Cellular respiration includes aerobic respiration and fer-mentation. Aerobic respiration, the oxidation of glu-cose to carbon dioxide and water, is an exergonic reactionthat drives ATP synthesis, an endergonic reaction. Oxi-dation involves the removal of hydrogen atoms (H+ + e–)from substrate molecules, usually by the coenzyme NAD+,but in one case by FAD. Four events are required: gly-colysis, the transition reaction, the Krebs cycle, and theelectron transport system.

During glycolysis, glucose is converted to pyru-vate in the cytosol. Glycolysis produces two ATP bysubstrate-level phosphorylation. When oxygen is avail-able, pyruvate from glycolysis enters mitochondria. Inmitochondria, the transition reaction and the Krebscycle are located in the matrix, and the electron transportsystem is located on the cristae. Both the transition re-action and the Krebs cycle release carbon dioxide as a re-sult of the oxidation of carbohydrate breakdown products.

The electrons carried by NADH and FADH2 enterthe electron transport system. The electrons pass downa chain of carriers until they are finally received by oxy-gen, which combines with H+, forming water. As electronspass down the electron transport system, energy is re-

leased and stored for ATP production. The term oxida-tive phosphorylation is sometimes used for 34 ATPsproduced as a result of the electron transport system.The protein complexes of the electron transport systempump H+ received from NADH and FADH2 into the in-termembrane space, setting up an electrochemical gradient.When H+ flows down this gradient through the ATP syn-thase complex, energy is released and used to form ATP.This process of producing ATP is called chemiosmosis.

Other carbohydrates, as well as protein and fat, canalso generate ATP by entering various steps in the degrada-tive paths of glycolysis and the Krebs cycle. These path-ways also provide metabolites needed for the synthesisof various important cellular substances.

Fermentation, which occurs when oxygen is not avail-able for aerobic respiration, involves glycolysis followedby the reduction of pyruvate by NADH. The end productcan be lactate or alcohol and carbon dioxide. Fermenta-tion produces a net yield of 2 ATP molecules per glu-cose molecule. Fermentation provides a quick, immediatesource of ATP, but lactate buildup is toxic to the cell andcreates an oxygen debt by the organism.

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7CELLULAR RESPIRATION

CHAPTER REVIEW

STUDY EXERCISES

Study the text section by section as you answer the questions that follow.

7.1 AEROBIC CELLULAR RESPIRATION (P. 118)

• During aerobic cellular respiration, the breakdown of glucose drives the synthesis of ATP.

1. Consider the following equation.

C6H12O6 + 6 O2 → 6 H2O + 6 CO2 + energy

The molecule glucose is (oxidized or reduced) a.______________ while oxygen is (oxidized or reduced)b.______________. This is an (endergonic or exergonic) c.______________ reaction and therefore is used by

cells to build up ATP.

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b.

d.

a. c.

2. Complete the adajacent diagram and answer thesequestions:

The (left or right) e.______________ side of the

diagram represents oxidation and the (left or

right) f.______________ side of the diagram

represents reduction of NAD. Why is NAD+

called a coenzyme of oxidation-reduction? g. ________________________________________

Phases of Aerobic Cellular Respiration (p. 119)

• Aerobic cellular respiration requires a number of reactions within three metabolic pathways.

3. Indicate whether the following statements pertain to glycolysis (GL), transition reaction (TR), Krebs cycle(KC), or the electron transport system (ETS). Some items require more than one answer.

a. Series of carriers that pass electrons from one to the other.b. Cyclical series of oxidation reactions that release CO2.c. Pyruvate is oxidized to an acetyl group.d. Breakdown of glucose to two molecules of pyruvate.e. Energy is released and stored for ATP production.f. Occurs inside mitochondria.g. Occurs outside the mitochondria in the cytosol.h. Results in only 2 ATP.

7.2 OUTSIDE THE MITOCHONDRIA: GLYCOLYSIS (P. 120)

• Glycolysis is a metabolic pathway that partially breaks down glucose outside the mitochondria.

4. Where does glycolysis occur? a.______________

Does it require oxygen? b.______________

Glycolysis begins with c.______________.

Glycolysis ends with d.______________.

How many ATP are produced per glucose molecule as a direct result of glycolysis? e.______________

What type of phosphorylation occurs during glycolysis? f.______________

What coenzyme carries out the oxidation of substrates during glycolysis? g.______________

Considering your answers to these questions, what is the output of glycolysis? h.______________,

______________, and ______________

When glycolysis is a part of fermentation, what is the end product in humans? i.___________________.

• The transition reaction and the Krebs cycle, which occur inside the mitochondria, continue the breakdown ofglucose until carbon dioxide and water result.

7.3 INSIDE THE MITOCHONDRIA (P. 122)

5. Label this diagram of a mitochondrion using the following terms:cristaecytosolinner membraneintermembrane spacematrixouter membrane

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c.

d. e.

b.a.

f.

6. Using your labels from question 5, where does each of the following processes occur?

glycolysis a.______________

Krebs cycle b.______________

electron transport system c.______________

7. The Krebs cycle begins and ends with what molecule? a.______________

A two-carbon molecule acetyl group enters the Krebs cycle. What carbon molecules leave the Krebs

cycle? b.______________

How many ATP are produced per glucose molecule as a direct result of the Krebs cycle? c.______________

What coenzymes carry out the oxidation of substrates in the Krebs cycle? d.______________

Considering your answers to these questions, what are the outputs of the Krebs cycle? e.______________,

______________, ______________, and ______________

Electron Transport System (p. 124)

• The electron transport system, which receives electrons from NAD+ and FAD, produces most of the ATPduring aerobic cellular respiration.

8. What coenzymes bring hydrogen atoms (H+ + e–) to the electron transport system? a.______________

What happens to the electrons? b.______________

What happens to the hydrogen ions? c.______________

What molecule is the final acceptor of electrons from the electron transport system? d.______________

Each pair of electrons carried by NADA from the Krebs cycle that passes down the electron transport system

accounts for the buildup of how many ATP? e.______________

What type of phosphorylation is associated with the electron transport system? f.______________

9. During chemiosmotic ATP synthesis in mitochondria, H+ build up in the a.______________ space.

When these H+ flow b.______________ their concentration gradient into the matrix, c.______________ is

produced from ADP + P .

10. Match the complexes with the following functions:1 passes on electrons and pumps H+ into inter-

membrane space2 carries out ATP synthesis3 receives electrons and passes them on to oxygen4 oxidizes NADH and pumps H+ into intermem-

brane space

_____ a. NADH dehydrogenase complex_____ b. cytochrome b-c complex_____ c. cytochrome oxidase complex_____ d. ATP synthase complex

Energy Yield from Glucose Metabolism (p. 126)

11. In the following diagram, fill in the blanks with the correct numbers and with the terms NADH, FADH2, andATP:

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Krebscycle

4 CO2

O2 H2O

2 CO2

Cyt

osol

Mito

chon

drio

n

ATP Yield +

Ele

ctro

n T

ran

spo

rt S

yste

m

glucose

2 pyruvate

glycolysis

2 acetyl-CoA

7.4 METABOLIC POOL AND BIOSYNTHESIS (P. 127)

• A number of metabolites in addition to glucose can be broken down to drive ATP synthesis.

12. The carbon skeleton of amino acids can be respired if the amino acid first undergoes a.______________.

When fats are respired, glycerol is converted to b.______________, fatty acids are converted to the two-carbon

molecule c.______________, and the acetyl group enters the Krebs cycle. Excess acetyl groups from glucose

metabolism can be used to build up fat. Explain why the consumption of carbohydrate makes us fat. d. ____

_______________________________________________________________________________________________

7.5 FERMENTATION (P. 129)

• Fermentation is a metabolic pathway that partially breaks down glucose under anaerobic conditions.

13. What happens to pyruvate during fermentation? in humans a.__________________ in yeast b.

Why is fermentation wasteful? c.

What is its advantage? d.

What is oxygen debt in humans? e.

14. Label the following processes I, II, and/or III, based on this pyruvate diagram:a. occurs under anaerobic conditionsb. fermentationc. glycolysisd. transition reaction

15. Consider III in the diagram for question 14.

Which has more hydrogen atoms, pyruvate or lactate? a.______________

In yeast, the product of this reaction is b.______________.

What happens to NAD+ produced by the reaction? c.______________

16. Consider II in the diagram for question 14.

What happens to NADH? a._______________

What happens to the acetyl group? b.______________

Cellular Respiration Roulette

On the table before you are three locations to place a bet. These locations are labeled:

G K E

Glycolysis Krebs cycle Electron Transport System

You can place a chip on more than one of these locations. For every chip properly placed, you would hypotheticallywin $5.00. Place your bets! Where should you place (a) chip(s) for each of the following:

1. occurs in cytoplasm2. occurs in mitochondrion3. glucose4. oxygen5. carbon dioxide6. NADH produced7. NADH received8. water produced9. 2 ATP

10. 32 ATP11. product from transition reaction enters here12. How much money did you win?

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glucose

pyruvate

NAD

H N

AD+ N

ADH

NAD +

acetyl group lactate

NAD+

NADH

I

II III

CHAPTER TEST

58

OBJECTIVE QUESTIONS

Do not refer to the text when taking this test.1. Fermentation is

a. glycolysis and the Krebs cycle.b. glycolysis and the reduction of pyruvate.c. glycolysis only.d. the reduction of pyruvate only.

2. Each of the following is a product of aerobic res-piration EXCEPTa. ATP.b. carbon dioxide.c. oxygen.d. water.

3. Per glucose molecule, the net gain of ATP mol-ecules from glycolysis isa. two.b. four.c. six.d. eight.

4. Fermentation suppliesa. glycolysis with free NAD+.b. hydrogen to the transition reaction.c. oxygen as an electron acceptor.d. the Krebs cycle with oxygen.

5. The process that occurs first isa. chemiosmosis.b. glycolysis.c. the electron transport system.d. the Krebs cycle.

6. Which is NOT an event of the transition reaction?a. breaks down pyruvateb. converts a citrate moleculec. oxidizes pyruvated. transfers an acetyl group

7. Select the incorrect association.a. electron transport system—cristaeb. fermentation—plasma membranec. glycolysis—cytosold. Krebs cycle—matrix

8. Select the process with the greatest yield per glu-cose molecule.a. glycolysisb. Krebs cyclec. substrate-level phosphorylationd. transition reaction

9. The energy yield by ATP molecules per glucosemolecule is closest toa. 25%.b. 40%.c. 50%.d. 60%.

10. Inside a cell, glycerol is broken down intoa. amino acids.b. acetyl-CoA.c. fatty acids.d. PGAL.

11. Which of the following reactions is NOT a partof aerobic respiration?a. glycolysisb. Krebs cyclec. electron transport systemd. transition reactione. fermentation

12. The coenzyme used in the transition reaction ofaerobic respiration isa. ATP.b. NAD+.c. NADH.d. coenzyme A.e. RuBP.

13. The carbon dioxide given off by aerobic respi-ration is produced bya. glycolysis.b. the transition reaction.c. the Krebs cycle.d. the electron transport system.e. Both b and c are correct.

14. The final acceptor for electrons in aerobic res-piration isa. ATP.b. NAD+.c. FAD.d. oxygen.e. carbon dioxide.

15. Which of the following reactions occurs on theinner membrane of mitochondria?a. the Krebs cycleb. the transition reactionc. the electron transport systemd. glycolysise. the Calvin cycle

16. The coenzymes NAD+ and FAD carry hydrogenatoms (H+ + e–) to thea. glycolysis reactions.b. transition reaction.c. Krebs cycle.d. Calvin cycle.e. electron transport system.

59

17. Which of the following statements is NOT trueabout fermentation?a. It is an anaerobic process.b. The end products are toxic to cells.c. It results in two ATPs per glucose molecule.d. In the absence of O2, muscle cells form CO2

and alcohol.e. It can be used to make bread rise.

18. Which of the following statements is NOT trueregarding fats?a. Fatty acids are converted to acetyl-CoA.b. Eighteen-carbon fatty acids are converted to

nine acetyl-CoA molecules.c. Glycerol is converted to PGAL.

d. Fats are the least efficient form of stored energy.

e. Carbohydrates can be converted to fats.19. The process directly responsible for most of the

ATP formed during aerobic respiration isa. the Krebs cycle.b. the transition reaction.c. the electron transport system.d. chemiosmosis.

20. A pathway that begins with glucose and endswith pyruvate isa. glycolysis.b. the Krebs cycle.c. the electron transport system.d. the transition reaction.

THOUGHT QUESTIONS

Answer in complete sentences.21. Explain how the human body obtains the reactants for aerobic respiration and what happens to the products.

22. Why would you expect the Krebs cycle to be located in the matrix of the mitochondrion and the electron trans-port system to be located on the cristae?

Test Results: ______ Number right ÷ 22 = ______ × 100 = ______ %

ANSWER KEY

STUDY EXERCISES

1. a. oxidized b. reduced c. exergonic 2. a. 2Hb. NADH + H+ c. 2H d. NAD+ e. right f. leftg. It becomes reduced when it accepts electrons from asubstrate and becomes oxidized when it passes electronson to another carrier. 3. a. ETS b. KC c. TR d. GLe. ETS f. TR, KC, ETS g. GL h. GL, KC (two turns perglucose molecule) 4. a. cytosol b. no c. glucosed. pyruvate e. two ATP f. substrate level g. NAD+

h. NADH, ATP, pyruvate i. lactate 5. a. cristae b. ma-trix c. outer membrane d. intermembrane space e. innermembrane f. cytosol 6. a. cytosol b. matrix c. cristae7. a. citrate b. CO2 c. two ATP d. NAD+ and FADe. NADH, FADH2, ATP, and CO2 8. a. NADH andFADH2 b. pass down the system c. pumped into inter-membrane space d. O2 e. three ATP f. oxidative9. a. intermembrane b. down c. ATP 10. a. 4 b. 1c. 3 d. 2 11. see Figure 7.8, page 126, in text12. a. deamination b. PGAL c. acetyl-CoA d. The acetylgroups, which result from carbohydrate breakdown, canbe used to make fat. 13. a. reduced to lactate b. re-duced to alcohol and CO2 c. produces only two ATPd. does not require oxygen e. O2 needed to metabolizelactate 14. a. I, III b. III c. I d. II 15. a. lactateb. alcohol and CO2 c. returns to glycolysis 16. a. goesto the electron transport system b. enters the Krebs cycle

CELLULAR RESPIRATION

ROULETTE

1. G 2. K, E 3. G 4. E 5. K 6. G, K 7. E 8. E9. G, K 10. E 11. K 12. Calculate your winnings.

CHAPTER TEST

1. b 2. c 3. a 4. a 5. b 6. b 7. b 8. b 9. b10. d 11. e 12. d 13. c 14. d 15. c 16. e17. d 18. d 19. d 20. a 21. Glucose enters the bodyat the digestive tract and oxygen enters at the lungs. Glu-cose and oxygen are delivered to cells by the circulatorysystem. Water from aerobic respiration enters the bloodand is utilized by the body or excreted; we breathe out thecarbon dioxide. 22. The Krebs cycle is a series of en-zymatic reactions, and one reaction is tied to the next be-cause the product of one reaction is the substrate for thenext. The electron transport system is the passage of elec-trons from one carrier to the next. These carriers are sta-tioned in their correct order on the cristae, otherwise thepassage of electrons would not release energy in the mostefficient manner.