energy for cells chapter outline - mcgraw-hill

In American society, we think a lot about weight loss. Everywhere we look, advertisements claim that some product will rev up our slow metabolism and cure our weight problems for good. In the meantime, our society has more obese individuals than ever. A “miracle cure” for being heavy often comes in the form of a very odd diet or a nutritional supplement. Have you ever considered the science behind how these products claim to work? When

97

C H A P T E REnergy for Cells 7

Anaerobic cellular respiration makes muscles ache.

Carbon monoxide and cyanide kill by stopping cellular respiration.

If a diet sounds too good to be true, it probably is.

O U T L I N E

7.1 Cellular Respiration • The breakdown of glucose to CO2 and H2O

during cellular respiration drives the synthesis of ATP. 98

• The complete breakdown of glucose requires four phases: three metabolic pathways and one individual enzymatic reaction. 99

7.2 Outside the Mitochondria: Glycolysis • Glycolysis is a metabolic pathway that

partially breaks down glucose outside the mitochondria. 101

7.3 Inside the Mitochondria • The preparatory reaction and the citric acid

cycle, which occur inside the mitochondria, continue the breakdown of glucose products until carbon dioxide and water result. 102

• The electron transport chain, which receives electrons from NADH and FADH2, produces most of the ATP during cellular respiration. 104

• Other nutrients in addition to glucose can be broken down to drive ATP synthesis. 106

7.4 Fermentation • Fermentation is a metabolic pathway that

partially breaks down glucose under anaerobic conditions. 107

negyxo

H2O

CO2

esoculg

double membraneinner membrane

outer membrane

matrix

cristae

intermembranespace

you do, you will see that many of these claims simply can’t be verifi ed. Yet millions of people spend a fortune hoping for a “miracle,” and many of them suffer health problems as a result of fad diets or the use of nutritional supplements.

Numerous nutritional supplements are supposed to allow you to eat everything you want, with no need to exercise. It sounds nice, but the reality is that our cells don’t work that way! In order to acquire the energy you need to live, cells break down glucose in a process called cellular respiration. Both the glucose and the oxygen needed for cellular respiration are provided by the process of photosynthesis in plants. In this chapter, you will learn how cells use glucose to produce the ATP they need. Understanding this process will then help you make informed decisions about weight loss, diets, and nutritional supplements.

98

C6H12O6

glucose6 O2 6CO2+ 6H2O+ energy+

Oxidation

Reduction

Figure 7.1 Cellular respiration.

Glucose from our food and the oxygen we breathe are

requirements for cellular respiration, a process completed

within the mitochondria.

7.1 Cellular RespirationWhether you go skiing, take an aerobics class, or just hang out, ATP molecules provide the energy needed for your muscles to contract. ATP molecules are produced during cellular respiration, a process that requires the participation of mitochondria. Cellular respiration is aptly named because just as you take in oxygen (O2) and give off carbon dioxide (CO2) during breathing, so do the mitochondria in your cells (Fig. 7.1). In fact, cellular respiration, which occurs in all cells of the body, is the reason you breathe.

Oxidation of substrates is a fundamental part of cellular respiration. In living things, oxidation doesn’t occur by the addition of oxygen (O2). Instead, oxidation is the removal of hydrogen atoms from a molecule. As cellular respiration occurs, hydrogen atoms are removed from glucose (and glucose products) and transferred to oxygen atoms, forming carbon dioxide (CO2) and water (H2O):

The breakdown of glucose releases a lot of energy. If you mistakenly burn sugar in a skillet, the energy escapes into the atmosphere as heat. A cell is more sophisticated than that. In a cell, glucose is broken down slowly—not all

at once—and the energy given off isn’t all lost as heat. Hydrogen atoms are removed bit by bit, and this allows energy to be captured and

used to make ATP molecules.

Chapter 7 Energy for Cells 99

Figure 7.2 The four phases of

complete glucose

breakdown.

a. The enzymatic

reactions of glycolysis

take place in the

cytoplasm. b. The

preparatory reaction,

(c) the citric acid cycle,

and (d) the electron

transport chain occur in

mitochondria.

Check Your Progress

1. Why is breathing necessary to cellular respiration? 2. Explain why glucose is broken down slowly, rather than

quickly, during cellular respiration. 3. List the four phases of complete glucose breakdown.

Answers: 1. Breathing takes in oxygen needed for cellular respiration and rids the body of carbon dioxide, a waste product of cellular respiration. 2. Slow breakdown allows much of the released energy to be captured and utilized by the cell. 3. Glycolysis, the preparatory reaction, the citric acid cycle, and the electron transport chain.

1 NAD = Nicotinamide adenine dinucleotide; FAD = Flavin adenine dinucleotide

a. d.c.

b.

Preparatory reactionglucose pyruvate

Cytoplasm

Glycolysis

Citric acidcycle

Electrontransportchain

ATPATPATP2 2 34

NADH andFADH2

e–

Phases of Complete Glucose BreakdownThe enzymes that carry out oxidation during cellular respiration are assisted by non-protein helpers called coenzymes. As glucose is oxidized, the coenzymes NAD� and FAD1 receive hydrogen atoms (H� � e�) and become NADH and FADH2, respectively (Fig. 7.2).

During these phases, notice where CO2 and H2O are produced.

• Glycolysis, which occurs in the cytoplasm outside the mitochondria, is the breakdown of glucose to 2 molecules of pyruvate. Oxidation results in NADH, and there is enough energy left over for a net gain of 2 ATP molecules.

• The preparatory (prep) reaction takes place in the matrix of mitochondria. Pyruvate is broken down to a 2-carbon acetyl group carried by coenzyme A (CoA). Oxidation of pyruvate results in not only NADH, but also CO2.

• The citric acid cycle also takes place in the matrix of mitochondria. As oxidation occurs, NADH and FADH2

result and more CO2 is released. The citric acid cycle is able to produce 2 ATP per glucose molecule.

• The electron transport chain requires a series of electron carriers in the cristae of mitochondria. NADH and FADH2 give up electrons to the chain. Energy is released and captured as the electrons move from a higher energy to a lower energy state. Later, this energy will be used for the production of ATP. Oxygen (O2 ) fi nally shows up here as the last acceptor of electrons from the chain. Combination with hydrogen ions (H�) produces water (H2O).

100 Part I The Cell

glucose

pyruvate

P

P

P

P

P

ATP

ATP

ADP

G3P

P P

BPG

G3P

ATP

ADP

NADH

+2 ATP

+2 ATP

–2 ATP

Net gain:2 ATP

NAD+ NAD+

P

P

ADP

ATP ADP

H2O

pyruvate

P

ATP

P P

BPG

NADH

P

P

3PG3PG

ADP

ATPADP

H2O

Energy-investment steps

Energy-harvesting steps

RuBP

3PG

BPG

G3P

ribulose 1,5-bisphosphate

3-phosphoglycerate

1,3-bisphosphoglycerate

glyceraldehyde-3-phosphate

Preparatory reaction

GlycolysisCitric acid

cycleElectrontransportchain

NADH andFADH2

e–

ATP2 2 34ATP ATP

Cytoplasm

Figure 7.3 Glycolysis.

This metabolic pathway begins with glucose and ends

with pyruvate. A net gain of 2 ATP molecules can be

calculated by subtracting those expended during the

energy-investment steps from those produced during the

energy-harvesting steps.


Check Your Progress

1. Contrast the energy-investment steps of glycolysis with the energy-harvesting steps.

2. What happens to pyruvate when oxygen is available in a cell?

Answers: 1. During the energy-investment steps, ATP breakdown provides the phosphate groups to activate substrates. During the energy-harvesting steps, NADH and ATP are produced. 2. Pyruvate enters the mitochondria for further breakdown.

Figure 7.4 Substrate-level ATP synthesis.

The net gain of 2 ATP from glycolysis is the result of substrate-level

ATP synthesis. At an enzyme’s active sites, ADP acquires a phosphate

group from a substrate, and ATP results.

Glycolysis

inputs outputs

glucose

2 NAD+

ADP +

2 NADH

2 pyruvate

2 ATP

4 ATP

2 ATP net

P

P

P

P

ADP

enzyme

ATP

7.2 Outside the Mitochondria: Glycolysis

In eukaryotes, such as plants and animals, glycolysis takes place within the cytoplasm outside the mitochondria. During glycolysis, glucose, a C6 mol-ecule, is broken down to 2 molecules of pyruvate, a C3 molecule. Glycolysis is divided into (1) the energy-investment steps when ATP is used, and (2) the energy-harvesting steps, when NADH and ATP are produced (Fig. 7.3).

Energy-Investment StepsDuring the energy-investment steps, 2 ATP transfer phosphate groups to sub-strates, and 2 ADP � �P result. In other words, ATP has been broken down, not built up. However, the phosphate groups activate the substrates so that they can undergo reactions.

Energy-Harvesting StepsDuring the energy-harvesting steps, substrates are oxidized by the removal of hydrogen atoms, and 2 NADH result.

Oxidation produces substrates with high-energy phosphate groups, which are used to synthesize 4 ATP. As a phosphate group is transferred to ADP, ATP results. The process is called substrate-level ATP synthesis (Fig. 7.4).

What is the net gain of ATP from glycolysis? Confi rm that 2 ATP are used to get started, and 4 ATP are produced by substrate-level ATP synthesis. Therefore, there is a net gain of 2 ATP from glycolysis.

If oxygen is available, pyruvate, the end product of glycolysis, enters mitochondria, where it undergoes further breakdown. If oxygen is not available, pyruvate undergoes reduction. In humans, if oxygen is not available, pyruvate is reduced to lactate, as discussed on page 107.

102 Part I The Cell

cytoplasmlocation of glycolysis

matrixlocation of the prepreaction and the citricacid cycle

cristaelocation of the electrontransport chain

outer membrane�70,000

matrix cristae

inner membraneforms cristae

intermembranespace

Citric acid cycle

inputs outputs

2 acetyl-CoA

6 NAD+

2 FAD

2 ADP + 2

6 NADH

2 FADH2

4 CO2

2 ATPP

During the citric acid cycle:

• The acetyl group is oxidized, and the rest of the CO2 we breathe out per glucose molecule is released.

• Both NAD� and FAD accept hydrogen atoms, resulting in NADH and FADH2.

• Substrate-level ATP synthesis occurs (see Fig. 7.4), and an ATP results.

Because the citric acid cycle turns twice for each original glucose molecule, the inputs and outputs of the citric acid cycle per glucose molecule are as follows:

7.3 Inside the MitochondriaThe other three phases of cellular respiration occur inside the mitochondria (Fig. 7.5).

Preparatory Reaction Occurring in the matrix, the preparatory (prep) reaction is so called because it produces a substrate that can enter the citric acid cycle. The preparatory reac-tion occurs twice per glucose molecule because glycolysis results in 2 pyruvate molecules. During the prep reaction:

• Pyruvate is oxidized, and a CO2 molecule is given off. This is part of the CO2 we breathe out!

• NAD� accepts a hydrogen atom, and NADH results. • A C2 acetyl group is attached to coenzyme A (CoA), forming acetyl-

CoA.

The Citric Acid CycleThe citric acid cycle is a cyclical metabolic pathway located in the matrix of mitochondria (Fig. 7.6). It was originally called the Krebs cycle to honor the man who fi rst studied it. At the start of the citric acid cycle, the C2 acetyl group carried by CoA joins with a C4 molecule, and a C6 citrate molecule results. The CoA returns to the preparatory reaction to be used again.

Figure 7.5 Mitochondrion structure and function.

A mitochondrion is bounded by a double membrane. The inner membrane

invaginates to form the shelfl ike cristae. Glycolysis takes place in the cytoplasm

outside the mitochondria. The preparatory reaction and the citric acid cycle

occur within the mitochondrial matrix. The electron transport chain is located

on the cristae of a mitochondrion.


Check Your Progress

1. A C2 acetyl group enters the citric acid cycle. Where does it come from?

2. What are the products of the citric acid cycle as a result of further breakdown of glucose?

Answers: 1. The C2 acetyl group comes from the prep reaction. 2. The citric acid cycle turns twice per glucose molecule, producing 2 CO2, 3 NADH, 1 FADH2, 1 ATP per turn.

Pyruvate from glycolysis is oxidized to a C2 acetyl group that is carried by CoA to the citric acid cycle.

1

The C2 acetyl group combines with a C4 molecule to produce citric acid, a C6 molecule.

2

The loss of two CO2 results in a new C4 molecule.

4ATP is produced by substrate-level ATP synthesis.

5

Twice over, oxidation reactions produce NADH, and CO2is released.

3

Additional oxidation reactions produce another NADH and an FADH2 and regenerate the original C4 molecule.

6

2 pyruvate

NADH2

CO2

2 NAD+

2 CoA

2 CoA

CO2

CO2

NADH

FADH2

NAD+

FAD

NAD+ NADH

NAD+ NADH

Citric acidcycle

Preparatoryreaction

ATP

ADP + P




NADH andFADH2

e–

ATP2 2 34matrix ATP ATP

Figure 7.6 The citric acid cycle.

The acetyl-CoA from the preparatory reaction enters the citric

acid cycle. The net result of one turn of this cycle of reactions is

the oxidation of the acetyl group to 2 molecules of CO2 and the

formation of 3 molecules of NADH and 1 molecule of FADH2.

Substrate-level ATP synthesis occurs, and the result is 1 ATP

molecule. The citric acid cycle turns twice per glucose molecule.

104 Part I The Cell

3 ATPH2O

O2

2e–

2 H+

NAD+

NADH

NADHand

FADH2

1

1

2

2

3

3

4

4

5

5

2 H+

O21–2

oxidized

oxidized

oxidized

oxidized

Figure 7.7 The electron transport chain.

An electron transport chain operates like a bucket brigade. Each electron carrier (#1–5)

is alternatively reduced (orange) and oxidized as if the electrons were a bucket being

passed from person to person. As oxidation-reduction occurs, energy is released that

will be used to make ATP.




NADH andFADH2

e–

ATP2 2 34ATP ATP

The Electron Transport ChainThe electron transport chain located in the cristae of mitochondria is a series of carriers that pass electrons from one to the other. NADH and FADH2 deliver electrons to the chain. Consider that the hydrogen atoms attached to NADH and FADH2 consist of an e� and an H�. The members of the electron transport chain accept only electrons (e�) and not hydrogen ions (H�).

In Figure 7.7, high-energy electrons enter the chain, and low-energy electrons leave the chain. When NADH gives up its electrons, the next carrier gains the electrons and is reduced. This oxidation-reduction reaction starts the process, and each of the carriers in turn becomes reduced and then oxidized as the electrons move down the system. As the pair of electrons is passed from carrier to carrier, energy is released and captured for ATP production. The fi nal acceptor of electrons is oxygen (O2), the very O2 we breathe in. It’s remarkable to think that the role of oxygen in cellular respiration is to keep the electrons moving from the fi rst to the last carrier. Why? Because oxygen attracts electrons to a greater degree than the carriers of the chain. Once oxygen accepts electrons it combines with H�, and the other end product of cellular respiration (i.e., water) results (see the equation on page 98).

When NADH delivers electrons to the fi rst carrier of the electron trans-port chain, enough energy is captured by the time the

electrons are received by O2 to permit the produc-tion of three ATP molecules. When FADH2

delivers electrons to the electron transport chain, only 2 ATP are produced.

Once NADH has delivered electrons to the electron transport

chain, NAD� is regenerated and can be used again. In the same manner, FAD is regenerated and can be used again. The recycling of coenzymes, and for that mat-ter ADP, increases cellular effi -ciency since it does away with the need to synthesize NAD�,

FAD, and ADP anew.

The Cristae of a Mitochondrion

The carriers of the electron transport chain are located in molecular complexes

within the inner mitochondrial membrane. ATP synthesis is carried out by ATP synthase complexes also located in this membrane (Fig. 7.8).

The carriers of the electron transport chain accept electrons from NADH or FADH2 and then pass them from one to the other by way of two additional


mobile carriers (orange arrow). What happens to the hydrogen ions (H�) carried by NADH and FADH2? The complexes use the energy released by oxidation-reduction to pump H� from the mitochondrial matrix into the intermembrane space located between the outer and inner membrane of a mitochondrion. The pumping of H� into the intermembrane space establishes an unequal distribu-tion of H� ions; in other words, there are many H� in the intermembrane space and few in the matrix of a mitochondrion.

The energy stored in the H� gradient is now used to drive forward ATP synthesis. The cristae of mitochondria (like the thylakoid membrane of chloro-plasts) contain an ATP synthase complex that allows H� to return to the matrix. The fl ow of H� through the ATP synthase complex brings about a conforma-tional change, which causes the enzyme ATP synthase to synthesize ATP from ADP � �P . ATP leaves the matrix by way of a channel protein. This ATP remains in the cell and is used for cellular work.

Check Your Progress

Explain how the electron transport chain results in the synthesis of ATP.

Answer: As electrons move from one carrier to another in the cristae, energy is released, and this energy is used to pump hydrogen ions from the matrix to the intermembrane space. The fl ow of hydrogen ions back down the concentration gradient into the matrix drives the synthesis of ATP by ATP synthase.

Figure 7.8 The organization of cristae.

Molecular complexes that contain the electron transport carriers are located in the cristae as are ATP synthase complexes. a. As electrons move from one carrier to

the other, hydrogen ions (H�) are pumped from the matrix into the intermembrane space. b. As hydrogen ions fl ow back down a concentration gradient through

an ATP synthase complex, ATP is synthesized by the enzyme ATP synthase.

H+

H+

H+

H+H+

H+

H+

H+H+

H+

H+

H+ H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

NAD+

H+

H+

H2O

NADH

ATPPADP +

e–

a. Electron transport chain

Intermembranespace

intermembranespace

electron transportcarriers in a molecularcomplex

mobile carrier

Matrix

b. ATP synthesis

ATPsynthasecomplex

O22

1–2

cristae

matrix

106 Part I The Cell

Proteins

amino acids glucose glycerol

Carbohydrates

Acetyl-CoA

Electrontransportchain

Glycolysis

Fats and oils

fatty acids

pyruvate

Citricacidcycle

ATP

ATP

ATP

H2O

O2

Food

NH3

Figure 7.10 Alternative metabolic pathways.

All the types of food in a pizza can be used to generate ATP.

Phase

Glycolysis

NADH ATP YieldFADH2

–

Prep reaction –

Electrontransport chain 2

Citric acidcycle

2

2

10

6

2

–

304

22

Total ATP 38

Energy Yield from Glucose MetabolismFigure 7.9 calculates the ATP yield for the complete breakdown of glucose to CO2 and H2O. Per glucose molecule, there is a net gain of 2 ATP from glycoly-sis, which takes place in the cytoplasm. The citric acid cycle, which occurs in the matrix of mitochondria, accounts for 2 ATP per glucose molecule. This means that a total of 4 ATP form due to substrate-level ATP synthesis outside the electron transport chain.

Most of the ATP produced comes from the electron transport chain and the ATP synthase complex. Per glucose molecule, 10 NADH and 2 FADH2 take electrons to the electron transport chain. The maximum number of ATP produced by the chain is therefore 34 ATP, and the maximum number produced by both the chain and substrate-level ATP synthesis is 38. However, for reasons beyond the scope of this book, the maximum number of ATP produced per glucose molecule in some cells is only 36 ATP or lower. A yield of 36–38 ATP represents about 40% of the available energy in a glucose molecule. The rest of the energy is lost in the form of heat.

Alternative Metabolic PathwaysLet’s say you are on a low-carbohydrate diet. Will you then run out of ATP? No, because your cells can also utilize other energy sources—the components of fats and oils, namely glycerol and fatty acids, and amino acids, which are derived from proteins (Fig. 7.10).

Because glycerol is a carbohydrate, it enters the process of cellular respi-ration during glycolysis. Fatty acids can be metabolized to acetyl groups, which enter the citric acid cycle. A fatty acid with a chain of 18 carbons can make three times the number of acetyl groups as does glucose. For this reason, fats are an effi cient form of stored energy—there are three long fatty acid chains per fat molecule. The complete breakdown of glycerol and fatty acids to carbon dioxide and water results in many more ATP molecules per molecule than does the breakdown of glucose.

Only the hydrocarbon backbone of amino acids, not the amino group, can be used by the cellular respiration pathways. The amino group becomes ammonia (NH3), which becomes part of urea, the primary excretory product of humans. Just where the hydrocarbon backbone from an amino acid begins degradation to produce ATP molecules depends on its length. Figure 7.10 shows that the hydrocarbon backbone from an amino acid can enter cellular respiration pathways at pyruvate, at acetyl-CoA, or during the citric acid cycle.

The smaller molecules in Figure 7.10 can also be used to synthesize larger molecules. In such instances ATP is used instead of generated. You already know that amino acids can be employed to synthesize proteins. Also, some sub-strates of the citric acid cycle can become amino acids through the addition of an amino group. Of the 20 most common amino acids, humans have the ability to synthesize 11 amino acids in this way, but we cannot synthesize the other 9. These nine are called the essential amino acids, meaning that they must be present in the diet or else we suffer a protein defi ciency.

Similarly, substrates from glycolysis can become glycerol, and acetyl groups can be used to produce fatty acids. When glycerol and three fatty acids join, a fat results. This explains why you can gain weight from eating carbohydrate-rich foods.

Figure 7.9 Calculating ATP energy yield per glucose

molecule.

Substrate-level ATP synthesis during glycolysis and the citric acid

cycle accounts for 4 ATP. The electron transport chain produces a

maximum of 34 ATP, and the maximum total is 38 ATP. Some cells,

however, produce only 36 ATP per glucose molecule or even less.

glucose

pyruvate

P2

P2

–2 ATP 2 ADP 2 ATP

+4 ATP

Net gain: 2 ATP

4 ADP4 ATP

P P2

2

2 NAD+

2 NADH

2 NADH

2 NAD+

2 lactate

AthleteWine

Bread

or

2 CO2

2 ethylalcohol

7.4 FermentationFermentation is the anaerobic breakdown of glucose resulting in the buildup of 2 ATP and lactate, a toxic by-product (Fig. 7.11). During fermentation in animal cells, the pyruvate formed by glycolysis accepts 2 hydrogen atoms and is reduced to lactate. Notice in Figure 7.11 that 2 NADH pass hydrogen atoms to pyruvate, reducing it. Why is it benefi cial for pyruvate to be reduced to lactate when oxygen is not available? The answer is that this reaction regener-ates NAD�, which can then pick up more electrons during the earlier reactions of glycolysis. This keeps glycolysis going, during which ATP is produced by substrate-level ATP synthesis.

The 2 ATP produced by fermentation represent only a small fraction of the potential energy stored in a glucose molecule. Following fermentation, most of this potential energy is still waiting to be released.

The inputs and outputs of fermentation are as follows:

Check Your Progress

What are the drawbacks and benefi ts of fermentation?

Answer: Drawbacks: Most of the energy in a glucose molecule is unused and it results in a toxic end product. Benefi ts: The 2 ATP gained can be used as a burst of energy when oxygen is not available for complete glucose breakdown.

Fermentation

inputs outputs

glucose

ADP +

2 alcohol and 2 CO2

2 lactate or

2 ATP net

2 ATP

P

Despite its low yield of only 2 ATP, fermentation is essential. It can provide a rapid burst of ATP, and muscle cells are more apt than other cells to carry on fermentation. When our muscles are working vigorously over a short period of time, as when we run, fermentation is a way to pro-duce ATP even though oxygen is temporarily in limited supply.

However, one of its by-products, lactate, is toxic to cells. At fi rst, blood carries away all the lactate formed in muscles. But eventually, lac-tate begins to build up, changing the pH and causing the muscles to “burn” and then to fatigue so that they no longer contract. When we stop running, our bodies are in oxygen defi cit, as signifi ed by the fact that we continue to breathe very heavily for a time. Recovery is complete when all the lactate is transported to the liver, where it is reconverted to pyruvate. Some of the pyruvate is oxidized completely, and the rest is converted back to glucose.

Microorganisms and FermentationBacteria utilize fermentation to produce an organic acid, such as lactate, or an alcohol and CO2, depending on the type of bacterium.

Yeasts are good examples of microorganisms that generate ethyl alcohol and CO2 when they carry out fermentation. When yeast is used to leaven bread, the CO2 makes the bread rise. When yeast is used to ferment grapes for wine production or to ferment wort—derived from barley—for beer production, ethyl alcohol is the desired product.

Figure 7.11 Fermentation.

Fermentation consists of glycolysis followed by a reduction of pyruvate by

NADH. This regenerates NAD�, which returns to the glycolytic pathway to

pick up more hydrogen atoms.

108 Part I The Cell

Summary7.1 Cellular Respiration

During cellular respiration, glucose from food is oxidized to CO2, which we exhale. Oxygen (O2), which we breathe in, is reduced to H2O. When glucose is oxidized, energy is released. Cellular respiration captures the energy of oxidation and uses it to produce ATP molecules. The following equation gives an overview of these events:

Citric Acid CycleAcetyl groups enter the citric acid cycle, a series of reactions occurring in the mitochondrial matrix. During one turn of the cycle, oxidation results in 2 CO2 molecules, 3 NADH molecules, and 1 FADH. One turn also produces 1 ATP molecule. The cycle must turn twice per glucose molecule.

Electron Transport ChainThe fi nal stage of cellular respiration involves the electron transport chain located in the cristae of the mitochondria. The chain is a series of electron carriers that accept electrons (e�) from NADH and FADH2 and pass them along until they are fi nally received by oxygen, which combines with H� to produce water.

The carriers of the electron transport chain are located in molecular complexes on the cristae of mitochondria. These carriers capture energy from the passage of electrons and use it to pump H� into the intermembrane space of the mitochondrion. When H� fl ows down its gradient into the matrix through an ATP synthase complex, energy is released and used to form ATP molecules from ADP and �P .

Energy YieldOf the maximum 38 ATP formed by complete glucose breakdown, 4 are the result of substrate-level ATP synthesis, and the rest are produced as a result of the electron transport chain and ATP synthase:

THE CHAPTER IN REVIEW

7.2 Outside the Mitochondria: GlycolysisGlycolysis, the breakdown of glucose to 2 molecules of pyruvate, is a series of enzymatic reactions that occur in the cytoplasm. During glycolysis:

• Glucose is oxidized by removal of hydrogen atoms. • When NAD� accepts these electrons, NADH results.

Breakdown releases enough energy to immediately give a net gain of 2 ATP by substrate-level ATP synthesis. The inputs and outputs of glycolysis are summarized here:

Alternative Metabolic PathwaysBesides carbohydrates, glycerol and fatty acids from fats, and amino acids from proteins can undergo cellular respiration by entering glycolysis and/or the citric acid cycle. These metabolic pathways also provide substrates for the synthesis of fats and proteins.

7.4 FermentationFermentation involves glycolysis followed by the reduction of pyruvate by NADH, either to lactate or to alcohol and CO2. The reduction of pyruvate regenerates NAD�, which can accept more hydrogen atoms from glycolysis.

• Although fermentation results in only 2 ATP molecules, it still provides a quick burst of ATP energy for short-term, strenuous muscular activity.

• The accumulation of lactate puts the individual in oxygen defi cit, which is the amount of oxygen needed when lactate is completely metabolized to CO2 and H2O.

ATPC6H12O6

glucose6 O2 6CO2+ 6H2O+ +

Glycolysis

inputs outputs

glucose

2 NAD+

ADP +

2 NADH

2 pyruvate

2 ATP

4 ATP

2 ATP net

P

2 acetyl-CoA

Cytoplasm

Glycolysisglucose pyruvate Citric acid


10 NADH and 2 FADH2

6 NADH and 2 FADH2

e–

2 ATP 2 ATP 34 ATP

2 NADH 2 NADH

When oxygen is available, pyruvate from glycolysis enters a mitochondrion.

7.3 Inside the Mitochondria

Preparatory ReactionDuring the preparatory reaction in the matrix:

• Oxidation occurs as CO2 is removed from pyruvate. • NAD� accepts hydrogen atoms, and NADH results. • An acetyl group, the end product, combines with CoA.

This reaction takes place twice per glucose molecule.


Thinking Scientifi cally 1. Occasionally, you’ll hear a news story about a bin of grain

that has undergone spontaneous combustion, resulting in a spectacular fi re. It may seem odd that wet grain is more likely to burn than dry grain. However, the grain contains living plant seeds that are physiologically more active when moist than when dry. In addition, the surfaces of the kernels of grain are covered with microorganisms that increase their growth rates when moist. Explain how the consumption of oxygen by these organisms can contribute to a grain-bin fi re.

2. One of the major risk factors for diabetes in the elderly is insulin resistance, which is decreased tissue sensitivity to the action of insulin. Tissues then compensate by increasing insulin secretion. Insulin resistance can result from the accumulation of fatty acids in muscle and liver tissue. Researchers have recently found a connection between fatty acid accumulation and mitochondrial function in elderly people. Logically, what might be this connection? Using this knowledge, how might elderly people reduce their risk of diabetes?

Testing YourselfChoose the best answer for each question.

1. During cellular respiration, _________ is oxidized and _________ is reduced.

a. glucose, oxygen b. glucose, water c. oxygen, water d. water, oxygen e. oxygen, carbon dioxide

2. The products of cellular respiration are energy and

a. water. b. oxygen. c. water and carbon dioxide. d. oxygen and carbon dioxide. e. oxygen and water.

3. During the energy-harvesting steps of glycolysis, which are produced?

a. ATP and NADH c. ATP and NAD b. ADP and NADH d. ADP and NAD

4. The end product of glycolysis is

a. phosphoenol pyruvate. c. phosphoglyceraldehyde. b. glucose. d. pyruvate.

5. Acetyl-CoA is the end product of

a. glycolysis. c. the citric acid cycle. b. the preparatory reaction. d. the electron transport chain.

6. The citric acid cycle results in the release of

a. carbon dioxide. c. oxygen. b. pyruvate. d. water.

7. The following reactions occur in the matrix of the mitochondria:

a. glycolysis and the preparatory reaction b. the preparatory reaction and the citric acid cycle c. the citric acid cycle and the electron transport chain d. the electron transport chain and glycolysis

8. Match the descriptions below to the lettered events in the preparatory reaction and citric acid cycle.

Pyruvate is broken down to an acetyl group. Acetyl group is taken up and a C6 molecule results. Oxidation results in NADH and CO2. ATP is produced by substrate-level ATP synthesis. Oxidation produces more NADH and FADH2.

9. The strongest and fi nal electron acceptor in the electron transport chain is

a. NADH. c. oxygen. b. FADH2. d. water.

For questions 10–16, match the items to those in the key. Answers can be used more than once, and each question can have more than one answer.

Key: a. glycolysis b. preparatory reaction c. citric acid cycle d. electron transport chain

10. Produces ATP.

11. Uses ATP.

12. Produces NADH.

13. Uses NADH.

14. Produces carbon dioxide.

15. Occurs in cytoplasm.

16. Occurs in mitochondria.

17. The carriers in the electron transport chain undergo

a. oxidation only. b. reduction only. c. oxidation and reduction. d. the loss of hydrogen ions. e. the gain of hydrogen ions.

2 pyruvate

NADH2

CO2

2 NAD+

2 CoA

2 CoA

CO2

CO2

NADH

FADH2

NAD+

FAD

NAD+ NADH

NAD+ NADH

Citric acidcycle

Preparatoryreaction

ATP

ADP + P

a.

b.

c.

d.

e.

110 Part I The Cell

18. The fi nal acceptor for hydrogen atoms during fermentation is

a. O2. c. FAD. b. acetyl CoA. d. pyruvic acid.

19. Which of the following do not enter the cellular respiration pathways?

a. fats c. nucleic acids b. amino acids d. carbohydrates

20. When animals carry out fermentation, they produce _________, while yeasts produce _________.

a. lactate, malate b. lactate, ethyl alcohol c. malate, ethyl alcohol d. malate, lactate e. ethyl alcohol, lactate

21. Fermentation does not yield as much ATP as cellular respiration does because fermentation

a. generates mostly heat. b. makes use of only a small amount of the potential energy in

glucose. c. creates by-products that require large amounts of ATP to

break down. d. creates ATP molecules that leak into the cytoplasm and are

broken down.

22. Which type of human cells carries on the most fermentation?

a. fat b. muscle c. nerve d. bone

23. Cellular respiration cannot occur without

a. sodium. b. oxygen. c. lactate. d. All of these are correct.

24. The metabolic process that produces the most ATP molecules is

a. glycolysis. b. the citric acid cycle. c. the electron transport chain. d. fermentation.

25. The greatest contributor of electrons to the electron transport chain is

a. oxygen. b. glycolysis. c. the citric acid cycle. d. the preparatory reaction. e. fermentation.

26. Substrate-level ATP synthesis takes place in

a. glycolysis and the citric acid cycle. b. the electron transport chain and the preparatory reaction. c. glycolysis and the electron transport chain. d. the citric acid cycle and the preparatory reaction.

27. Which of the following is not true of fermentation? Fermentation

a. has a net gain of only 2 ATP. b. occurs in the cytoplasm. c. donates electrons to the electron transport chain. d. begins with glucose. e. is carried on by yeast.

28. Match the terms to their defi nitions. Only four of these terms are needed:

anaerobic oxygen defi citcitric acid cycle pyruvatefermentation preparatory reaction

a. Occurs in mitochondria and produces CO2, ATP, NADH, and FADH2.

b. Growing or metabolizing in the absence of oxygen. c. End product of glycolysis. d. Anaerobic breakdown of glucose that results in a gain of

2 ATP and end products such as alcohol and lactate.

Go to www.mhhe.com/maderessentials for more quiz questions.

Bioethical IssueFor millennia, humans have taken advantage of a product of fermentation in microbes—ethyl alcohol. However, alcohol is toxic to human cells, and alcohol abuse is a serious problem from many perspectives. For example, a woman who consumes large amounts of alcohol during pregnancy may cause her child to have fetal alcohol syndrome. Children with this syndrome suffer from mental retardation and physical problems. Some people believe the unborn child has a right to be protected from harm and argue that intervention is justifi ed if a pregnant woman drinks heavily despite her doctor’s orders. In extreme cases, it may be necessary to incarcerate the woman throughout the pregnancy. The alternative point of view is that every person has a right to freedom of choice, and society has no authority to intervene in the life of a pregnant woman. In addition, forcing a pregnant woman to follow medical treatment against her will, for the sake of her fetus, is imposing an obligation that we do not impose on others—that is, other members of our society are not forced to change their lifestyles purely for the sake of others.

Do you think society has an obligation to do whatever is necessary to prevent women from drinking excessively during pregnancy? Will this lead to attempts to control the lives of pregnant women in other ways as well, such as requiring them to exercise more or to abstain from caffeine?

Understanding the Termsacetyl-CoA 102citric acid cycle 99, 102coenzyme A (CoA) 99electron transport chain 99fermentation 107

glycolysis 99intermembrane space 105oxygen defi cit 107preparatory (prep) reaction 99, 102

Match the terms to these defi nitions:

a. _______________ First step in cellular respiration.

b. _______________ This metabolic pathway breaks down pyruvate in the mitochondrial matrix.

c. _______________ Reduction of pyruvate when oxygen is not available.

d. _______________ Hydrogen ions are pumped into this region during the electron transport chain.

e. _______________ Acetyl-CoA is needed during this phase of cellular respiration.

energy for cells chapter outline - mcgraw-hill

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