how cells harvest energy - usp159 9 how cells harvest energy concept outline 9.1 cells harvest the...

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9How Cells Harvest Energy

Concept Outline

9.1 Cells harvest the energy in chemical bonds.

Using Chemical Energy to Drive Metabolism. Theenergy in C—H, C—O, and other chemical bonds can becaptured and used to fuel the synthesis of ATP.

9.2 Cellular respiration oxidizes food molecules.

An Overview of Glucose Catabolism. The chemicalenergy in sugar is harvested by both substrate-levelphosphorylation and by aerobic respiration.Stage One: Glycolysis. The 10 reactions of glycolysiscapture energy from glucose by reshuffling the bonds.Stage Two: The Oxidation of Pyruvate. Pyruvate, theproduct of glycolysis, is oxidized to acetyl-CoA.Stage Three: The Krebs Cycle. In a series of reactions,electrons are stripped from acetyl-CoA.Harvesting Energy by Extracting Electrons. Therespiration of glucose is a series of oxidation-reductionreactions which involve stripping electrons from glucose andusing the energy of these electrons to power the synthesis ofATP. Stage Four: The Electron Transport Chain. Theelectrons harvested from glucose pass through a chain ofmembrane proteins that use the energy to pump protons,driving the synthesis of ATP.Summarizing Aerobic Respiration. The oxidation ofglucose by aerobic respiration in eukaryotes produces up tothree dozen ATP molecules, over half the energy in thechemical bonds of glucose.Regulating Aerobic Respiration. High levels of ATPtend to shut down cellular respiration by feedback-inhibitingkey reactions.

9.3 Catabolism of proteins and fats can yieldconsiderable energy.

Glucose Is Not the Only Source of Energy. Proteinsand fats are dismantled and the products fed into cellularrespiration.

9.4 Cells can metabolize food without oxygen.

Fermentation. Fermentation allows continued metabolismin the absence of oxygen by donating the electrons harvestedin glycolysis to organic molecules.

Life is driven by energy. All the activities organismscarry out—the swimming of bacteria, the purring of a

cat, your reading of these words—use energy. In this chap-ter, we will discuss the processes all cells use to derivechemical energy from organic molecules and to convertthat energy to ATP. We will consider photosynthesis,which uses light energy rather than chemical energy, in de-tail in chapter 10. We examine the conversion of chemicalenergy to ATP first because all organisms, both photosyn-thesizers and the organisms that feed on them (like thefield mice in figure 9.1), are capable of harvesting energyfrom chemical bonds. As you will see, though, this processand photosynthesis have much in common.

FIGURE 9.1Harvesting chemical energy. Organisms such as these harvestmice depend on the energy stored in the chemical bonds of thefood they eat to power their life processes.

fireplace. In both instances, the reactants are carbohydratesand oxygen, and the products are carbon dioxide, water,and energy:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (heat or ATP)

The change in free energy in this reaction is –720 kilo-calories (–3012 kilojoules) per mole of glucose under theconditions found within a cell (the traditional value of–686 kilocalories, or –2870 kJ, per mole refers to stan-dard conditions—room temperature, one atmosphere ofpressure, etc.). This change in free energy results largelyfrom the breaking of the six C—H bonds in the glucosemolecule. The negative sign indicates that the productspossess less free energy than the reactants. The sameamount of energy is released whether glucose is catabo-lized or burned, but when it is burned most of the energyis released as heat. This heat cannot be used to performwork in cells. The key to a cell’s ability to harvest usefulenergy from the catabolism of food molecules such asglucose is its conversion of a portion of the energy into amore useful form. Cells do this by using some of the en-ergy to drive the production of ATP, a molecule that canpower cellular activities.

160 Part III Energetics

Using Chemical Energy to Drive MetabolismPlants, algae, and some bacteria harvest the en-ergy of sunlight through photosynthesis, convert-ing radiant energy into chemical energy. Theseorganisms, along with a few others that usechemical energy in a similar way, are called au-totrophs (“self-feeders”). All other organismslive on the energy autotrophs produce and arecalled heterotrophs (“fed by others”). At least95% of the kinds of organisms on earth—all ani-mals and fungi, and most protists and bacteria—are heterotrophs.

Where is the chemical energy in food, andhow do heterotrophs harvest it to carry out themany tasks of living (figure 9.2)? Most foodscontain a variety of carbohydrates, proteins,and fats, all rich in energy-laden chemicalbonds. Carbohydrates and fats, for example,possess many carbon-hydrogen (C—H), as wellas carbon-oxygen (C—O) bonds. The job of ex-tracting energy from this complex organic mix-ture is tackled in stages. First, enzymes breakthe large molecules down into smaller ones, aprocess called digestion. Then, other enzymesdismantle these fragments a little at a time, har-vesting energy from C—H and other chemicalbonds at each stage. This process is called ca-tabolism.

While you obtain energy from many of the constituentsof food, it is traditional to focus first on the catabolism ofcarbohydrates. We will follow the six-carbon sugar, glucose(C6H12O6), as its chemical bonds are progressively har-vested for energy. Later, we will come back and examinethe catabolism of proteins and fats.

Cellular Respiration

The energy in a chemical bond can be visualized as poten-tial energy borne by the electrons that make up the cova-lent bond. Cells harvest this energy by putting the elec-trons to work, often to produce ATP, the energy currencyof the cell. Afterward, the energy-depleted electron (associ-ated with a proton as a hydrogen atom) is donated to someother molecule. When oxygen gas (O2) accepts the hydro-gen atom, water forms, and the process is called aerobicrespiration. When an inorganic molecule other than oxy-gen accepts the hydrogen, the process is called anaerobicrespiration. When an organic molecule accepts the hydro-gen atom, the process is called fermentation.

Chemically, there is little difference between the catabo-lism of carbohydrates in a cell and the burning of wood in a


FIGURE 9.2Start every day with a good breakfast. The carbohydrates, proteins, and fatsin this fish contain energy that the bear’s cells can use to power their dailyactivities.

The ATP Molecule

Adenosine triphosphate (ATP) is the energy currency ofthe cell, the molecule that transfers the energy capturedduring respiration to the many sites that use energy in thecell. How is ATP able to transfer energy so readily? Recallfrom chapter 8 that ATP is composed of a sugar (ribose)bound to an organic base (adenine) and a chain of threephosphate groups. As shown in figure 9.3, each phosphategroup is negatively charged. Because like charges repeleach other, the linked phosphate groups push against thebond that holds them together. Like a cocked mousetrap,the linked phosphates store the energy of their electrostaticrepulsion. Transferring a phosphate group to another mol-ecule relaxes the electrostatic spring of ATP, at the sametime cocking the spring of the molecule that is phosphory-lated. This molecule can then use the energy to undergosome change that requires work.

How Cells Use ATP

Cells use ATP to do most of those activities that requirework. One of the most obvious is movement. Some bacte-ria swim about, propelling themselves through the water byrapidly spinning a long, tail-like flagellum, much as a shipmoves by spinning a propeller. During your developmentas an embryo, many of your cells moved about, crawlingover one another to reach new positions. Movement alsooccurs within cells. Tiny fibers within muscle cells pullagainst one another when muscles contract. Mitochondriapass a meter or more along the narrow nerve cells that con-nect your feet with your spine. Chromosomes are pulled bymicrotubules during cell division. All of these movementsby cells require the expenditure of ATP energy.

A second major way cells use ATP is to drive endergonicreactions. Many of the synthetic activities of the cell are en-dergonic, because building molecules takes energy. Thechemical bonds of the products of these reactions containmore energy, or are more organized, than the reactants.The reaction can’t proceed until that extra energy is sup-plied to the reaction. It is ATP that provides this neededenergy.

How ATP Drives Endergonic Reactions

How does ATP drive an endergonic reaction? The en-zyme that catalyzes the endergonic reaction has two bind-ing sites on its surface, one for the reactant and anotherfor ATP. The ATP site splits the ATP molecule, liberat-ing over 7 kcal (30 kJ) of chemical energy. This energypushes the reactant at the second site “uphill,” driving theendergonic reaction. (In a similar way, you can makewater in a swimming pool leap straight up in the air, de-spite the fact that gravity prevents water from rising spon-taneously—just jump in the pool! The energy you addgoing in more than compensates for the force of gravityholding the water back.)

When the splitting of ATP molecules drives an energy-requiring reaction in a cell, the two parts of the reaction—ATP-splitting and endergonic—take place in concert. Insome cases, the two parts both occur on the surface of thesame enzyme; they are physically linked, or “coupled,” liketwo legs walking. In other cases, a high-energy phosphatefrom ATP attaches to the protein catalyzing the ender-gonic process, activating it (figure 9.4). Coupling energy-requiring reactions to the splitting of ATP in this way isone of the key tools cells use to manage energy.

The catabolism of glucose into carbon dioxide andwater in living organisms releases about 720 kcal(3012 kJ) of energy per mole of glucose. This energy iscaptured in ATP, which stores the energy by linkingcharged phosphate groups near one another. When thephosphate bonds in ATP are hydrolyzed, energy isreleased and available to do work.

Chapter 9 How Cells Harvest Energy 161

Triphosphate group

Sugar

Adenine

NH2

OP CH2

O

O

O–

P

O

O

O–

P

O

–O

O–

OH OH

O

N

N

N

N

FIGURE 9.3Structure of the ATP molecule. ATP is composed of an organicbase and a chain of phosphates attached to opposite ends of a five-carbon sugar. Notice that the charged regions of the phosphatechain are close to one another. These like charges tend to repelone another, giving the bonds that hold them together aparticularly high energy transfer potential.

ADP

Inactive ActiveATP

P

FIGURE 9.4How ATP drives an endergonic reaction. In many cases, aphosphate group split from ATP activates a protein, catalyzing anendergonic process.

An Overview of GlucoseCatabolismCells are able to make ATP from thecatabolism of organic molecules in twodifferent ways.

1. Substrate-level phosphoryla-tion. In the first, calledsubstrate- level phosphoryla-tion, ATP is formed by transfer-ring a phosphate group directlyto ADP from a phosphate-bear-ing intermediate (figure 9.5).During glycolysis, discussedbelow, the chemical bonds of glu-cose are shifted around in reac-tions that provide the energy re-quired to form ATP.

2. Aerobic respiration. In thesecond, called aerobic respira-tion, ATP forms as electrons areharvested, transferred along the electron transportchain, and eventually donated to oxygen gas. Eukary-otes produce the majority of their ATP from glucosein this way.

In most organisms, these two processes are combined.To harvest energy to make ATP from the sugar glucose inthe presence of oxygen, the cell carries out a complex se-ries of enzyme-catalyzed reactions that occur in fourstages: the first stage captures energy by substrate-levelphosphorylation through glycolysis, the following threestages carry out aerobic respiration by oxidizing the endproduct of glycolysis.

Glycolysis

Stage One: Glycolysis. The first stage of extracting en-ergy from glucose is a 10-reaction biochemical pathwaycalled glycolysis that produces ATP by substrate-levelphosphorylation. The enzymes that catalyze the glycolyticreactions are in the cytoplasm of the cell, not bound to anymembrane or organelle. Two ATP molecules are used upearly in the pathway, and four ATP molecules are formedby substrate-level phosphorylation. This yields a net oftwo ATP molecules for each molecule of glucose catabo-lized. In addition, four electrons are harvested as NADHthat can be used to form ATP by aerobic respiration. Still,the total yield of ATP is small. When the glycolyticprocess is completed, the two molecules of pyruvate thatare formed still contain most of the energy the originalglucose molecule held.

Aerobic Respiration

Stage Two: Pyruvate Oxidation. In the second stage,pyruvate, the end product from glycolysis, is converted intocarbon dioxide and a two-carbon molecule called acetyl-CoA. For each molecule of pyruvate converted, one mole-cule of NAD+ is reduced to NADH.

Stage Three: The Krebs Cycle. The third stage intro-duces this acetyl-CoA into a cycle of nine reactions calledthe Krebs cycle, named after the British biochemist, SirHans Krebs, who discovered it. (The Krebs cycle is alsocalled the citric acid cycle, for the citric acid, or citrate,formed in its first step, and less commonly, the tricar-boxylic acid cycle, because citrate has three carboxylgroups.) In the Krebs cycle, two more ATP molecules areextracted by substrate-level phosphorylation, and a largenumber of electrons are removed by the reduction ofNAD+ to NADH.

Stage Four: Electron Transport Chain. In the fourthstage, the energetic electrons carried by NADH are em-ployed to drive the synthesis of a large amount of ATP bythe electron transport chain.

Pyruvate oxidation, the reactions of the Krebs cycle, andATP production by electron transport chains occur withinmany forms of bacteria and inside the mitochondria of alleukaryotes. Recall from chapter 5 that mitochondria arethought to have evolved from bacteria. Although plants andalgae can produce ATP by photosynthesis, they also pro-duce ATP by aerobic respiration, just as animals and othernonphotosynthetic eukaryotes do. Figure 9.6 provides anoverview of aerobic respiration.



P

PEP

Enzyme

ADP

Adenosine

P

P

Pyruvate

ATP

Adenosine

P

P

P

FIGURE 9.5Substrate-level phosphorylation. Some molecules, such as phosphoenolpyruvate (PEP),possess a high-energy phosphate bond similar to the bonds in ATP. When PEP’sphosphate group is transferred enzymatically to ADP, the energy in the bond is conservedand ATP is created.

Anaerobic Respiration

In the presence of oxygen, cells can respire aerobically,using oxygen to accept the electrons harvested from foodmolecules. In the absence of oxygen to accept the electrons,some organisms can still respire anaerobically, using inor-ganic molecules to accept the electrons. For example,many bacteria use sulfur, nitrate, or other inorganic com-pounds as the electron acceptor in place of oxygen.

Methanogens. Among the heterotrophs that practiceanaerobic respiration are primitive archaebacteria such asthe thermophiles discussed in chapter 4. Some of these,called methanogens, use CO2 as the electron acceptor, re-ducing CO2 to CH4 (methane) with the hydrogens derivedfrom organic molecules produced by other organisms.

Sulfur Bacteria. Evidence of a second anaerobic respi-ratory process among primitive bacteria is seen in agroup of rocks about 2.7 billion years old, known as theWoman River iron formation. Organic material in theserocks is enriched for the light isotope of sulfur, 32S, rela-

tive to the heavier isotope 34S. No known geochemicalprocess produces such enrichment, but biological sulfurreduction does, in a process still carried out today by cer-tain primitive bacteria. In this sulfate respiration, thebacteria derive energy from the reduction of inorganicsulfates (SO4) to H2S. The hydrogen atoms are obtainedfrom organic molecules other organisms produce. Thesebacteria thus do the same thing methanogens do, butthey use SO4 as the oxidizing (that is, electron-accepting)agent in place of CO2.

The sulfate reducers set the stage for the evolution ofphotosynthesis, creating an environment rich in H2S. Asdiscussed in chapter 8, the first form of photosynthesis ob-tained hydrogens from H2S using the energy of sunlight.

In aerobic respiration, the cell harvests energy fromglucose molecules in a sequence of four majorpathways: glycolysis, pyruvate oxidation, the Krebscycle, and the electron transport chain. Oxygen is thefinal electron acceptor. Anaerobic respiration donatesthe harvested electrons to other inorganic compounds.


NADH

NADH

H2O

CO2

Extracellular fluid

Lactate

Mitochondrion

Plasma membrane

ATP

ATP

ATP

Electron transport system

O2

NADH

Krebscycle

Acetyl-CoA

Pyruvate

Cytoplasm

GlycolysisGlucose

FIGURE 9.6An overview of aerobic respiration.

Stage One: GlycolysisThe metabolism of primitive organisms focused on glu-cose. Glucose molecules can be dismantled in many ways,but primitive organisms evolved a glucose-catabolizingprocess that releases enough free energy to drive the syn-thesis of ATP in coupled reactions. This process, calledglycolysis, occurs in the cytoplasm and involves a se-quence of 10 reactions that convert glucose into 2 three-carbon molecules of pyruvate (figure 9.7). For each mole-cule of glucose that passes through this transformation,the cell nets two ATP molecules by substrate-level phos-phorylation.

Priming

The first half of glycolysis consists of five sequential reac-tions that convert one molecule of glucose into two mole-cules of the three-carbon compound, glyceraldehyde 3-phosphate (G3P). These reactions demand the expenditureof ATP, so they are an energy-requiring process.

Step A: Glucose priming. Three reactions “prime”glucose by changing it into a compound that can becleaved readily into 2 three-carbon phosphorylated mole-cules. Two of these reactions require the cleavage of ATP,so this step requires the cell to use two ATP molecules.Step B: Cleavage and rearrangement. In the first ofthe remaining pair of reactions, the six-carbon productof step A is split into 2 three-carbon molecules. One isG3P, and the other is then converted to G3P by the sec-ond reaction (figure 9.8).


OVERVIEW OF GLYCOLYSIS1 2 3

(Starting material)

6-carbon sugar diphosphate

6-carbon glucose

2

P P

6-carbon sugar diphosphate

P P

3-carbon sugarphosphate

P


P


P


P

Priming reactions. Glycolysis beginswith the addition of energy. Two high-energy phosphates from two moleculesof ATP are added to the six-carbon molecule glucose, producing a six-carbonmolecule with two phosphates.

3-carbonpyruvate

2

NADH

Cleavage reactions. Then, the six-carbon molecule with two phosphates is split in two, forming two three-carbon sugar phosphates.

Energy-harvesting reactions. Finally, ina series of reactions, each of the two three-carbon sugar phosphates is converted to pyruvate. In the process, an energy-rich hydrogen is harvested as NADH, and two ATP molecules are formed.

3-carbonpyruvate

ATP

ATP 2

NADH

ATP

FIGURE 9.7How glycolysis works.


1. Phosphorylation of glucose by ATP.

Glucose

Hexokinase

Phosphoglucoisomerase

Phosphofructokinase

Glyceraldehyde 3-phosphate (G3P)

Pi Pi

Dihydroxyacetonephosphate

Glucose 6-phosphate

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Isomerase

Triose phosphatedehydrogenase

Aldolase

1,3-Bisphosphoglycerate(BPG)

1,3-Bisphosphoglycerate(BPG)

3-Phosphoglycerate(3PG)




Phosphoenolpyruvate(PEP)

Phosphoenolpyruvate(PEP)

Pyruvate Pyruvate

2–3. Rearrangement, followed by a second ATP phosphorylation.

4–5. The six-carbon molecule is split into two three-carbon molecules—one G3P, another that is converted into G3P in another reaction.

6. Oxidation followed by phosphorylation produces two NADH molecules and two molecules of BPG, each with one high-energy phosphate bond.

7. Removal of high-energy phosphate by two ADP molecules produces two ATP molecules and leaves two 3PG molecules.

8–9. Removal of water yields two PEP molecules, each with a high-energy phosphate bond.

10. Removal of high-energy phosphate by two ADP molecules produces two ATP molecules and two pyruvate molecules.

1

2

3

6

Phosphoglycerokinase

7

Phosphoglyceromutase

8

EnolaseH2O H2O9

Pyruvate kinase

10

4,5

ADP

ATP

ADP

ATP

ADP

ATP

ADP

ATP

ADP

ATP

ADP

ATP

NADH

NAD+

NADH

NAD+

O

C O

O

CH2OH

CH2P

C O

C

O–

O

CH3

PC O

C

O–

O

CH2

PC OH

C

O–

O

CH2OH

CHOH

PO

C

O–

O

CH2

CHOH

PO

C

H

O

CH2

POCH2P O CH2

OPO

CH2OH

CH2OH

CH2

POCH2

CHOH

P

P

O

O C O

CH2

O

O

FIGURE 9.8The glycolytic pathway. The first five reactions convert a molecule of glucose into two molecules of G3P. The second five reactionsconvert G3P into pyruvate.

Substrate-Level Phosphorylation

In the second half of glycolysis, five more reactions convertG3P into pyruvate in an energy-yielding process that gen-erates ATP. Overall, then, glycolysis is a series of 10 en-zyme-catalyzed reactions in which some ATP is invested inorder to produce more.

Step C: Oxidation. Two electrons and one proton aretransferred from G3P to NAD+, forming NADH. Notethat NAD+ is an ion, and that both electrons in the newcovalent bond come from G3P.Step D: ATP generation. Four reactions convertG3P into another three-carbon molecule, pyruvate. Thisprocess generates two ATP molecules (see figure 9.5).Because each glucose molecule is split into two G3Pmolecules, the overall reaction sequence yields two mol-ecules of ATP, as well as two molecules of NADH andtwo of pyruvate:

4 ATP (2 ATP for each of the 2 G3P molecules in step D)– 2 ATP (used in the two reactions in step A )

2 ATP

Under the nonstandard conditions within a cell, eachATP molecule produced represents the capture of about 12kcal (50 kJ) of energy per mole of glucose, rather than the7.3 traditionally quoted for standard conditions. Thismeans glycolysis harvests about 24 kcal/mole (100kJ/mole). This is not a great deal of energy. The total en-ergy content of the chemical bonds of glucose is 686 kcal(2870 kJ) per mole, so glycolysis harvests only 3.5% of thechemical energy of glucose.

Although far from ideal in terms of the amount of en-ergy it releases, glycolysis does generate ATP. For morethan a billion years during the anaerobic first stages oflife on earth, it was the primary way heterotrophic organ-isms generated ATP from organic molecules. Like manybiochemical pathways, glycolysis is believed to haveevolved backward, with the last steps in the process beingthe most ancient. Thus, the second half of glycolysis, theATP-yielding breakdown of G3P, may have been theoriginal process early heterotrophs used to generateATP. The synthesis of G3P from glucose would have ap-peared later, perhaps when alternative sources of G3Pwere depleted.

All Cells Use Glycolysis

The glycolytic reaction sequence is thought to have beenamong the earliest of all biochemical processes to evolve. Ituses no molecular oxygen and occurs readily in an anaero-bic environment. All of its reactions occur free in the cyto-plasm; none is associated with any organelle or membranestructure. Every living creature is capable of carrying out

glycolysis. Most present-day organisms, however, can ex-tract considerably more energy from glucose through aero-bic respiration.

Why does glycolysis take place even now, since its en-ergy yield in the absence of oxygen is comparatively sopaltry? The answer is that evolution is an incrementalprocess: change occurs by improving on past successes. Incatabolic metabolism, glycolysis satisfied the one essentialevolutionary criterion: it was an improvement. Cells thatcould not carry out glycolysis were at a competitive disad-vantage, and only cells capable of glycolysis survived theearly competition of life. Later improvements in catabolicmetabolism built on this success. Glycolysis was not dis-carded during the course of evolution; rather, it served asthe starting point for the further extraction of chemicalenergy. Metabolism evolved as one layer of reactionsadded to another, just as successive layers of paint coverthe walls of an old building. Nearly every present-day or-ganism carries out glycolysis as a metabolic memory of itsevolutionary past.

Closing the Metabolic Circle: The Regenerationof NAD+

Inspect for a moment the net reaction of the glycolytic se-quence:

Glucose + 2 ADP + 2 Pi + 2 NAD+ →2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

You can see that three changes occur in glycolysis: (1) glu-cose is converted into two molecules of pyruvate; (2) twomolecules of ADP are converted into ATP via substratelevel phosphorylation; and (3) two molecules of NAD+ arereduced to NADH.

The Need to Recycle NADH

As long as food molecules that can be converted into glu-cose are available, a cell can continually churn out ATP todrive its activities. In doing so, however, it accumulatesNADH and depletes the pool of NAD+ molecules. A celldoes not contain a large amount of NAD+, and for glycoly-sis to continue, NADH must be recycled into NAD+. Someother molecule than NAD+ must ultimately accept the hy-drogen atom taken from G3P and be reduced. Two mole-cules can carry out this key task (figure 9.9):

1. Aerobic respiration. Oxygen is an excellent elec-tron acceptor. Through a series of electron transfers,the hydrogen atom taken from G3P can be donatedto oxygen, forming water. This is what happens in thecells of eukaryotes in the presence of oxygen. Becauseair is rich in oxygen, this process is also referred to asaerobic metabolism.


2. Fermentation. When oxygen is unavailable, an or-ganic molecule can accept the hydrogen atom instead.Such fermentation plays an important role in the me-tabolism of most organisms (figure 9.10), even thosecapable of aerobic respiration.

The fate of the pyruvate that is produced by glycolysisdepends upon which of these two processes takes place.The aerobic respiration path starts with the oxidation ofpyruvate to a molecule called acetyl-CoA, which is thenfurther oxidized in a series of reactions called the Krebscycle. The fermentation path, by contrast, involves the re-duction of all or part of pyruvate. We will start by examin-ing aerobic respiration, then look briefly at fermentation.

Glycolysis generates a small amount of ATP byreshuffling the bonds of glucose molecules. Inglycolysis, two molecules of NAD+ are reduced toNADH. NAD+ must be regenerated for glycolysis tocontinue unabated.


NADH

Pyruvate

With oxygen Without oxygen

Acetyl-CoA

Lactate

Ethanol

NAD+

O2

NAD+

NADH

NAD+

NADH

CO2

Acetaldehyde

H2O

Krebscycle

FIGURE 9.9What happens to pyruvate, the product of glycolysis? In the presence of oxygen, pyruvate is oxidized to acetyl-CoA, which enters theKrebs cycle. In the absence of oxygen, pyruvate is instead reduced, accepting the electrons extracted during glycolysis and carried byNADH. When pyruvate is reduced directly, as in muscle cells, the product is lactate. When CO2 is first removed from pyruvate and theproduct, acetaldehyde, is then reduced, as in yeast cells, the product is ethanol.

FIGURE 9.10Fermentation. The conversion of pyruvate to ethanol takes placenaturally in grapes left to ferment on vines, as well as infermentation vats of crushed grapes. Yeasts carry out the process,but when their conversion increases the ethanol concentration toabout 12%, the toxic effects of the alcohol kill the yeast cells.What is left is wine.

Stage Two: The Oxidation of PyruvateIn the presence of oxygen, the oxidation of glucose that be-gins in glycolysis continues where glycolysis leaves off—with pyruvate. In eukaryotic organisms, the extraction ofadditional energy from pyruvate takes place exclusively in-side mitochondria. The cell harvests pyruvate’s consider-able energy in two steps: first, by oxidizing pyruvate toform acetyl-CoA, and then by oxidizing acetyl-CoA in theKrebs cycle.

Producing Acetyl-CoA

Pyruvate is oxidized in a single “decarboxylation” reactionthat cleaves off one of pyruvate’s three carbons. This car-bon then departs as CO2 (figure 9.11, top). This reactionproduces a two-carbon fragment called an acetyl group, aswell as a pair of electrons and their associated hydrogen,which reduce NAD+ to NADH. The reaction is complex,involving three intermediate stages, and is catalyzedwithin mitochondria by a multienzyme complex. As chapter8 noted, such a complex organizes a series of enzymaticsteps so that the chemical intermediates do not diffuseaway or undergo other reactions. Within the complex,component polypeptides pass the substrates from one en-zyme to the next, without releasing them. Pyruvate dehy-drogenase, the complex of enzymes that removes CO2 frompyruvate, is one of the largest enzymes known: it contains60 subunits! In the course of the reaction, the acetylgroup removed from pyruvate combines with a cofactorcalled coenzyme A (CoA), forming a compound known asacetyl-CoA:

Pyruvate + NAD+ + CoA → Acetyl-CoA + NADH + CO2

This reaction produces a molecule of NADH, which islater used to produce ATP. Of far greater significance thanthe reduction of NAD+ to NADH, however, is the produc-tion of acetyl-CoA (figure 9.11, bottom). Acetyl-CoA is im-portant because so many different metabolic processes gen-erate it. Not only does the oxidation of pyruvate, anintermediate in carbohydrate catabolism, produce it, butthe metabolic breakdown of proteins, fats, and other lipidsalso generate acetyl-CoA. Indeed, almost all molecules ca-tabolized for energy are converted into acetyl-CoA. Acetyl-CoA is then channeled into fat synthesis or into ATP pro-duction, depending on the organism’s energyrequirements. Acetyl-CoA is a key point of focus for themany catabolic processes of the eukaryotic cell.

Using Acetyl-CoA

Although the cell forms acetyl-CoA in many ways, only alimited number of processes use acetyl-CoA. Most of it iseither directed toward energy storage (lipid synthesis, forexample) or oxidized in the Krebs cycle to produce ATP.Which of these two options is taken depends on the levelof ATP in the cell. When ATP levels are high, the oxida-

tive pathway is inhibited, and acetyl-CoA is channeledinto fatty acid synthesis. This explains why many animals(humans included) develop fat reserves when they con-sume more food than their bodies require. Alternatively,when ATP levels are low, the oxidative pathway is stimu-lated, and acetyl-CoA flows into energy-producing oxida-tive metabolism.

In the second energy-harvesting stage of glucosecatabolism, pyruvate is decarboxylated, yielding acetyl-CoA, NADH, and CO2. This process occurs within themitochondrion.


Coenzyme A

Acetylgroup

Acetyl coenzyme A

ATPFat

CO2

Protein Lipid

NADH

NAD+

Pyruvate

Glycolysis

FIGURE 9.11The oxidation of pyruvate. This complex reaction involves thereduction of NAD+ to NADH and is thus a significant source ofmetabolic energy. Its product, acetyl-CoA, is the starting materialfor the Krebs cycle. Almost all molecules that are catabolized forenergy are converted into acetyl-CoA, which is then channeledinto fat synthesis or into ATP production.

Stage Three: The Krebs CycleAfter glycolysis catabolizes glucose to produce pyruvate,and pyruvate is oxidized to form acetyl-CoA, the thirdstage of extracting energy from glucose begins. In this thirdstage, acetyl-CoA is oxidized in a series of nine reactionscalled the Krebs cycle. These reactions occur in the matrixof mitochondria. In this cycle, the two-carbon acetyl groupof acetyl-CoA combines with a four-carbon molecule calledoxaloacetate (figure 9.12). The resulting six-carbon mole-cule then goes through a sequence of electron-yielding oxi-dation reactions, during which two CO2 molecules split off,restoring oxaloacetate. The oxaloacetate is then recycled tobind to another acetyl group. In each turn of the cycle, anew acetyl group replaces the two CO2 molecules lost, and

more electrons are extracted to drive proton pumps thatgenerate ATP.

Overview of the Krebs Cycle

The nine reactions of the Krebs cycle occur in two steps:

Step A: Priming. Three reactions prepare the six-carbon molecule for energy extraction. First, acetyl-CoA joins the cycle, and then chemical groups arerearranged.Step B: Energy extraction. Four of the six reactionsin this step are oxidations in which electrons are re-moved, and one generates an ATP equivalent directly bysubstrate-level phosphorylation.


OVERVIEW OF THE KREBS CYCLE1 2 3

Finally, the resulting four-carbon moleculeis further oxidized (hydrogens removedto form FADH2 and NADH). Thisregenerates the four-carbon startingmaterial, completing the cycle.

Then, the resulting six-carbon mole-cule is oxidized (a hydrogen removedto form NADH) and decarboxylated(a carbon removed to form CO2). Next,the five-carbon molecule is oxidized and decarboxylated again, and a coupled reaction generates ATP.

The Krebs cycle begins when a two-carbon fragment is transferred fromacetyl-CoA to a four-carbon molecule(the starting material).

5-carbon molecule4-carbon molecule

(Acetyl-CoA)

6-carbon molecule4-carbon molecule(Starting material)

CoA–

CoA

NADH

CO2

NADH

CO2

ATP

NADH

FADH2

6-carbon molecule

4-carbon molecule

4-carbon molecule(Starting material)

FIGURE 9.12How the Krebs cycle works.

The Reactions of the Krebs Cycle

The Krebs cycle consists of nine sequential reactions thatcells use to extract energetic electrons and drive the synthe-sis of ATP (figure 9.13). A two-carbon group from acetyl-CoA enters the cycle at the beginning, and two CO2 mole-cules and several electrons are given off during the cycle.

Reaction 1: Condensation. The two-carbon group fromacetyl-CoA joins with a four-carbon molecule, oxaloac-etate, to form a six-carbon molecule, citrate. This conden-sation reaction is irreversible, committing the two-carbonacetyl group to the Krebs cycle. The reaction is inhibitedwhen the cell’s ATP concentration is high and stimulatedwhen it is low. Hence, when the cell possesses ampleamounts of ATP, the Krebs cycle shuts down and acetyl-CoA is channeled into fat synthesis.

Reactions 2 and 3: Isomerization. Before the oxidationreactions can begin, the hydroxyl (—OH) group of citratemust be repositioned. This is done in two steps: first, awater molecule is removed from one carbon; then, water isadded to a different carbon. As a result, an —H group andan —OH group change positions. The product is an iso-mer of citrate called isocitrate.

Reaction 4: The First Oxidation. In the first energy-yielding step of the cycle, isocitrate undergoes an oxidativedecarboxylation reaction. First, isocitrate is oxidized, yield-ing a pair of electrons that reduce a molecule of NAD+ toNADH. Then the oxidized intermediate is decarboxylated;the central carbon atom splits off to form CO2, yielding afive-carbon molecule called α-ketoglutarate.

Reaction 5: The Second Oxidation. Next, α-ketoglutarateis decarboxylated by a multienzyme complex similar topyruvate dehydrogenase. The succinyl group left after theremoval of CO2 joins to coenzyme A, forming succinyl-CoA. In the process, two electrons are extracted, and theyreduce another molecule of NAD+ to NADH.

Reaction 6: Substrate-Level Phosphorylation. Thelinkage between the four-carbon succinyl group and CoA isa high-energy bond. In a coupled reaction similar to thosethat take place in glycolysis, this bond is cleaved, and the

energy released drives the phosphorylation of guanosinediphosphate (GDP), forming guanosine triphosphate(GTP). GTP is readily converted into ATP, and the four-carbon fragment that remains is called succinate.

Reaction 7: The Third Oxidation. Next, succinate isoxidized to fumarate. The free energy change in this re-action is not large enough to reduce NAD+. Instead,flavin adenine dinucleotide (FAD) is the electron accep-tor. Unlike NAD+, FAD is not free to diffuse within themitochondrion; it is an integral part of the inner mito-chondrial membrane. Its reduced form, FADH2, con-tributes electrons to the electron transport chain in themembrane.

Reactions 8 and 9: Regeneration of Oxaloacetate. Inthe final two reactions of the cycle, a water molecule isadded to fumarate, forming malate. Malate is then oxi-dized, yielding a four-carbon molecule of oxaloacetate andtwo electrons that reduce a molecule of NAD+ to NADH.Oxaloacetate, the molecule that began the cycle, is nowfree to combine with another two-carbon acetyl groupfrom acetyl-CoA and reinitiate the cycle.

The Products of the Krebs Cycle

In the process of aerobic respiration, glucose is entirelyconsumed. The six-carbon glucose molecule is first cleavedinto a pair of three-carbon pyruvate molecules during gly-colysis. One of the carbons of each pyruvate is then lost asCO2 in the conversion of pyruvate to acetyl-CoA; twoother carbons are lost as CO2 during the oxidations of theKrebs cycle. All that is left to mark the passing of the glu-cose molecule into six CO2 molecules is its energy, some ofwhich is preserved in four ATP molecules and in the re-duced state of 12 electron carriers. Ten of these carriers areNADH molecules; the other two are FADH2.

The Krebs cycle generates two ATP molecules permolecule of glucose, the same number generated byglycolysis. More importantly, the Krebs cycle and theoxidation of pyruvate harvest many energized electrons,which can be directed to the electron transport chain todrive the synthesis of much more ATP.



Oxaloacetate (4C)

Krebs cycle

Mitochondrial membrane

Oxidation of pyruvatePyruvate

Acetyl-CoA (2C)

CoA-SH

CoA-SH

Malate (4C)

Fumarate (4C)

Succinate (4C)

α-Ketoglutarate (5C)

Isocitrate (6C)

Citrate (6C)

Citratesynthetase

Succinyl-CoA (4C)The oxidationof succinateproduces FADH2.

The dehydrogenationof malate produces athird NADH, and thecycle returns to itsstarting point.

A second oxidativedecarboxylation producesa second NADH with therelease of a second CO2.

Oxidativedecarboxylationproduces NADHwith the release ofCO2.

The cycle beginswhen a 2C unit fromacetyl-CoA reactswith a 4C molecule(oxaloacetate) toproduce citrate (6C).

H2O

NADH

NAD+

NAD+

NADH

NAD+

NADH

NAD+

NADH

FAD

FADH2

CO

COO–

COO–

CH2

C

S CoA

O

CH3

COO–

COO–

CH2

CH2

CHHO

COO–

COO–

CH2

CH

COO–

COO–

HC

COO–

C O

S CoA

CH2

CH2

COO–

COO–

C O

CH2

CH2

COO–HC

CHHO

COO–

COO–

CH2

COO–CHO

COO–

COO–

CH2

CH2

CO2

CO2

CO2

1

Isocitratedehydrogenase4

Malatedehydrogenase9

Succinatedehydrogenase7

Fumarase8

α-Ketoglutaratedehydrogenase

Succinyl-CoAsynthetase

5

6

Aconitase2

3

CoA-SH

GTP GDP + Pi

ATPADP

FIGURE 9.13The Krebs cycle. This series of reactions takes place within the matrix of the mitochondrion. For the complete breakdown of a moleculeof glucose, the two molecules of acetyl-CoA produced by glycolysis and pyruvate oxidation will each have to make a trip around the Krebscycle. Follow the different carbons through the cycle, and notice the changes that occur in the carbon skeletons of the molecules as theyproceed through the cycle.

Harvesting Energy by ExtractingElectronsTo understand how cells direct some of the energy releasedduring glucose catabolism into ATP production, we needto take a closer look at the electrons in the C—H bonds ofthe glucose molecule. We stated in chapter 8 that when anelectron is removed from one atom and donated to an-other, the electron’s potential energy of position is alsotransferred. In this process, the atom that receives the elec-tron is reduced. We spoke of reduction in an all-or-nonefashion, as if it involved the complete transfer of an elec-tron from one atom to another. Often this is just what hap-pens. However, sometimes a reduction simply changes thedegree of sharing within a covalent bond. Let us now revisitthat discussion and consider what happens when the trans-fer of electrons is incomplete.

A Closer Look at Oxidation-Reduction

The catabolism of glucose is an oxidation-reduction reac-tion. The covalent electrons in the C—H bonds of glucoseare shared approximately equally between the C and Hatoms because carbon and hydrogen nuclei have about thesame affinity for valence electrons (that is, they exhibit sim-ilar electronegativity). However, when the carbon atoms ofglucose react with oxygen to form carbon dioxide, the elec-trons in the new covalent bonds take a different position.Instead of being shared equally, the electrons that were as-sociated with the carbon atoms in glucose shift far towardthe oxygen atom in CO2 because oxygen is very electroneg-ative. Since these electrons are pulled farther from the car-bon atoms, the carbon atoms of glucose have been oxidized(loss of electrons) and the oxygen atoms reduced (gain ofelectrons). Similarly, when the hydrogen atoms of glucosecombine with oxygen atoms to form water, the oxygenatoms draw the shared electrons strongly toward them;again, oxygen is reduced and glucose is oxidized. In this re-action, oxygen is an oxidizing (electron-attracting) agentbecause it oxidizes the atoms of glucose.

Releasing Energy

The key to understanding the oxidation of glucose is tofocus on the energy of the shared electrons. In a covalentbond, energy must be added to remove an electron from anatom, just as energy must be used to roll a boulder up a hill.The more electronegative the atom, the steeper the energyhill that must be climbed to pull an electron away from it.However, energy is released when an electron is shiftedaway from a less electronegative atom and closer to a moreelectronegative atom, just as energy is released when aboulder is allowed to roll down a hill. In the catabolism ofglucose, energy is released when glucose is oxidized, aselectrons relocate closer to oxygen (figure 9.14).

Glucose is an energy-rich food because it has an abun-dance of C—H bonds. Viewed in terms of oxidation-reduction, glucose possesses a wealth of electrons held farfrom their atoms, all with the potential to move closer to-ward oxygen. In oxidative respiration, energy is releasednot simply because the hydrogen atoms of the C—Hbonds are transferred from glucose to oxygen, but be-cause the positions of the valence electrons shift. Thisshift releases energy that can be used to make ATP.

Harvesting the Energy in Stages

It is generally true that the larger the release of energy inany single step, the more of that energy is released as heat(random molecular motion) and the less there is availableto be channeled into more useful paths. In the combustionof gasoline, the same amount of energy is released whetherall of the gasoline in a car’s gas tank explodes at once, orwhether the gasoline burns in a series of very small explo-sions inside the cylinders. By releasing the energy in gaso-line a little at a time, the harvesting efficiency is greater and


Electrontransportchain

Low energy

High energyEnergy forsynthesis of

Electrons from food

Formation of water

ATP

e–

e–

FIGURE 9.14How electron transport works. This diagram shows how ATP isgenerated when electrons transfer from one energy level toanother. Rather than releasing a single explosive burst of energy,electrons “fall” to lower and lower energy levels in steps, releasingstored energy with each fall as they tumble to the lowest (mostelectronegative) electron acceptor.

more of the energy can be used to push the pistons andmove the car.

The same principle applies to the oxidation of glucoseinside a cell. If all of the hydrogens were transferred to oxy-gen in one explosive step, releasing all of the free energy atonce, the cell would recover very little of that energy in auseful form. Instead, cells burn their fuel much as a cardoes, a little at a time. The six hydrogens in the C—Hbonds of glucose are stripped off in stages in the series ofenzyme-catalyzed reactions collectively referred to as gly-colysis and the Krebs cycle. We have had a great deal to sayabout these reactions already in this chapter. Recall that thehydrogens are removed by transferring them to a coenzymecarrier, NAD+ (figure 9.15). Discussed in chapter 8, NAD+ isa very versatile electron acceptor, shuttling energy-bearingelectrons throughout the cell. In harvesting the energy ofglucose, NAD+ acts as the primary electron acceptor.

Following the Electrons

As you examine these reactions, try not to become confusedby the changes in electrical charge. Always follow the elec-trons. Enzymes extract two hydrogens—that is, two elec-trons and two protons—from glucose and transfer bothelectrons and one of the protons to NAD+. The other pro-ton is released as a hydrogen ion, H+, into the surroundingsolution. This transfer converts NAD+ into NADH; that is,

two negative electrons and one positive proton are added toone positively charged NAD+ to form NADH, which iselectrically neutral.

Energy captured by NADH is not harvested all at once.Instead of being transferred directly to oxygen, the twoelectrons carried by NADH are passed along the electrontransport chain if oxygen is present. This chain consists ofa series of molecules, mostly proteins, embedded within theinner membranes of mitochondria. NADH delivers elec-trons to the top of the electron transport chain and oxygencaptures them at the bottom. The oxygen then joins withhydrogen ions to form water. At each step in the chain, theelectrons move to a slightly more electronegative carrier,and their positions shift slightly. Thus, the electrons movedown an energy gradient. The entire process releases a totalof 53 kcal/mole (222 kJ/mole) under standard conditions.The transfer of electrons along this chain allows the energyto be extracted gradually. In the next section, we will dis-cuss how this energy is put to work to drive the productionof ATP.

The catabolism of glucose involves a series ofoxidation-reduction reactions that release energy byrepositioning electrons closer to oxygen atoms. Energyis thus harvested from glucose molecules in gradualsteps, using NAD+ as an electron carrier.


N+

O CH2

H

P

O

O

O

H O

OP

H

-O

-O

O

P-O

O

HO OH

O

CH2

H

HO OH HO OH

O

C NH2 + 2HReduction

Oxidation

NAD+: oxidized form of nicotinamide

Adenine

N

N

N

N H

NH2

N

O CH2

H

P

O

-O

O

O

H H O

O

H

HO OH

O

CH2

H

O

C NH2 + H+

NADH: reduced form of nicotinamide

Adenine

N

N

N

N H

NH2

FIGURE 9.15NAD+ and NADH.This dinucleotideserves as an “electronshuttle” duringcellular respiration.NAD+ acceptselectrons fromcatabolizedmacromolecules andis reduced to NADH.

Stage Four: The Electron Transport ChainThe NADH and FADH2 molecules formed during the firstthree stages of aerobic respiration each contain a pair ofelectrons that were gained when NAD+ and FAD were re-duced. The NADH molecules carry their electrons to theinner mitochondrial membrane, where they transfer theelectrons to a series of membrane-associated proteins col-lectively called the electron transport chain.

Moving Electrons through the ElectronTransport Chain

The first of the proteins to receive the electrons is a com-plex, membrane-embedded enzyme called NADH dehy-drogenase. A carrier called ubiquinone then passes theelectrons to a protein-cytochrome complex called the bc1complex. This complex, along with others in the chain, oper-ates as a proton pump, driving a proton out across the mem-brane. Cytochromes are respiratory proteins that containheme groups, complex carbon rings with many alternatingsingle and double bonds and an iron atom in the center.

The electron is then carried by another carrier, cy-tochrome c, to the cytochrome oxidase complex. This com-plex uses four such electrons to reduce a molecule of oxy-gen, each oxygen then combines with two hydrogen ions toform water:

O2 + 4 H+ + 4 e- → 2 H2O

This series of membrane-associated electron carriers iscollectively called the electron transport chain (figure 9.16).

NADH contributes its electrons to the first protein ofthe electron transport chain, NADH dehydrogenase.FADH2, which is always attached to the inner mitochon-drial membrane, feeds its electrons into the electron trans-port chain later, to ubiquinone.

It is the availability of a plentiful electron acceptor(often oxygen) that makes oxidative respiration possible. Aswe’ll see in chapter 10, the electron transport chain used inaerobic respiration is similar to, and may well have evolvedfrom, the chain employed in aerobic photosynthesis.

The electron transport chain is a series of fivemembrane-associated proteins. Electrons delivered byNADH and FADH2 are passed from protein to proteinalong the chain, like a baton in a relay race.


Intermembrane space

Mitochondrial matrix

Innermitochondrialmembrane

NAD+

Q

C

NADH

H2O 2H+ + O2

+ H+

H+ H+ H+

NADH dehydrogenase

bc1 complexCytochrome

oxidase complex

FADH2

�12

FIGURE 9.16The electron transport chain. High-energy electrons harvested from catabolized molecules are transported (red arrows) by mobileelectron carriers (ubiquinone, marked Q, and cytochrome c, marked C) along a chain of membrane proteins. Three proteins use portionsof the electrons’ energy to pump protons (blue arrows) out of the matrix and into the intermembrane space. The electrons are finallydonated to oxygen to form water.

Building an Electrochemical Gradient

In eukaryotes, aerobic metabolism takes place within themitochondria present in virtually all cells. The internalcompartment, or matrix, of a mitochondrion contains theenzymes that carry out the reactions of the Krebs cycle.As the electrons harvested by oxidative respiration arepassed along the electron transport chain, the energy theyrelease transports protons out of the matrix and into theouter compartment, sometimes called the intermembranespace. Three transmembrane proteins in the inner mito-chondrial membrane (see figure 9.16) actually accomplishthe transport. The flow of excited electrons induces achange in the shape of these pump proteins, which causesthem to transport protons across the membrane. Theelectrons contributed by NADH activate all three of theseproton pumps, while those contributed by FADH2 acti-vate only two.

Producing ATP: Chemiosmosis

As the proton concentration in the intermembrane spacerises above that in the matrix, the matrix becomes slightlynegative in charge. This internal negativity attracts thepositively charged protons and induces them to reenter thematrix. The higher outer concentration tends to drive pro-tons back in by diffusion; since membranes are relativelyimpermeable to ions, most of the protons that reenter thematrix pass through special protonchannels in the inner mitochondrialmembrane. When the protons passthrough, these channels synthesizeATP from ADP + Pi within the ma-trix. The ATP is then transported byfacilitated diffusion out of the mito-chondrion and into the cell’s cyto-plasm. Because the chemical forma-tion of ATP is driven by a diffusionforce similar to osmosis, this processis referred to as chemiosmosis (fig-ure 9.17).

Thus, the electron transport chainuses electrons harvested in aerobicrespiration to pump a large number ofprotons across the inner mitochon-drial membrane. Their subsequentreentry into the mitochondrial matrixdrives the synthesis of ATP bychemiosmosis. Figure 9.18 summa-rizes the overall process.

The electrons harvested fromglucose are pumped out of themitochondrial matrix by theelectron transport chain. Thereturn of the protons into thematrix generates ATP.


Intermembrane space


Na-Kpump

ATP

ADP + Pi

NADH

H+

H+

H+H+

H+

H+

NAD+

Innermitochondrial

membrane

Proton pump ATPsynthase

FIGURE 9.17Chemiosmosis. NADH transports high-energy electronsharvested from the catabolism of macromolecules to “protonpumps” that use the energy to pump protons out of themitochondrial matrix. As a result, the concentration of protons inthe intermembrane space rises, inducing protons to diffuse backinto the matrix. Many of the protons pass through special channelsthat couple the reentry of protons to the production of ATP.

H+

H+H+

H+

O2O2+

Q C

ATP32ATP2

Pyruvate fromcytoplasm

Electrontransportsystem

Channelprotein

H2O

CO2

Acetyl-CoA

Krebscycle FADH2

NADH

NADH

Intermembranespace


Innermitochondrialmembrane

1. Electrons are harvested and carried to the transport system.

2. Electrons provide energy to pump protons across the membrane.

3. Oxygen joins with protons to form water.

4. Protons diffuse back in, driving the synthesis of ATP.

2H+

�12

FIGURE 9.18ATP generation during the Krebs cycle and electron transport chain. This processbegins with pyruvate, the product of glycolysis, and ends with the synthesis of ATP.

Summarizing AerobicRespirationHow much metabolic energy does a cellactually gain from the electrons har-vested from a molecule of glucose, usingthe electron transport chain to produceATP by chemiosmosis?

Theoretical Yield

The chemiosmotic model suggests thatone ATP molecule is generated foreach proton pump activated by theelectron transport chain. Since the elec-trons from NADH activate threepumps and those from FADH2 activatetwo, we would expect each molecule ofNADH and FADH2 to generate threeand two ATP molecules, respectively.However, because eukaryotic cells carryout glycolysis in their cytoplasm andthe Krebs cycle within their mitochon-dria, they must transport the two mole-cules of NADH produced during gly-colysis across the mitochondrialmembranes, which requires one ATPper molecule of NADH. Thus, the netATP production is decreased by two.Therefore, the overall ATP productionresulting from aerobic respiration theo-retically should be 4 (from substrate-level phosphorylation during glycolysis)+ 30 (3 from each of 10 molecules ofNADH) + 4 (2 from each of 2 mole-cules of FADH2) – 2 (for transport ofglycolytic NADH) = 36 molecules of ATP (figure 9.19).

Actual Yield

The amount of ATP actually produced in a eukaryotic cellduring aerobic respiration is somewhat lower than 36, fortwo reasons. First, the inner mitochondrial membrane issomewhat “leaky” to protons, allowing some of them toreenter the matrix without passing through ATP-generatingchannels. Second, mitochondria often use the proton gra-dient generated by chemiosmosis for purposes other thanATP synthesis (such as transporting pyruvate into the ma-trix). Consequently, the actual measured values of ATPgenerated by NADH and FADH2 are closer to 2.5 foreach NADH and 1.5 for each FADH2. With these correc-tions, the overall harvest of ATP from a molecule of glu-cose in a eukaryotic cell is closer to 4 (from substrate-levelphosphorylation) + 25 (2.5 from each of 10 molecules ofNADH) + 3 (1.5 from each of 2 molecules of FADH2) – 2(transport of glycolytic NADH) = 30 molecules of ATP.

The catabolism of glucose by aerobic respiration, incontrast to glycolysis, is quite efficient. Aerobic respirationin a eukaryotic cell harvests about 7.3 × 30 ÷ 686 = 32% ofthe energy available in glucose. (By comparison, a typicalcar converts only about 25% of the energy in gasoline intouseful energy.) The efficiency of oxidative respiration atharvesting energy establishes a natural limit on the maxi-mum length of food chains.

The high efficiency of aerobic respiration was one of thekey factors that fostered the evolution of heterotrophs. Withthis mechanism for producing ATP, it became feasible fornonphotosynthetic organisms to derive metabolic energy ex-clusively from the oxidative breakdown of other organisms.As long as some organisms captured energy by photosynthe-sis, others could exist solely by feeding on them.

Oxidative respiration produces approximately 30molecules of ATP from each molecule of glucose ineukaryotic cells. This represents more than half of theenergy in the chemical bonds of glucose.


Glycolysis2

2 6 ATP

Pyruvate

Glucose

Acetyl-CoA

NADH

2 4 ATPNADH

2 ATP

2 ATP

6 18 ATPNADH

2 4 ATP

Total net ATP yield = 36 ATP

FADH2

Krebscycle

ATP

FIGURE 9.19Theoretical ATP yield. The theoretical yield of ATP harvested from glucose by aerobicrespiration totals 36 molecules.

Regulating Aerobic RespirationWhen cells possess plentiful amounts of ATP, the key reac-tions of glycolysis, the Krebs cycle, and fatty acid break-down are inhibited, slowing ATP production. The regula-tion of these biochemical pathways by the level of ATP isan example of feedback inhibition. Conversely, when ATPlevels in the cell are low, ADP levels are high; and ADP ac-tivates enzymes in the pathways of carbohydrate catabolismto stimulate the production of more ATP.

Control of glucose catabolism occurs at two key pointsof the catabolic pathway (figure 9.20). The control pointin glycolysis is the enzyme phosphofructokinase, whichcatalyzes reaction 3, the conversion of fructose phosphateto fructose bisphosphate. This is the first reaction of gly-colysis that is not readily reversible, committing the sub-strate to the glycolytic sequence. High levels of ADP rela-tive to ATP (implying a need to convert more ADP toATP) stimulate phosphofructokinase, committing moresugar to the catabolic pathway; so do low levels of citrate(implying the Krebs cycle is not running at full tilt andneeds more input). The main control point in the oxida-tion of pyruvate occurs at the committing step in theKrebs cycle with the enzyme pyruvate decarboxylase. It isinhibited by high levels of NADH (implying no more isneeded).

Another control point in the Krebs cycle is the enzymecitrate synthetase, which catalyzes the first reaction, theconversion of oxaloacetate and acetyl-CoA into citrate.High levels of ATP inhibit citrate synthetase (as well aspyruvate decarboxylase and two other Krebs cycle en-zymes), shutting down the catabolic pathway.

Relative levels of ADP and ATP regulate the catabolismof glucose at key committing reactions.


A Vocabulary ofATP Generation

which the final electron acceptor is or-ganic; it includes aerobic and anaerobicrespiration.chemiosmosis The passage of high-energy electrons along the electron trans-port chain, which is coupled to the pump-ing of protons across a membrane and thereturn of protons to the original side ofthe membrane through ATP-generatingchannels.fermentation Alternative ATP-producingpathway performed by some cells in the ab-sence of oxygen, in which the final electronacceptor is an organic molecule.maximum efficiency The maximumnumber of ATP molecules generated by

oxidizing a substance, relative to the freeenergy of that substance; in organisms, theactual efficiency is usually less than themaximum.oxidation The loss of an electron. In cel-lular respiration, high-energy electrons arestripped from food molecules, oxidizingthem.photosynthesis The chemiosmotic gen-eration of ATP and complex organic mole-cules powered by the energy derived fromlight.substrate-level phosphorylation Thegeneration of ATP by the direct transfer ofa phosphate group to ADP from anotherphosphorylated molecule.

aerobic respiration The portion of cellu-lar respiration that requires oxygen as anelectron acceptor; it includes pyruvate oxi-dation, the Krebs cycle, and the electrontransport chain.anaerobic respiration Cellular respirationin which inorganic electron acceptors otherthan oxygen are used; it includes glycolysis.cellular respiration The oxidation oforganic molecules to produce ATP in

ATP

Krebscycle

Electron transportchain and

chemiosmosis

NADH

Citrate

Glucose ADP

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Pyruvate

Pyruvate decarboxylase

Acetyl-CoA

Phosphofructokinase

Inhibits Activates

Activates

Inhibits

FIGURE 9.20Control of glucose catabolism. The relative levels of ADP andATP control the catabolic pathway at two key points: thecommitting reactions of glycolysis and the Krebs cycle.

Glucose Is Not the Only Source of EnergyThus far we have discussed oxidativerespiration of glucose, which organismsobtain from the digestion of carbohy-drates or from photosynthesis. Otherorganic molecules than glucose, partic-ularly proteins and fats, are also impor-tant sources of energy (figure 9.21).

Cellular Respiration of Protein

Proteins are first broken down into theirindividual amino acids. The nitrogen-containing side group (the aminogroup) is then removed from eachamino acid in a process called deamina-tion. A series of reactions convert thecarbon chain that remains into a mole-cule that takes part in glycolysis or theKrebs cycle. For example, alanine isconverted into pyruvate, glutamate intoα-ketoglutarate (figure 9.22), and aspar-tate into oxaloacetate. The reactions ofglycolysis and the Krebs cycle then ex-tract the high-energy electrons fromthese molecules and put them to workmaking ATP.


9.3 Catabolism of proteins and fats can yield considerable energy.

Macromoleculedegradation

Cellbuilding blocks

Nucleicacids

Proteins Lipidsand fats

Polysaccharides

Nucleotides Amino acids Fatty acidsSugars

NH3 H2O CO2

Deamination �-oxidationGlycolysis

Oxidativerespiration

Ultimatemetabolicproducts

Pyruvate

Acetyl-CoA

Krebscycle

FIGURE 9.21How cells extract chemical energy. All eukaryotes and many prokaryotes extract energyfrom organic molecules by oxidizing them. The first stage of this process, breaking downmacromolecules into their constituent parts, yields little energy. The second stage,oxidative or aerobic respiration, extracts energy, primarily in the form of high-energyelectrons, and produces water and carbon dioxide.

�-Ketoglutarate

C

———

O

O

H — C —

——

—

H — C —

HO

—

—

—

C

———

HO

— C — H

H

H

Glutamate

H2N

C

———

O

O

O

H — C —

——

——

—

H — C —

—

—

—

C

———

HO

HO

C

H

H

NH2

Urea

FIGURE 9.22Deamination. Afterproteins are broken downinto their amino acidconstituents, the aminogroups are removed fromthe amino acids to formmolecules that participate inglycolysis and the Krebscycle. For example, theamino acid glutamatebecomes α-ketoglutarate, aKrebs cycle molecule, whenit loses its amino group.

Cellular Respiration of Fat

Fats are broken down into fatty acids plus glycerol. Thetails of fatty acids typically have 16 or more —CH2 links,and the many hydrogen atoms in these long tails provide arich harvest of energy. Fatty acids are oxidized in the ma-trix of the mitochondrion. Enzymes there remove the two-carbon acetyl groups from the end of each fatty acid tailuntil the entire fatty acid is converted into acetyl groups(figure 9.23). Each acetyl group then combines with coen-zyme A to form acetyl-CoA. This process is known as �-oxidation.

How much ATP does the catabolism of fatty acids pro-duce? Let’s compare a hypothetical six-carbon fatty acidwith the six-carbon glucose molecule, which we’ve saidyields about 30 molecules of ATP in a eukaryotic cell. Tworounds of β-oxidation would convert the fatty acid intothree molecules of acetyl-CoA. Each round requires onemolecule of ATP to prime the process, but it also producesone molecule of NADH and one of FADH2. These mole-cules together yield four molecules of ATP (assuming 2.5ATPs per NADH and 1.5 ATPs per FADH2). The oxida-tion of each acetyl-CoA in the Krebs cycle ultimately pro-duces an additional 10 molecules of ATP. Overall, then,the ATP yield of a six-carbon fatty acid would be approxi-mately 8 (from two rounds of β-oxidation) – 2 (for primingthose two rounds) + 30 (from oxidizing the three acetyl-CoAs) = 36 molecules of ATP. Therefore, the respirationof a six-carbon fatty acid yields 20% more ATP than therespiration of glucose. Moreover, a fatty acid of that sizewould weigh less than two-thirds as much as glucose, so agram of fatty acid contains more than twice as many kilo-calories as a gram of glucose. That is why fat is a storagemolecule for excess energy in many types of animals. If ex-cess energy were stored instead as carbohydrate, as it is inplants, animal bodies would be much bulkier.

Proteins, fats, and other organic molecules are alsometabolized for energy. The amino acids of proteins arefirst deaminated, while fats undergo a process called β-oxidation.


OH

OFatty acid C

H

H

C

H

H

C

Fatty acid C

H

H

C

H

H

C

O

FAD

FADH2

ATP

AMP + PPi

CoA

CoAFatty acid C

H

C

H

C

O

CoA

Fatty acid C

HO

H

C

H

H

C

O

CoA

Fatty acid C C

H

H

C

OO

CoA

NAD+

NADH

H2O

Acetyl-CoA

Krebscycle

CoA

FIGURE 9.23β-oxidation. Through a series of reactions known as β-oxidation, thelast two carbons in a fatty acid tail combine with coenzyme A to formacetyl-CoA, which enters the Krebs cycle. The fatty acid, now twocarbons shorter, enters the pathway again and keeps reentering untilall its carbons have been used to form acetyl-CoA molecules. Eachround of β-oxidation uses one molecule of ATP and generates onemolecule each of FADH2 and NADH, not including the moleculesgenerated from the Krebs cycle.


Even with oxidative metabolism, approx-imately two-thirds of the available energy islost at each trophic level, and that puts alimit on how long a food chain can be. Mostfood chains, like the one illustrated in figure9.A, involve only three or rarely four trophiclevels. Too much energy is lost at eachtransfer to allow chains to be much longerthan that. For example, it would be impossi-ble for a large human population to subsistby eating lions captured from the SerengetiPlain of Africa; the amount of grass availablethere would not support enough zebras andother herbivores to maintain the number oflions needed to feed the human population.Thus, the ecological complexity of ourworld is fixed in a fundamental way by thechemistry of oxidative respiration.

When organisms became able to extractenergy from organic molecules by oxidativemetabolism, this constraint became far lesssevere, because the efficiency of oxidativerespiration is estimated to be about 52 to63%. This increased efficiency results inthe transmission of much more energyfrom one trophic level to another than doesglycolysis. (A trophic level is a step in themovement of energy through an ecosys-tem.) The efficiency of oxidative metabo-lism has made possible the evolution offood chains, in which autotrophs are con-sumed by heterotrophs, which are con-sumed by other heterotrophs, and so on.You will read more about food chains inchapter 28.

It has been estimated that a heterotrophlimited to glycolysis captures only 3.5% ofthe energy in the food it consumes. Hence,if such a heterotroph preserves 3.5% of theenergy in the autotrophs it consumes, thenany other heterotrophs that consume thefirst hetertroph will capture through glycol-ysis 3.5% of the energy in it, or 0.12% ofthe energy available in the original au-totrophs. A very large base of autotrophswould thus be needed to support a smallnumber of heterotrophs.

Metabolic Efficiencyand the Length of

Food Chains

FIGURE 9.AA food chain in the savannas, or open grasslands, of East Africa. Stage 1: Photosynthesizer. The grass under these palm trees growsactively during the hot, rainy season, capturing the energy of the sun and storing it in molecules of glucose, which are then converted intostarch and stored in the grass. Stage 2: Herbivore. These large antelopes, known as wildebeests, consume the grass and transfer some of itsstored energy into their own bodies. Stage 3: Carnivore. The lion feeds on wildebeests and other animals, capturing part of their storedenergy and storing it in its own body. Stage 4: Scavenger. This hyena and the vultures occupy the same stage in the food chain as the lion.They are also consuming the body of the dead wildebeest, which has been abandoned by the lion. Stage 5: Refuse utilizer. These butterflies,mostly Precis octavia, are feeding on the material left in the hyena’s dung after the food the hyena consumed had passed through itsdigestive tract. At each of these four levels, only about a third or less of the energy present is used by the recipient.

Stage 1: Photosynthesizers Stage 2: Herbivores

Stage 3: Carnivore Stage 4: Scavengers Stage 5: Refuse utilizers

FermentationIn the absence of oxygen, aerobic metabolism cannotoccur, and cells must rely exclusively on glycolysis to pro-duce ATP. Under these conditions, the hydrogen atomsgenerated by glycolysis are donated to organic molecules ina process called fermentation.

Bacteria carry out more than a dozen kinds of fermen-tations, all using some form of organic molecule to ac-cept the hydrogen atom from NADH and thus recycleNAD+:

Organic molecule→

Reduced organic molecule+ NADH + NAD+

Often the reduced organic compound is an organic acid—such as acetic acid, butyric acid, propionic acid, or lacticacid—or an alcohol.

Ethanol Fermentation

Eukaryotic cells are capable of only a few types of fermen-tation. In one type, which occurs in single-celled fungicalled yeast, the molecule that accepts hydrogen fromNADH is pyruvate, the end product of glycolysis itself.Yeast enzymes remove a terminal CO2 group from pyru-vate through decarboxylation, producing a two-carbonmolecule called acetaldehyde. The CO2 released causesbread made with yeast to rise, while bread made withoutyeast (unleavened bread) does not. The acetaldehyde ac-cepts a hydrogen atom from NADH, producing NAD+ andethanol (ethyl alcohol). This particular type of fermenta-tion is of great interest to humans, since it is the source ofthe ethanol in wine and beer (figure 9.24). Ethanol is a by-product of fermentation that is actually toxic to yeast; as itapproaches a concentration of about 12%, it begins to killthe yeast. That explains why naturally fermented wine con-tains only about 12% ethanol.

Lactic Acid Fermentation

Most animal cells regenerate NAD+ without decarboxyla-tion. Muscle cells, for example, use an enzyme called lactatedehydrogenase to transfer a hydrogen atom from NADHback to the pyruvate that is produced by glycolysis. Thisreaction converts pyruvate into lactic acid and regeneratesNAD+ from NADH. It therefore closes the metabolic cir-cle, allowing glycolysis to continue as long as glucose isavailable. Circulating blood removes excess lactate (the ion-ized form of lactic acid) from muscles, but when removalcannot keep pace with production, the accumulating lacticacid interferes with muscle function and contributes tomuscle fatigue.

In fermentation, which occurs in the absence of oxygen,the electrons that result from the glycolytic breakdownof glucose are donated to an organic molecule,regenerating NAD+ from NADH.



Glucose

2 Pyruvate

Alcohol fermentation in yeast

G

L

Y

C

O

L

Y

S

I

S

2 ATP

2 ADP

C O

O-

C O

CH3

C O

O–

CH3

C

2 Ethanol

2 Acetaldehyde

2 Lactate

H OH

H

CH OH

CH3

Glucose

2 Pyruvate

Lactic acid fermentation in muscle cells

G

L

Y

C

O

L

Y

S

I

S

2 ATP

2 ADP

C O

O-

C O

CH3

2 NAD+

OC

H

CH3

2 NADH

2 NAD+

2 NADH

CO2

FIGURE 9.24How wine is made. Yeasts carry out the conversion of pyruvate toethanol. This takes place naturally in grapes left to ferment onvines, as well as in fermentation vats containing crushed grapes.When the ethanol concentration reaches about 12%, its toxiceffects kill the yeast; what remains is wine. Muscle cells convertpyruvate into lactate, which is less toxic than ethanol. However,lactate is still toxic enough to produce a painful sensation inmuscles during heavy exercise, when oxygen in the muscles isdepleted.


Chapter 9Summary Questions Media Resources


• The reactions of cellular respiration are oxidation-reduction (redox) reactions. Those that require a netinput of free energy are coupled to the cleavage ofATP, which releases free energy.

• The mechanics of cellular respiration are oftendictated by electron behavior, which is in turninfluenced by the presence of electron acceptors.Some atoms, such as oxygen, are very electronegativeand thus behave as good oxidizing agents.

1. What is the differencebetween an autotroph and aheterotroph? How does eachobtain energy?2. What is the differencebetween digestion andcatabolism? Which providesmore energy?

• In eukaryotic cells, the oxidative respiration ofpyruvate takes place within the matrix ofmitochondria.

• The electrons generated in the process are passedalong the electron transport chain, a sequence ofelectron carriers in the inner mitochondrialmembrane.

• Some of the energy released by passage of electronsalong the electron transport chain is used to pumpprotons out of the mitochondrial matrix. The reentryof protons into the matrix is coupled to theproduction of ATP. This process is calledchemiosmosis. The ATP then leaves themitochondrion by facilitated diffusion.

3. Where in a eukaryotic celldoes glycolysis occur? What isthe net production of ATPduring glycolysis, and why is itdifferent from the number ofATP molecules synthesizedduring glycolysis?4. By what two mechanisms canthe NADH that results fromglycolysis be converted back intoNAD+? 5. What is the theoreticalmaximum number of ATPmolecules produced during theoxidation of a glucose moleculeby these processes? Why is theactual number of ATP moleculesproduced usually lower than thetheoretical maximum?


• The catabolism of fatty acids begins with �-oxidationand provides more energy than the catabolism ofcarbohydrates.

6. How is acetyl-CoA producedduring the aerobic oxidation ofcarbohydrates, and what happensto it? How is it produced duringthe aerobic oxidation of fattyacids, and what happens to itthen?

9.3 Catabolism of proteins and fats can yield considerable energy.

• Fermentation is an anaerobic process that uses anorganic molecule instead of oxygen as a final electronacceptor.

• It occurs in bacteria as well as eukaryotic cells,including yeast and the muscle cells of animals.

7. How do the amounts of ATPproduced by the aerobicoxidation of glucose and fattyacids compare? Which type ofsubstance contains more energyon a per-weight basis?


http://www.mhhe.com/raven6e http://www.biocourse.com

• Exploration:Oxidative Respiration

• Art Activity: AerobicCellular Respiration

• Art Activity:Organization ofCristae

• Electron Transportand ATP

• Glycolysis• Krebs Cycle• Electron Transport

• Fermentation

how cells harvest energy - usp159 9 how cells harvest energy concept outline 9.1 cells harvest the...

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