raises 1 L of water 100°C. Note: It takes much more energy to melt ice or evaporate water as steam.)
2.The location within a cell of each of the following reactions is often lost in the details of the processes; yet the locations are important. The “Evolution Connection” section at the end of this chapter discusses the significance of glycolysis occurring in the cytosol. Consider pointing to a diagram of a cell, with mitochondrial detail, as you lecture on cellular respiration to emphasize the location of each stage.
3.As you relate the structure of the inner mitochondrial membrane to its functions, challenge the students to suggest an adaptive advantage of the many folds of this inner membrane. These folds greatly increase the membrane region available for the associated reactions.
4.The production of NADH by glycolysis and the citric acid cycle, instead of just the direct production of ATP, can get confusing for students. Help students understand that NADH molecules have energy “value,” to be “cashed in” by the electron transport chain. The NADH can therefore be thought of as casino chips, accumulated along the way to be cashed in at the “electron transport” cashier.
5.The authors developed an analogy between the function of the inner mitochondrial membrane and a dam. A reservoir of hydrogen ions is built up between the two mitochondrial membranes, like a dam holding back water. As the hydrogen ions move down their concentration gradient, they “spin” the ATP synthase, which helps generate ATP. In a dam, water rushing downhill turns giant turbines, which generate electricity.
6.Students should be reminded that the ATP yield per glucose molecule of up to 38 ATP is only a potential. The complex chemistry of aerobic metabolism can only yield this amount under ideal conditions, when every substrate and enzyme is immediately available. Such circumstances may only rarely occur in a working cell.
Fermentation: Anaerobic Harvest of Food Energy
Student Misconceptions and Concerns
1.Some students might expect that fermentation produces alcohol and maybe even carbon dioxide. Care should be taken to clarify the different possible products of fermentation in muscle cells and alcoholic fermentation used in the food and beverage industry.
2.The text notes that some microbes are useful in the dairy industry because they produce lactic acid. However, the impact of acids on milk may not be obvious to many students. Consider a simple demonstration mixing about equal portions of milk (skim or 2%) with some acid (vinegar will work). Notice the accumulation of strands of milk curd (protein) on the side of the container and stirring device.
Teaching Tips
1.The carbon dioxide released from fermentation also makes beer and champagne bubbly.
2.Dry wines are produced when the yeast cells use up all or most of the sugar available. Sweet wines result when the alcohol accumulates enough to inhibit fermentation before the sugar is depleted.
3.Exposing fermenting yeast to oxygen will slow or stop the process, because the yeast will switch back to aerobic respiration. When fermentation is rapid, the carbon dioxide produced drives away the immediate oxygen above the wine. However, as fermentation slows down, the wine must be sealed to prevent oxygen exposure and permit the fermentation process to finish.
Answers to End-of-Chapter Questions
The Process of Science
12.Suggested answer: NADH and FADH2 are needed to pick up the electrons and hydrogen atoms from a glucose molecule. Ten NAD+ molecules are needed to break down one glucose molecule. However, NADH and FADH2 are recycled between electron transport, glycolysis, and the citric acid cycle. Therefore, we need a small additional supply to replace those that are lost or damaged.
Biology and Society
13.Some issues and questions to consider: Perhaps microbes such as yeast had accidentally been sealed in an anaerobic environment. This environment would be conducive for alcoholic fermentation to occur. Alcoholic beverages were probably easier to store for longer periods without worry of bacterial contamination due to the high alcohol content.
14.Some issues and questions to consider: Is your customer aware of the danger? Do you have an obligation to protect the customer, even against her wishes? Does your employer have the right to dismiss you for informing the customer or for refusing to serve the customer? Could you or the restaurant later be held liable for injury to the fetus? Is the mother responsible for willfully disregarding warnings about drinking?
Additional Critical Thinking Questions
The Process of Science
1.For pyruvic acid to enter the citric acid cycle, it must first be oxidized to acetyl CoA. The acetyl CoA then joins with a molecule of oxaloacetic acid to form citric acid in the citric acid cycle. After one turn of the citric acid cycle, the citric acid regenerates the oxaloacetic acid molecule consumed. Many of the intermediates of the citric acid cycle are used for other purposes. Succinyl CoA, for example, is used in the production of heme, the iron-containing molecule in hemoglobin and in the cytochromes of electron transport. This use of citric acid cycle intermediates reduces the amount of oxaloacetic acid available to join with acetyl CoA. If acetyl CoA cannot enter the citric acid cycle, it is converted to ketone bodies that are eliminated in the urine. When oxaloacetic acid is low, an enzyme converts pyruvic acid to oxaloacetic acid. Dr. Atkins’ diet is a popular weight-loss program. The principle is for dieters to consume large quantities of fats and protein, but to avoid carbohydrates. How does this diet cause weight loss? Hint: Consider the steps of cellular respiration and think about what other molecules would be missing if there was no glucose to begin the process.
Suggested answer: Fats and some amino acids are converted directly to acetyl CoA. There is no way for the acetyl CoA to be converted directly to pyruvic acid. If the diet is low in carbohydrates, little pyruvic acid is made by glycolysis, and the oxaloacetic acid cannot be replaced. The result is that the acetyl CoA produced by the fats and protein cannot be oxidized. Instead, the acetyl CoA is converted to ketone bodies, and the Calories are excreted from the body. This diet can be dangerous since many of the ketone bodies are acids that can lower the pH of the blood. Uncontrolled diabetics cannot make pyruvic acid because without the hormone insulin, glucose cannot enter the cells. They are subject to diabetic comas caused by ketone bodies. One of the ketone bodies is acetone. Acetone is very volatile and has a distinctly sweet smell. Some dieters complain of breath odors when they are on this diet.
2.An average adult human requires 2,200 kilocalories (kcal) of energy per day. Suppose your diet provides an average of 2,300 kcal per day. How many hours per week would you have to walk to burn off the extra 100 Calories consumed?
Suggested answer: 100 kcal per day is 700 kcal per week. Based on Figure 5.3 in your textbook, walking 3 miles per hour would use 245 Calories per hour. This would require 700 Calories / 245 Calories per hour = 2.86 hours of walking.
3.Although your body can convert excess carbohydrates in the diet to fats, it cannot convert a carbohydrate into protein. What is missing in the carbohydrate that is necessary to convert it to a protein?
Suggested answer: The amino acids that make up proteins have amino groups containing nitrogen atoms. Amino groups must be added to fats or carbohydrates to make amino acids for making proteins.
Biology and Society
4.Obesity is a serious epidemic in the United States. According to the Centers for Disease Control and Prevention (CDC), obesity in the U.S. population increased from about 12% in 1991 to about 34% in 2006. The highest increase occurred in 18- to 29-year-olds. Although part of the reason is our increasingly sedentary lifestyle, the principal cause is our overconsumption of abundant food products, many of which are high in fat content. Fat tastes good to most people, and the temptation to eat too much is ever present. Why do you think this is so? Consider the fact that our appetites and digestive systems evolved over time under very different circumstances. Are there advantages to our tendency to eat too much that would have been selected for in the course of human evolution? How can we, as a society, deal with the problem?
Some issues and questions to consider: Take an anonymous survey of the class using a website ( that calculates a person’s body mass index (BMI). A BMI over 30 is considered obese; a BMI between 25 and 30 is defined as overweight. What percentage of the class is obese? Does this fit the CDC data? Consider the conditions experienced by our prehistoric ancestors. Was food plentiful or scarce for hunter-gatherers? If a group came upon an infrequent source of food, would there be an advantage to eating heartily? Fatty foods have the largest concentration of Calories. Would this give a selective advantage to those individuals who found fat flavorful? Women tend to have a higher fat content than men. Is this related to pregnancy? Consider the fact that women with a very low body-fat content, for example, those who suffer from anorexia, stop menstruating. Why?
5.We are regularly bombarded with advertisements for supplements that will help us lose that ugly fat the fast and easy way with no diet or exercise required. Your roommate insists on purchasing every new supplement that hits the market in the hopes of shedding a few pounds. As a scientifically educated person, you know that the Food and Drug Administration does not regulate these supplement companies and that their claims are typically not backed by good scientific evidence. Explain to your roommate some of the risks associated with taking these supplements (based on what you know about cellular respiration and metabolism) and what will work best to lose weight.