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1 Energy, Life, and Biochemical Reactions

1 Energy, Life, and Biochemical Reactions

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694



CHAPTER 21



The Generation of Biochemical Energy



Our bodies do not produce energy by burning up a meal all at once because the release of a large quantity of energy (primarily as heat) would be harmful to us. Furthermore, it is difficult to capture energy for storage once it has been converted to heat. We

need energy that can be stored and then released in the right amounts when and where

it is needed, whether we are jogging, studying, or sleeping. We, therefore, have some

specific requirements for energy.













Energy must be released from food gradually.

Energy must be stored in readily accessible forms as glycogen and fat

(triacylglycerides).

Release of energy from storage must be finely controlled so that it is available

exactly when and where it is needed.

Just enough energy must be released as heat to maintain constant body

temperature.

Energy in a form other than heat must be available to drive chemical reactions that

are not favorable at body temperatures.



This chapter looks at some of the ways these requirements for energy regulation

are met. We begin by reviewing basic concepts about energy and then learn about

metabolism. Next, we look at the citric acid cycle and oxidative phosphorylation,

which together form the common pathway for the production of energy.



Biochemical reactions

ConCEPtS to rEViEW Review

entropy, enthalpy, endergonic,

exergonic, and free-energy change in

Sections 7.2–7.4.



Chemical reactions either release or absorb energy. Whether a reaction is favorable or

not depends on either the release or absorption of energy as heat (the change in enthalpy, ∆H), together with the increase or decrease in disorder (∆S, the entropy change)

caused by the reaction. The net effect of these changes is given by the free-energy

change of a reaction: ∆G = ∆H - T∆S .

Reactions in living organisms are no different from reactions in a chemistry laboratory. Both follow the same laws, and both have the same energy requirements. Spontaneous reactions—that is, those that are favorable in the forward direction—release

free energy, and the energy released is available to do work. Such reactions, described

as exergonic, are the source of our biochemical energy.

As shown by the energy diagram in Figure 7.3 the products of a favorable, exergonic

reaction are farther downhill on the energy scale than the reactants. That is, the products are more stable than the reactants, and as a result the free-energy change 1 ∆G2

has a negative value. Oxidation reactions, for example, are usually downhill reactions

that release energy. Oxidation of glucose, the principal source of energy for animals,

produces 2870 kJ of free energy per mole of glucose.

C6H12O6 + 6 O2 ¡ 6 CO2 + 6 H2O



∆G = - 2870 kJ>mol



The greater the amount of free energy released, the farther a reaction proceeds

toward product formation before reaching equilibrium.

Reactions in which the products are higher in energy than the reactants can also

take place, but such unfavorable reactions cannot occur without the input of energy

from an external source; such reactions are endergonic.

The free-energy change switches sign for the reverse of a reaction, but the value

does not change. Photosynthesis, the process whereby plants convert CO2 and H2O to

glucose and O2, is the reverse of the oxidation of glucose. Its ∆G is therefore positive

and equal to the value for the oxidation of glucose (see the Chemistry in Action “Plants

and Photosynthesis” on p. 696). The sun provides the necessary external energy for

photosynthesis (2870 kJ>mol of glucose formed).



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SECTION 21.1

Photosynthesis



6CO2 + 6H2O



Energy, Life, and Biochemical Reactions



695



∆G = + 2870 kJ/mol (endergonic, energy required)



C6H12O6 + 6O2

Oxidation



∆G = −2870 kJ/mol (exergonic, energy released)



Living systems make constant use of this principle in the series of chemical reactions we know as the biochemical pathways. Energy is stored in the products of an

overall endergonic reaction pathway. This stored energy is released as needed in an

overall exergonic reaction pathway that regenerates the original reactants. It is not necessary that every reaction in the pathways between the reactants and products be the

same, so long as the pathways connect the same reactants and products.



Pathway A series of enzymecatalyzed chemical reactions that are

connected by their intermediates, that

is, the product of the first reaction is

the reactant for the second reaction,

and so on.



Worked Example 21.1 Determining Reaction Energy

Are the following reactions exergonic or endergonic?

(a) Glucose 6@phosphate S Fructose 6@phosphate

∆G = + 2.09 kJ>mol

(b) Fructose 6@phosphate + ATP S Fructose 1,6@bisphosphate + ADP

∆G = - 14.2 kJ>mol

AnAlySiS Exergonic reactions release free energy, and ∆G is negative. Endergonic reactions gain free energy,



and so ∆G is positive.



Solution

Reaction (a), the conversion of glucose 6-phosphate to fructose 6-phosphate has a positive ∆G; therefore, it is

endergonic. Reaction (b), the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate has a negative

∆G; therefore it is exergonic.



KEy ConCEPt ProBlEM 21.1

In a cell, glucose can be oxidized via metabolic pathways. Alternatively, you could

burn glucose in the laboratory. Which of these methods consumes or produces more

energy? (Hint: All of the energy comes from converting the energy stored in the reduced bonds in glucose into the most oxidized form, carbon dioxide.)



KEy ConCEPt ProBlEM 21.2

The overall equation in this section,

6CO2 + 6H2O



photosynthesis

oxidation



C6H12O6 + 6O2,



shows the cycle between photosynthesis and oxidation. Pathways operating in opposite

directions cannot be exergonic in both directions.

(a) Which of the two pathways in this cycle is exergonic and which is endergonic?

(b) Where does the energy for the endergonic pathway come from?



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696



CHAPTER 21



The Generation of Biochemical Energy



CHEMiStry in ACtion

Plants and Photosynthesis

The principal biochemical difference between humans and

plants is that plants derive energy directly from sunlight and

we cannot. In the process of photosynthesis, plants use solar

energy to synthesize oxygen and energy-rich carbohydrates

from energy-poor reactants: CO2 and water. Our metabolism

breaks down energy-rich reactants to extract the useful energy and produce energy-poor products: CO2 and water. Is it

surprising to discover that despite this difference in the direction of their reactions, plants rely on biochemical pathways

very much like our own?

The energy-capturing phase of photosynthesis takes place

mainly in green leaves. Plant cells contain chloroplasts, which,

though larger and more complex in structure, resemble mitochondria. Embedded in membranes within the chloroplasts are

large groups of chlorophyll molecules and the enzymes of an

electron-transport chain. Chlorophyll is similar in structure to

heme but contains magnesium ions 1Mg2+ 2 instead of iron

ions 1Fe2+ 2 .

As solar energy is absorbed, chlorophyll molecules pass it

along to specialized reaction centers, where it is used to boost

the energy of electrons. The excited electrons then give up their

extra energy as they pass down a pair of electron-transport

chains.

Some of this energy is used to oxidize water, splitting it into

oxygen, hydrogen ions, and electrons (which replace those

entering the electron-transport chain). At the end of the chain,

the hydrogen ions, together with the electrons, are used to reduce NADP+ to NADPH. Along the way, part of the energy of the

electrons is used to pump hydrogen ions across a membrane

to create a concentration gradient. As in mitochondria, the hydrogen ions can only return across the membrane at enzyme

complexes that convert ADP to ATP. Water needed for these

light-dependent reactions enters the plant through the roots

and leaves, and the oxygen that is formed is released through

openings in the leaves.

The energy-carrying ATP and NADPH enter the fluid interior of

the chloroplasts. Here their energy is used to drive the synthesis

of carbohydrate molecules. So long as ATP and NADPH are available, this part of photosynthesis is light-independent—it can

proceed in the absence of sunlight.

Plants have mitochondria as well as chloroplasts, so they

can also carry out the release of energy from stored carbohydrates. Because the breakdown of carbohydrates continues

in many harvested fruits and vegetables, the goal in storage

is to slow it down. Refrigeration is one measure that is taken,

since (like most chemical reactions) the rate of respiration

decreases at lower temperatures. Another is replacement of

air over stored fruits and vegetables with carbon dioxide or

nitrogen.



These flowers are converting the potential energy of the sun into

chemical potential energy stored in the bonds of carbohydrates.





H2O



LIGHT-DEPENDENT

REACTIONS



Depleted

carriers

(ADP, NADP+)



Glucose





O2



Energized

carriers

(ATP, NADPH)



LIGHT-INDEPENDENT

REACTIONS



CO2 + H2O



The coupled reactions of photosynthesis.



CiA Problem 21.1 Chlorophyll is similar in structure to heme in

red blood cells but does not have an iron atom. What metal

ion is present in chlorophyll?

CiA Problem 21.2 Photosynthesis consists of both light-dependent

and light-independent reactions. What is the purpose of each

type of reaction?

CiA Problem 21.3 One step of the cycle that incorporates

CO2 into glyceraldehyde in plants is the production

of two 3-phosphoglycerates. ∆G = - 3.5 kJ>mol for this

reaction. Is this process endergonic or exergonic?

CiA Problem 21.4 What general process does refrigeration of

harvested fruits and vegetables slow? What cellular processes

are slowed by refrigeration?



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SECTION 21.2



Cells and Their Structure



697



21.2 Cells and Their Structure

Learning Objective:

• Describe the eukaryotic cell and explain the function of each structure.

Before learning about metabolism, it is important to see where the energy-generating

reactions take place within the cells of living organisms. There are two main categories

of cells: prokaryotic cells, found in single-celled organisms (e.g., bacteria and bluegreen algae), and eukaryotic cells, found in some single-celled organisms, such as

yeast, and all plants and animals.

Eukaryotic cells are about 1000 times larger than bacterial cells, have a membraneenclosed nucleus that contains their deoxyribonucleic acid (DNA), and include several

other kinds of internal structures known as organelles—small, functional units that perform specialized tasks. A generalized eukaryotic cell is shown in Figure 21.2 with short

descriptions of the functions of some of its major parts. Everything between the cell membrane and the nuclear membrane in a eukaryotic cell, including the various organelles,

is the cytoplasm. The organelles are surrounded by the fluid part of the cytoplasm, the

cytosol, which contains electrolytes, nutrients, and many enzymes, all in aqueous solution.



Cytoplasm The region between the

cell membrane and the nuclear membrane in a eukaryotic cell.

Cytosol The fluid part of the

cytoplasm surrounding the organelles

within a cell, contains dissolved

proteins and nutrients.



Cilia

(movement of materials)

Microvilli

(absorption of extracellular substances)

Cytosol

(intracellular fluid)

Mitochondrion

(synthesis of ATP)

Nucleus

(replication of DNA)



Golgi apparatus

(synthesis of macromolecules)

Smooth endoplasmic reticulum

(synthesis of lipids and carbohydrates)



Rough endoplasmic reticulum

(protein synthesis and transport)



Lysosome

(breakdown of unwanted molecules and cellular components)



Ribosomes

(protein synthesis)



Cell membrane

(separates cell contents from exterior; permits exchange of

molecules with exterior fluid and delivers signals to interior)







Figure 21.2



A generalized eukaryotic cell.

Major cell components are labeled with a description of their primary function.



The mitochondria (singular, mitochondrion), often called the cell’s “power plants,”

are the most important of the organelles for energy production and produce about 90%

of the body’s energy-carrying molecule, ATP.

A mitochondrion is a roughly egg-shaped structure composed of a smooth outer

membrane and a folded inner membrane (Figure 21.3). The space enclosed by the inner membrane is the mitochondrial matrix. Within the matrix, the citric acid cycle

(Section 21.7) and production of most of the body’s adenosine triphosphate (ATP)

take place. The coenzymes and proteins that manage the transfer of energy to the

chemical bonds of ATP (Section 21.8) are embedded in the inner membrane of the

mitochondrion.



Mitochondrion (plural,

mitochondria) An egg-shaped organelle where small molecules are broken

down to provide the energy for an

organism.

Mitochondrial matrix The space surrounded by the inner membrane of a

mitochondrion.

Adenosine triphosphate (ATP) The

principal energy-carrying molecule,

removal of a phosphoryl group to give

ADP releases free energy.



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CHAPTER 21



The Generation of Biochemical Energy

Intermembrane space

Outer membrane

ATP



Inner membrane



O2



CO2



Glucose, O2,

ADP, HOPO32−



Matrix



CO2



Citric

acid

cycle



ATP



Mitochondrion



ATP synthase

enzymes—

sites of ATP production



Matrix



Enzymes

ADP+

and

phosphate coenzymes

of cristae

H



Cristae







Figure 21.3



The mitochondrion.

Cells have many mitochondria. The citric acid cycle takes place in the matrix. Electron transport and ATP production, the final

stage in biochemical energy generation (described in Section 21.8), take place at the inner surface of the inner membrane. The

numerous folds in the inner membrane—known as cristae—increase the surface area over which these pathways can take place.



Mitochondria contain their own DNA, synthesize some of their own proteins, and

multiply using chemicals moved from the cell cytosol into the mitochondrial matrix.

The number of mitochondria is greatest in eye, brain, heart, and muscle cells, where the

need for energy is greatest. The ability of mitochondria to reproduce is seen in athletes

who put heavy energy demands on their bodies—they develop an increased number of

mitochondria to aid in energy production.



21.3 An Overview of Metabolism and Energy Production

Learning Objective:

• List the stages in catabolism of food and describe the role of each stage.

Metabolism The sum of all of the

chemical reactions that take place in an

organism.



Together, all of the chemical reactions that take place in an organism constitute its

metabolism. Most of these reactions occur in the reaction sequences of metabolic

pathways, a sequence of reactions where the product of one reaction serves as the starting material for the next. Such pathways may be linear (a series of reactions that convert

a reactant into a specific product through a series of intermediate molecules and reactions), cyclic (a series of reactions that regenerates one of the first reactants), or spiral

(the same set of enzymes progressively builds up or breaks down a molecule).

A linear

sequence



A



Enzyme 1



Enzyme 2



C



Enzyme 3



. . .



A

Enzyme 4



A cyclic

sequence



B



A



D

Enzyme 3



Enzyme 1



B

C



Enzyme 2



A spiral

sequence



Enzymes 1



4



Enzymes 1



4



Enzymes 1



4



B

C

Final product



Catabolism Metabolic reaction pathways that break down food molecules

and release biochemical energy.



As we study metabolism we will encounter each of these types of pathways. Those

pathways that break molecules apart are known collectively as catabolism, whereas

those that put building blocks back together to assemble larger molecules are known



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SECTION 21.3



An Overview of Metabolism and Energy Production



collectively as anabolism. The purpose of catabolism is to release energy from food,

and the purpose of anabolism is to synthesize new biomolecules, including those that

store energy.



699



Anabolism Metabolic reactions that

build larger biological molecules from

smaller pieces.



CATABOLISM



+



Energy



Smaller

molecules



Larger

molecules



A N A B O L IS M



The overall picture of digestion, catabolism, and energy production is simple: eating

provides fuel, breathing provides oxygen, and our bodies oxidize the fuel to extract energy. The process can be roughly divided into the four stages described here and shown

in Figure 21.4.

StAgE 1: Digestion Enzymes in saliva, the stomach, and the small intestine convert the



large molecules of carbohydrates, proteins, and lipids to smaller molecules. Carbohydrates are broken down to glucose and other sugars; proteins are broken down to amino

acids; and triacylglycerols, the lipids commonly known as fats and oils, are broken

down to glycerol plus long-chain carboxylic acids, termed fatty acids. These smaller

molecules are transferred into the blood for transport to cells throughout the body.

StAgE 2: Acetyl-coenzyme A production The small molecules from digestion follow

separate pathways that separate their carbon atoms into two-carbon acetyl groups. The

acetyl groups are attached to coenzyme A by a high-energy bond between the sulfur

atom of the thiol 1 ¬ SH2 group at the end of the coenzyme A molecule and the carbonyl carbon atom of the acetyl group.



See the chemical structure of

coenzyme A in Figure 19.10.



Acetyl-coenzyme A (acetyl-CoA)

Acetyl-substituted coenzyme A—the

common intermediate that carries acetyl groups into the citric acid cycle.



Acetyl group



Attachment of acetyl group to coenzyme A

Acetyl group



CH3



O

C



S



[Coenzyme A]



The resultant compound, acetyl-coenzyme A, which is abbreviated acetyl-CoA, is an

intermediate in the breakdown of all classes of food molecules. It carries the acetyl

groups into the common pathways of catabolism—Stage 3, the citric acid cycle and

Stage 4, electron transport and ATP production.

StAgE 3: Citric acid cycle Within mitochondria, the acetyl-group carbon atoms are

oxidized to the carbon dioxide that we exhale. Most of the energy released in the

oxidation leaves the citric acid cycle in the chemical bonds of reduced coenzymes

(NADH, FADH2). Some energy also leaves the cycle stored in the chemical bonds of

ATP or a related triphosphate.

StAgE 4: ATP production Electrons from the reduced coenzymes are passed from

molecule to molecule down an electron-transport chain. Along the way, their energy

is harnessed to produce more ATP. At the end of the process, these electrons—along

with hydrogen ions from the reduced coenzymes—combine with oxygen we breathe in

to produce water. Thus, the reduced coenzymes are in effect oxidized by atmospheric

oxygen, and the energy that they carried is stored in the chemical bonds of ATP

molecules.



Acetyl-coenzyme A



looKing AHEAD



Digestion and

conversion of food molecules to acetylCoA, Stages 1 and 2 in Figure 21.4,

occur by different metabolic pathways

for carbohydrates, lipids, and proteins.

Each of these pathways is discussed

separately in later chapters: carbohydrate metabolism in Chapter 22, lipid

metabolism in Chapter 24, and protein

metabolism in Chapter 25.



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700



CHAPTER 21



The Generation of Biochemical Energy





Figure 21.4



Pathways for the digestion of food and

the production of biochemical energy.

This diagram summarizes pathways

covered in this chapter (the citric acid

cycle and electron transport) and also

the pathways discussed in Chapter 22

for carbohydrate metabolism, in

Chapter 24 for lipid metabolism, and in

Chapter 25 for protein metabolism.



FOOD



LIPIDS

Stage 1. Digestion

Bulk food is digested in

the mouth, stomach, and

small intestine to yield

Fatty acids

small molecules.

and glycerol



CARBOHYDRATES



PROTEINS



Glucose and

other sugars



Amino acids



Glycolysis



Amino acid

catabolism



Fatty acid

oxidation



ATP

Stage 2. Acetyl-CoA Production

Sugar and amino acid molecules

are degraded in the cytoplasm of

cells to yield acetyl-CoA. Fatty acid

molecules are degraded in the

mitochondria of cells to yield acetyl-CoA.



Stage 3. Citric Acid Cycle

Acetyl-CoA is oxidized

inside mitochondria by

the citric acid cycle to

yield CO2 and reduced

coenzymes.



Pyruvate



Acetyl-CoA



Citric

acid

cycle



CO2



ATP



REDUCED

COENZYMES

Stage 4. ATP Production

The energy transferred to the

reduced coenzymes in stage

3 is used to make ATP by the

coupled pathways of electron

transport and oxidative

phosphorylation.



Electron

transport

chain



ATP



O2



H2O



Worked Example 21.2 Identifying Metabolic Pathways That Convert Basic Molecules to Energy

(a) In Figure 21.4, identify the stages in the catabolic pathway in which lipids ultimately yield ATP.

(b) In Figure 21.4, identify the place at which the products of lipid catabolism can join the common

metabolism pathway.

AnAlySiS Look at Figure 21.4 and find the pathway for lipids. Follow the arrows to trace the flow of energy.

Note that Stage 3 is the point at which the products of lipid, carbohydrate, and protein catabolism all feed into

a central, common metabolic pathway, the citric acid cycle. The lipid molecules that feed into Stage 3 do so

via acetyl-CoA (Stage 2). Note also that most products of Stage 3 catabolism feed into Stage 4 catabolism to

produce ATP.



Solution

The lipids in food are broken down in Stage 1 (digestion) to fatty acids and glycerol. Stage 2 (acetyl-CoA

production) results in fatty acid oxidation to acetyl-CoA. In Stage 3 (citric acid cycle), acetyl-CoA enters

the citric acid cycle (the common metabolism pathway), which produces ATP, reduced coenzymes, and

CO2. In Stage 4 (ATP production), the energy stored in the reduced coenzymes (from the citric acid cycle) is

converted to ATP energy.



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SECTION 21.4



Strategies of Metabolism: ATP and Energy Transfer



ProBlEM 21.3

(a) In Figure 21.4, identify the stages in the pathway for the conversion of the energy

from carbohydrates to energy stored in ATP molecules.

(b) In Figure 21.4, identify the three places at which the products of amino acid

catabolism can join the central metabolism pathway.



21.4 Strategies of Metabolism: ATP and Energy Transfer

Learning Objective:

• Describe the role of ATP in energy transfer.

ATP is the body’s energy-transporting molecule. What exactly does that mean? Consider that the molecule has three ¬ PO3- groups.

Adenosine



NH2



Triphosphate group



O





O



P



O





P



O





O



O



Bond broken

in hydrolysis to ADP



N



O



O



P



O



CH2







O



N



O



OH



N

N



OH



Adenosine triphosphate (ATP)



Removal of the terminal ¬ PO3 - 2 group from ATP by hydrolysis gives adenosine

diphosphate (ADP). The ATP S ADP reaction is exergonic; it releases chemical energy that was held in the bond to the ¬ PO32- group.

ATP + H2O ¡ ADP + HOPO32- + H +



∆G = - 30.5 kJ>mol



The reverse of ATP hydrolysis—a phosphorylation reaction—is endergonic.

ADP + HOPO32- + H + ¡ ATP + H2O



∆G = + 30.5 kJ>mol



(In equations for biochemical reactions, we represent ATP and other energycarrying molecules in red and their lower-energy equivalent molecules in blue.)

ATP is an energy transporter because its production from ADP requires an input

of energy that is released when the reverse reaction occurs. Biochemical energy is

gathered from exergonic reactions and stored in the bonds of the ATP molecule. ATP

hydrolysis releases energy for energy-requiring work. Biochemical energy production,

transport, and use, all depend upon the ATP ÷ ADP interconversion.

P P P



O



Adenosine



ATP

Energy

from

food



Energy

for

work



ADP

Phosphate



P P



O



Adenosine



Phosphate



The hydrolysis of ATP to give ADP and its reverse, the phosphorylation of ADP,

are reactions perfectly suited to their role in metabolism for two major reasons. Firstly,

ATP hydrolysis occurs slowly in the absence of a catalyst, so the stored energy is released only in the presence of the appropriate enzymes.



701



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CHAPTER 21



The Generation of Biochemical Energy



Secondly, the free energy of hydrolysis of ATP is an intermediate value for energy

carriers (Table 21.1). Since the primary metabolic function of ATP is to transport energy, it is often referred to as a “high-energy” molecule or as containing “high-energy”

phosphorus–oxygen bonds. These terms are misleading because they promote the idea

that ATP is somehow different from other compounds. The terms mean only that ATP

is reactive and that a useful amount of energy is released when a phosphoryl group is

removed from it by hydrolysis.

table 21.1  Free Energies of Hydrolysis of Some Phosphates

O

R



O



P



O

O2 1



H2O



ROH 1



HO



O2



O2



P



∆G 1kJ>mol2



O2



Compound Name



Function



Phosphoenol pyruvate



Final intermediate in conversion of

glucose to pyruvate (glycolysis)—

Stage 2, Figure 21.5



1, 3-Bisphosphoglycerate



Another intermediate in glycolysis



- 49.4



Energy storage in muscle cells



- 43.1



Principal energy carrier



- 30.5



Glucose 1-phosphate



First intermediate in breakdown of

carbohydrates stored as starch or

glycogen



- 20.9



Glucose 6-phosphate



First intermediate in glycolysis



- 13.8



Fructose 6-phosphate



Second intermediate in glycolysis



- 13.8



ATP 1 ¡ ADP2



Creatine phosphate



- 61.9



In fact, if removal of a phosphoryl group from ATP released unusually large

amounts of energy, other reactions would not be able to provide enough energy to convert ADP back to ATP. ATP is a convenient energy carrier in metabolism because its

free energy of hydrolysis has an intermediate value among high energy carriers. For

this reason, the phosphorylation of ADP can be driven by coupling this reaction with a

more exergonic reaction.



ProBlEM 21.4

Acetyl phosphate, whose structure is given here, is another compound with a relatively

high free energy of hydrolysis.

O



O

CH3



C



O



P



O−



O−



Using structural formulas, write the equation for the hydrolysis of this phosphate.



ProBlEM 21.5

A common metabolic strategy is the lack of reactivity—that is, the slowness to react—

of compounds whose breakdown is exergonic. For example, hydrolysis of ATP to ADP

or adenosine monophosphate (AMP) is exergonic but does not take place without an

appropriate enzyme present. Why would the cell use this metabolic strategy?



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