CHAPTER
6
How Cells Harvest Chemical Energy
Chapter Objectives
Opening Essay
Describe the main reactants and products of cellular respiration.
Cellular Respiration: Aerobic Harvesting of Energy
6.1 Compare the processes and locations of cellular respiration and photosynthesis. Explain why it is accurate to say that life on Earth is solar-powered.
6.2 Explain how breathing and cellular respiration are related.
6.3 Provide the overall chemical equation for cellular respiration. Compare the efficiency of this process in cells to the efficiency of a gasoline automobile engine.
6.4 Explain how the human body uses its daily supply of ATP.
6.5 Explain how the energy in a glucose molecule is released during cellular respiration.
6.5 Explain how redox reactions are used in cellular respiration.
6.5 Describe the general roles of dehydrogenase, NADH, and the electron transport chain in cellular respiration.
Stages of Cellular Respiration
6.6 List the cellular regions where glycolysis, the citric acid cycle, and oxidative phosphorylation occur. Note whether substrate-level phosphorylation or chemiosmosis occur at each of these sites.
6.7–6.12 Compare the reactants, products, and energy yield of the three stages of cellular respiration.
6.11 Explain how rotenone, cyanide, carbon monoxide, oligomycin, and uncouplers interrupt critical events in cellular respiration.
Fermentation: Anaerobic Harvesting of Energy
6.13 Compare the reactants, products, and energy yield of alcohol and lactic acid fermentation. Distinguish between strict anaerobes and facultative anaerobes. 6.14 Describe the evolutionary history of glycolysis.
Connections Between Metabolic Pathways
6.15 Explain how carbohydrates, fats, and proteins are used as fuel for cellular respiration. Explain why a gram of fat yields more ATP than a gram of starch or protein.
Lecture Outline
I. Introduction
A.In eukaryotes, cellular respiration 1. harvests energy from food, 2. yields large amounts of ATP, and 3. uses ATP to drive cellular work.
B. A similar process takes place in many prokaryotic organisms.
II. Cellular Respiration: Aerobic Harvesting of Energy
A.6.1 Photosynthesis and cellular respiration provide energy for life 1. Life requires energy.
2. In almost all ecosystems, energy ultimately comes from the sun. 3. In photosynthesis,
a. some of the energy in sunlight is captured by chloroplasts, b.atoms of carbon dioxide and water are rearranged, and c. glucose and oxygen are produced.
4. In cellular respiration
a. glucose is broken down to carbon dioxide and water, and b.the cell captures some of the released energy to make ATP.
5. Cellular respiration takes place in the mitochondria of eukaryotic cells. B. 6.2 Breathing supplies O2 for use in cellular respiration and removes CO2
1. Respiration, as it relates to breathing, and cellular respiration are not the same. a. Respiration, in the breathing sense, refers to an exchange of gases. Usually an
organism brings in oxygen from the environment and releases waste CO2. b.Cellular respiration is the aerobic (oxygen requiring) harvesting of energy from
food molecules by cells.
C.6.3 Cellular respiration banks energy in ATP molecules
1. Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to form ATP.
2. Cellular respiration
a. produces up to 32 ATP molecules from each glucose molecule and b.captures only about 34% of the energy originally stored in glucose. 3. Other foods (organic molecules) can also be used as a source of energy. D.6.4 CONNECTION: The human body uses energy from ATP for all its activities
1. The average adult human needs about 2,200 kcal of energy per day. a. About 75% of these calories are used to maintain a healthy body. b.The remaining 25% is used to power physical activities.
2. A kilocalorie (kcal) is
a. the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by 1oC,
b.the same as a food Calorie, and
c. used to measure the nutritional values indicated on food labels. E. 6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen
1. The energy necessary for life is contained in the arrangement of electrons in chemical bonds in organic molecules.
3. When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen.
a. Oxygen has a strong tendency to attract electrons.
b.An electron loses potential energy when it “falls” to oxygen. 4. Energy can be released from glucose by simply burning it.
5. The energy is dissipated as heat and light and is not available to living organisms. 6. On the other hand, cellular respiration is the controlled breakdown of organic
molecules. 7. Energy is
a. gradually released in small amounts, b.captured by a biological system, and c. stored in ATP.
8. The movement of electrons from one molecule to another is an oxidation-reduction reaction, or redox reaction. In a redox reaction,
a. the loss of electrons from one substance is called oxidation, b.the addition of electrons to another substance is called reduction, c. a molecule is oxidized when it loses one or more electrons, and d.reduced when it gains one or more electrons.
9. A cellular respiration equation is helpful to show the changes in hydrogen atom distribution.
10. Glucose
a. loses its hydrogen atoms and b.becomes oxidized to CO2. 11. Oxygen
a. gains hydrogen atoms and b.becomes reduced to H2O.
12. Enzymes are necessary to oxidize glucose and other foods. 13. NAD+
a. is an important enzyme in oxidizing glucose, b.accepts electrons, and
c. becomes reduced to NADH.
14. There are other electron “carrier” molecules that function like NAD+.
a. They form a staircase where the electrons pass from one to the next down the staircase.
b.These electron carriers collectively are called the electron transport chain. c. As electrons are transported down the chain, ATP is generated.
III. Stages of Cellular Respiration
A.6.6 Overview: Cellular respiration occurs in three main stages
1. Cellular respiration consists of a sequence of steps that can be divided into three stages.
a. Stage 1 – Glycolysis
b.Stage 2 – Pyruvate oxidation and citric acid cycle c. State 3 – Oxidative phosphorylation
c. breaks down glucose into two molecules of a three-carbon compound called pyruvate.
3. Stage 2: The citric acid cycle a. takes place in mitochondria,
b.oxidizes pyruvate to a two-carbon compound, and c. supplies the third stage with electrons.
4. Stage 3: Oxidative phosphorylation
a. involves electrons carried by NADH and FADH2,
b.shuttles these electrons to the electron transport chain embedded in the inner mitochondrial membrane,
c. involves chemiosmosis, and
d.generates ATP through oxidative phosphorylation associated with chemiosmosis. B. 6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
1. In glycolysis,
a. a single molecule of glucose is enzymatically cut in half through a series of steps, b.two molecules of pyruvate are produced,
c. two molecules of NAD+ are reduced to two molecules of NADH, and d.a net of two molecules of ATP is produced.
2. ATP is formed in glycolysis by substrate-level phosphorylation during which a. an enzyme transfers a phosphate group from a substrate molecule to ADP and b.ATP is formed.
3. The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are called intermediates.
4. The steps of glycolysis can be grouped into two main phases. a. In steps 1–4, the energy investment phase,
i. energy is consumed as two ATP molecules are used to energize a glucose molecule,
ii. which is then split into two small sugars that are now primed to release energy. b.In steps 5–9, the energy payoff,
i. two NADH molecules are produced for each initial glucose molecule, and ii. ATP molecules are generated.
C.6.8 Pyruvate is oxidized prior to the citric acid cycle
1. The pyruvate formed in glycolysis is transported from the cytoplasm into a mitochon-drion where
a. the citric acid cycle and
b.oxidative phosphorylation will occur.
2. Two molecules of pyruvate are produced for each molecule of glucose that enters glycolysis.
3. Pyruvate does not enter the citric acid cycle, but undergoes some chemical grooming in which
a. a carboxyl group is removed and given off as CO2,
b.the two-carbon compound remaining is oxidized while a molecule of NAD+ is reduced to NADH,
c. coenzyme A joins with the two-carbon group to form acetyl coenzyme A, abbrevi-ated as acetyl CoA, and
D.6.9 The citric acid cycle completes the oxidation of organic molecules, generating many NADH and FADH2 molecules
1. The citric acid cycle
a. is also called the Krebs cycle (after the German-British researcher Hans Krebs, who worked out much of this pathway in the 1930s),
b.completes the oxidation of organic molecules, and c. generates many NADH and FADH2 molecules. 2. During the citric acid cycle
a. the two-carbon group of acetyl CoA is added to a four-carbon compound, forming citrate,
b.citrate is degraded back to the four-carbon compound, c. two CO2 are released, and
d.1 ATP, 3 NADH, and 1 FADH2 are produced.
3. Remember that the citric acid cycle processes two molecules of acetyl CoA for each initial glucose molecule.
4. Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is a. 2 ATP,
b.6 NADH, and c. 2 FADH2.
E. 6.10 Most ATP production occurs by oxidative phosphorylation 1. Oxidative phosphorylation
a. involves electron transport and chemiosmosis and b.requires an adequate supply of oxygen.
2. Electrons from NADH and FADH2 travel down the electron transport chain to O2. 3. Oxygen picks up H+ to form water.
4. Energy released by these redox reactions is used to pump H+ from the mitochondrial matrix into the intermembrane space.
5. In chemiosmosis, the H+ diffuses back across the inner membrane through ATP synthase complexes, driving the synthesis of ATP.
F. 6.11 CONNECTION: Interrupting cellular respiration can have both harmful and benefi-cial effects
1. Three categories of cellular poisons obstruct the process of oxidative phosphorylation. These poisons
a. block the electron transport chain (for example, rotenone, cyanide, and carbon monoxide),
b.inhibit ATP synthase (for example, the antibiotic oligomycin), or
c. make the membrane leaky to hydrogen ions (called uncouplers, examples include dinitrophenol).
2. Brown fat is
a. a special type of tissue associated with the generation of heat and b.more abundant in hibernating mammals and newborn infants. 3. In brown fat,
a. the cells are packed full of mitochondria,
b.the inner mitochondrial membrane contains an uncoupling protein, which allows H+ to flow back down its concentration gradient without generating ATP, and
G.6.12 Review: Each molecule of glucose yields many molecules of ATP 1. Recall that the energy payoff of cellular respiration involves
a. glycolysis,
b.alteration of pyruvate, c. the citric acid cycle, and d.oxidative phosphorylation.
2. The total yield is about 32 ATP molecules per glucose molecule. 3. This is about 34% of the potential energy of a glucose molecule. 4. In addition, water and CO2 are produced.
IV. Fermentation: Anaerobic Harvesting of Energy
A.6.13 Fermentation enables cells to produce ATP without oxygen
1. Fermentation is a way of harvesting chemical energy that does not require oxygen. Fermentation
a. takes advantage of glycolysis,
b.produces two ATP molecules per glucose, and c. reduces NAD+ to NADH.
2. The trick of fermentation is to provide an anaerobic path for recycling NADH back to NAD+.
3. Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation, in which
a. NADH is oxidized to NAD+, and b.pyruvate is reduced to lactate.
4. Lactate is carried by the blood to the liver, where it is converted back to pyruvate and oxidized in the mitochondria of liver cells.
5. The dairy industry uses lactic acid fermentation by bacteria to make cheese and yogurt.
6. Other types of microbial fermentation turn a. soybeans into soy sauce and
b.cabbage into sauerkraut.
7. The baking and winemaking industries have used alcohol fermentation for thousands of years.
8. In this process yeasts (single-celled fungi) a. oxidize NADH back to NAD+ and b.convert pyruvate to CO2 and ethanol. 9. Obligate anaerobes
a. are poisoned by oxygen, requiring anaerobic conditions, and b.live in stagnant ponds and deep soils.
10. Facultative anaerobes
a. include yeasts and many bacteria and
b.can make ATP by fermentation or oxidative phosphorylation.
B. 6.14 EVOLUTION CONNECTION: Glycolysis evolved early in the history of life on Earth
1. Glycolysis is the universal energy-harvesting process of life. 2. The role of glycolysis in fermentation and respiration dates back to
3. The ancient history of glycolysis is supported by its a. occurrence in all the domains of life and
b.location within the cell, using pathways that do not involve any membrane-bounded organelles.
V. Connections Between Metabolic Pathways
A.6.15 Cells use many kinds of organic molecules as fuel for cellular respiration
1. Although glucose is considered to be the primary source of sugar for respiration and fermentation, ATP is generated using
a. carbohydrates, b.fats, and c. proteins.
2. Fats make excellent cellular fuel because they
a. contain many hydrogen atoms and thus many energy-rich electrons and b.yield more than twice as much ATP per gram than a gram of carbohydrate or
protein.
B. 6.16 Food molecules provide raw materials for biosynthesis
1. Cells use intermediates from cellular respiration for the biosynthesis of other organic molecules.
2. Metabolic pathways are often regulated by feedback inhibition in which an accumula-tion of product suppresses the process that produces the product.
Chapter Guide to Teaching Resources
Cellular Respiration: Aerobic Harvesting of Energy (6.1–6.5)
Student Misconceptions and Concerns
Students should be cautioned against the assumption 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 addressed in Module 5.10). (6.1– 6.5)
Students often fail to realize that aerobic metabolism is a process generally similar to the burning of wood or the burning of gasoline in an automobile engine. Noting these general similarities can help students comprehend the overall reaction and heat generation
associated with these processes. (6.3)
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. (6.5)
Teaching Tips
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 only paid its employees 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 allows us to make purchases in distant locations). This idea of earning and spending is a common concept we all know well. (6.1–6.3)
During cellular respiration, our cells convert about 34% of our food energy to useful work (Module 6.3). The other 66% 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 75-watt incandescent light bulb. If you choose to include a discussion of heat generation from aerobic metabolism, consider the following. (6.3) A.Ask your students why they feel warm when it is 30°C (86°F) outside, if their core body
temperature is about 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 helps us get rid of the extra heat from cellular respiration.
B.Share this calculation with your students. Depending upon a person’s size and level of activity, a human might burn 2,000 dietary calories (kilocalories) a day. This is enough energy to raise the temperature of 20 liters 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-liter bottle as a visual aid, or ten 2-liter bottles to make the point above. It takes 100 calories to raise 1 liter of water 100°C; it takes much more energy to melt ice or evaporate water as steam.)
You might share with your students that it takes about 10 million ATP molecules per second to power one active muscle cell. (6.4)
The use of the word “falling” when discussing the movement of electrons in a redox reaction can be confusing. Consider explaining the use of the term falling, in reference to potential energy of a falling object. (6.5)
Stages of Cellular Respiration (6.6–6.12)
Student Misconceptions and Concerns
Perhaps more than anywhere else in general biology, students studying aerobic metabolism may fail to see the forest for the trees. Students may focus on the details of each stage of aerobic metabolism and devote little attention to the overall process and products.
Consider emphasizing the products and energy yields associated with glycolysis, the citric acid cycle, and oxidative phosphorylation before detailing the specifics of each reaction. (6.6–6.12)
Students frequently think that plants have chloroplasts instead of mitochondria. Take care to point out the need for mitochondria in plants when photosynthesis is not efficient or possible (such as during the night). (6.6–6.12)
Teaching Tips
The production of NADH through glycolysis and the Krebs cycle, as compared to the direct production of ATP, can get confusing for students. Help students understand that NADH molecules have 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. (6.7–6.10)
As you relate the structure of the inner mitochondrial membrane to its functions, challenge students to explain the adaptive advantage of the many folds of this inner membrane (see Figure 6.6). (These folds greatly increase the surface area available for the associated reactions.) (6.10)
The authors develop an analogy between the function of the inner mitochondrial
membrane and a dam. A reservoir of hydrogen ions is built up between the inner and outer 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. (6.10)
Module 6.11 explores the many points in cellular respiration where poisons may produce their deadly effects. Like any complex process, such as an engine of a car or the
cooperation of athletes on a team, the results depend upon the proper functioning of each part.
Poisons can stop a metabolic pathway by disrupting a single step in the process. (6.11) Students should be reminded that the ATP yield of up to 32 ATP per glucose molecule is
only a potential. The complex chemistry of aerobic metabolism can yield this amount only under ideal conditions, when every substrate and enzyme is immediately available. Such circumstances may occur only rarely in a working cell. (6.12)
Fermentation: Anaerobic Harvesting of Energy (6.13–6.14)
Student Misconceptions and Concerns
Students may expect that fermentation will produce alcohol and maybe even carbon dioxide. Take the time to clarify the different possible products of fermentation and correct this general misconception. (6.13)
Teaching Tips
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. (6.13)
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 oxygen immediately above the wine. However, as fermentation slows down, the wine must be sealed to prevent oxygen exposure and permit the
fermentation process to finish. (6.13)
The widespread occurrence of glycolysis, which takes place in the cytosol and independent of organelles, suggests that this process had an early evolutionary origin. Since
atmospheric oxygen was not available in significant amounts during the early stages of Earth’s history, and glycolysis does not require oxygen, it is likely that this chemical pathway was used by the prokaryotes in existence at that time. Students focused on the evolution of large, readily apparent structures such as wings and teeth may have never considered the evolution of cellular chemistry. (6.14)
Connections Between Metabolic Pathways (6.15–6.16)
Student Misconceptions and Concerns
Some students may only view nutrients as sources of calories. As noted in Module 6.16, the monomers of many nutrients are recycled into synthetic pathways of organic
molecules. (6.16)
Teaching Tips
The same mass of fat stores nearly twice as many calories (about 9 kcal per gram) as an equivalent mass of protein or carbohydrates (about 4.5–5 kcal per gram). Fat is therefore an efficient way to store energy in animals and many plants. To store an equivalent amount of energy in the form of carbohydrates or proteins would require about twice the mass, adding a significant burden to the organism’s structure. (For example, if you were 20 lbs overweight, you would be nearly 40 lbs overweight if the same energy were stored as carbohydrates or proteins instead of fat). (6.15)
Figure 6.15 is an important visual synthesis of the diverse fuels that can enter into cellular respiration and the various stages of this process. Figures such as this can serve as a visual anchor to integrate the many aspects of this chapter. (6.15)
The final modules in this chapter may raise questions about obesity and proper diet. The Center for Disease Control and Prevention website, www.cdc.gov/nccdphp/dnpa, discusses many aspects of nutrition, obesity, and general physical fitness and is a useful reference for teachers and students. (6.15–6.16)
Key Terms
acetyl CoA (acetyl coenzyme A) alcohol fermentation ATP synthase chemiosmosis citric acid cycle
electron transport chain glycolysis
intermediates kilocalorie (kcal) lactic acid fermentation oxidation
Word Roots
aero- = air (aerobic: using oxygen) an- = not (anaerobic: not using oxygen)
chemi- = chemical (chemiosmosis: the production of ATP using the energy of hydrogen ion gradients across membranes to phosphorylate ADP)
de- = without; -hydro = water (dehydrogenase: an enzyme that removes water when catalyzing a chemical reaction)
