chapter 5 introduction to energy transfer

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C H A P T E R

5

CHAPTER OBJECTIVES

➤ Describe the first law of thermodynamics relatedto energy balance and work within biologicsystems

➤ Define potential energy and kinetic energy andgive examples of each

➤ Discuss the role of free energy in biologic work

➤ Give examples of exergonic and endergonicchemical reactions within the body and indicatetheir importance

➤ State the second law of thermodynamics and givea practical application of this law

➤ Discuss the role of coupled reactions in biologicprocesses

➤ Differentiate between photosynthesis and respira-tion and give the biologic significance of each

➤ Identify and give examples of the three forms ofbiologic work

➤ Describe how enzymes and coenzymes affectenergy metabolism

➤ Differentiate between hydrolysis and condensa-tion and explain their importance in physiologicfunction

➤ Discuss the role of redox chemical reactions inenergy metabolism

Introduction to Energy Transfer

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CHAPTER 5 Introduction to Energy Transfer 119The capacity to extract energy from the food macronutrientsand continually transfer it at a high rate to the contractileelements of skeletal muscle determines one’s capacity forswimming, running, or skiing long distances. Likewise,specific energy-transferring capacities that demand all-out,“explosive” power output for brief durations determine suc-cess in weightlifting, sprinting, jumping, and football lineplay. Although muscular activity represents the main frame ofreference in this text, all forms of biologic work requirepower generated from the direct transfer of chemical energy.

The sections that follow introduce general conceptsabout bioenergetics that form the basis for understanding en-ergy metabolism during physical activity.

ENERGY—THE CAPACITY FOR WORKUnlike the physical properties of matter, one cannot defineenergy in concrete terms of size, shape, or mass. Rather theterm energy reflects a dynamic state related to change; thus,energy emerges only when change occurs. Within this con-text, energy relates to the performance of work—as work in-creases so also does energy transfer and thus change. From aNewtonian (mechanical) perspective, work is the product ofa given force acting through a given distance. In the body, cellsmore commonly accomplish chemical and electrical workthan mechanical work. Because it is possible to exchange andconvert energy from one form to another, we commonly ex-press biologic work in mechanical units.

Bioenergetics refers to the flow and exchange of energywithin a living system. The first law of thermodynamicsdescribes a principle related to biologic work. Its basic tenetstates that energy cannot be created or destroyed but trans-forms from one form to another without being depleted. Inessence, this law describes the important conservation ofenergy principle that applies to both living and nonlivingsystems. In the body, chemical energy within the bonds of themacronutrients does not immediately dissipate as heat duringenergy metabolism; instead, a large portion remains as chem-ical energy, which the musculoskeletal system then changesinto mechanical energy (and ultimately to heat energy). Thefirst law of thermodynamics dictates that the body does notproduce, consume, or use up energy; instead it transforms itfrom one state into another as physiologic systems undergocontinual change.

INTEGRATIVE QUESTION

Based on the first law of thermodynamics, why isit imprecise to refer to energy “production” in thebody?

Potential and Kinetic EnergyPotential energy and kinetic energy constitute the total en-ergy of a system. FIGURE 5.1 shows potential energy as energyof position, similar to a boulder tottering atop a cliff or water

before it flows downstream. In the example of flowing water,the energy change is proportional to the water’s verticaldrop—the greater the vertical drop, the greater the potentialenergy at the top. The waterwheel harnesses a portion of theenergy from the falling water to produce useful work. In thecase of the falling boulder, all potential energy transforms tokinetic energy and dissipates as unusable heat.

Other examples of potential energy include boundenergy within the internal structure of a battery, a stick ofdynamite, or a macronutrient before releasing its stored en-ergy in metabolism. Releasing potential energy transformsit into kinetic energy of motion. In some cases, bound en-ergy in one substance directly transfers to other substancesto increase their potential energy. Energy transfers of thistype provide the necessary energy for the body’s chemicalwork of biosynthesis. In this process, specific building-block atoms of carbon, hydrogen, oxygen, and nitrogenbecome activated and join other atoms and molecules tosynthesize important biologic compounds and tissues.Some newly created compounds provide structure as inbone or the lipid-containing plasma membrane that en-closes each cell. Other synthesized compounds such asadenosine triphosphate (ATP) and phosphocreatine (PCr)serve the cell’s energy requirements.

Potential energy

Kinetic energy

Heat energy

Workresults fromharnessingpotentialenergy

Potential energydissipates to kineticenergy as the waterflows down the hill

Lower potentialenergy

Figure 5.1 • High-grade potential energy capable ofperforming work degrades to a useless form of kineticenergy. In the example of falling water, the waterwheelharnesses potential energy to perform useful work. For thefalling boulder, all of the potential energy dissipates tokinetic energy (heat) as the boulder crashes to the surfacebelow.

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120 Section 2 Energy for Physical Activity

Energy-Releasing and Energy-Conserving ProcessesThe term exergonic describes any physical or chemicalprocess that releases (frees) energy to its surroundings. Suchreactions represent “downhill” processes because of a declinein free energy—“useful” energy for biologic work that en-compasses all of the cell’s energy-requiring, life-sustainingprocesses. Within a cell, where pressure and volume remainrelatively stable, free energy (denoted by the symbol G tohonor Willard Gibbs [1839–1903] whose research providedthe foundation of biochemical thermodynamics) equals thepotential energy within a molecule’s chemical bonds (calledenthalpy, or H) minus the energy unavailable because of ran-domness (S) times the absolute temperature (°C � 273). Theequation G � H � TS describes free energy quantitatively.

Chemical reactions that store or absorb energy are ender-gonic; these reactions represent “uphill” processes and proceedwith an increase in free energy for biologic work. Exergonicprocesses sometimes link or couple with endergonic reactionsto transfer some energy to the endergonic process. In the body,coupled reactions conserve in usable form a large portion of thechemical energy stored within the macronutrients.

FIGURE 5.2 illustrates the flow of energy in exergonic andendergonic chemical reactions. Changes in free energy occurwhen the bonds in the reactant molecules form new productmolecules with different bonding. The equation that expressesthese changes, under conditions of constant temperature,pressure, and volume, takes the following form:

�G � �H � T�S

The symbol � designates change. The change in free en-ergy represents a keystone of chemical reactions. In exergonicreactions, �G is negative; the products contain less freeenergy than the reactants, with the energy differential releasedas heat. For example, the union of hydrogen and oxygen to

form water releases 68 kCal per mole (molecular weight of asubstance in grams) of free energy in the following reaction:

H2 � O : H2O � �G 68 kCal � mole�1

In the reverse endergonic reaction, �G remains positivebecause the product contains more free energy than the reac-tants. The infusion of 68 kCal of energy per mole of watercauses the chemical bonds of the water molecule to splitapart, freeing the original hydrogen and oxygen atoms. This“uphill” process of energy transfer provides the hydrogen andoxygen atoms with their original energy content to satisfy theprinciple of the first law of thermodynamics—the conserva-tion of energy.

H2 + O ; H2O � �G 68 kCal � mole�1

Energy transfer in cells follows the same principles asthose in the waterfall–waterwheel example. Carbohydrate,lipid, and protein macronutrients possess considerable potentialenergy within their chemical bonds. The formation of productsubstances progressively reduces the nutrient molecule’s origi-nal potential energy with a corresponding increase in kineticenergy. Enzyme-regulated transfer systems harness or conservea portion of this chemical energy in new compounds for use inbiologic work. In essence, living cells serve as transducers withthe capacity to extract and use chemical energy stored within acompound’s atomic structure. Conversely, and equally impor-tant, cells also bond atoms and molecules together to raise themto a higher level of potential energy.

The transfer of potential energy in any spontaneousprocess always proceeds in a direction that decreases the ca-pacity to perform work. The tendency of potential energy todegrade to kinetic energy of motion with a lower capacity forwork (i.e., increased entropy) reflects the second law ofthermodynamics. A flashlight battery provides a good illus-tration. The electrochemical energy stored within its cellsslowly dissipates, even if the battery remains unused. The

Endergonic

Energysupplied

Product

Product

Reactant

Reaction Progress Reaction Progress

Ener

gy

Ener

gy

A

Exergonic

Energyreleased

Reactant

B

Figure 5.2 • Energy flow in chemical reactions. A. Energy supply prepares an endergonic reaction to proceed because thereaction’s product contains more energy than the reactant. B. Exergonic reaction releases energy, resulting in less energy in theproduct than in the reactant.

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CHAPTER 5 Introduction to Energy Transfer 121energy from sunlight also continually degrades to heat energywhen light strikes and becomes absorbed by a surface. Foodand other chemicals represent excellent stores of potentialenergy, yet this energy continually decreases as the compoundsdecompose through normal oxidative processes. Energy, likewater, always runs downhill so potential energy decreases.Ultimately, all of the potential energy in a system degrades tothe unusable form of kinetic or heat energy.

INTERCONVERSIONS OF ENERGYThe total energy in an isolated system remains constant; a de-crease in one form of energy matches an equivalent increase inanother form. During energy conversions, a loss of potentialenergy from one source often produces a temporary increase in

the potential energy of another source. In this way, nature har-nesses vast quantities of potential energy for useful purposes.Even under such favorable conditions, the net flow of energy inthe biologic world moves toward entropy, ultimately producinga loss of potential energy.

In 1877, Austrian physicist Ludwig Boltzmann (1844–1906) established the relationship between entropy and thestatistical analysis of molecular motion. Entropy reflects thecontinual process of energy change. All chemical and physi-cal processes proceed in a direction where total randomnessor disorder increases and the energy available for workdecreases. In coupled reactions during biosynthesis, part of asystem may show a decrease in entropy while another partshows an increase. No way exists to circumvent the secondlaw—the entire system always shows a net increase in

H2O

CO2

GlucoseH2O CO2

O2

Light energy(Sun)

Nuclear energy(reactor)

Electricalenergy

Mechanical energy(hydroelectricgenerating plant)

Heat energy(solar panels)

Chemical energy(fossil fuel, oilburner)

Forms of Energy • Chemical • Mechanical • Heat • Light • Electrical • Nuclear

Figure 5.3 • Interconversions among six forms of energy.

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entropy. In a more global sense, the biochemical reactionswithin the body’s trillions of cells (as within the universe as awhole) “tilt” in the direction of spontaneity that favors disor-der and randomness of an irreversible process (i.e., entropy)as originally theorized by Boltzmann.

Forms of EnergyFIGURE 5.3 shows energy categorized into one of six forms:chemical, mechanical, heat, light, electrical, and nuclear.

Examples of Energy Conversions

The conversion of energy from one form to anotheroccurs readily in the inanimate and animate worlds.Photosynthesis and respiration represent the most funda-mental examples of energy conversion in living cells.

Photosynthesis. In the sun, nuclear fusion releases partof the potential energy stored in the nucleus of the hydrogenatom. This energy, in the form of gamma radiation, then con-verts to radiant energy.

FIGURE 5.4 depicts the dynamics of photosynthesis, an en-dergonic process powered by energy from sunlight. The pig-ment chlorophyll, contained in large chloroplast organelleswithin the leaf’s cells, absorbs radiant (solar) energy to syn-thesize glucose from carbon dioxide and water, while oxygenflows to the environment. The plant also converts carbohy-drates to lipids and proteins for storage as a future reserve forenergy and to sustain growth. Animals then ingest plant nutri-ents to serve their own energy and growth needs. In essence,solar energy coupled with photosynthesis powers the animalworld with food and oxygen.

Respiration. FIGURE 5.5 shows the exergonic reactionsof respiration, the reverse of photosynthesis, as the plant’sstored energy is recovered for biologic work. In the presenceof oxygen, the cells extract the chemical energy stored inthe carbohydrate, lipid, and protein molecules. For glucose,respiration releases 689 kCal per mole (180 g) oxidized. Aportion of the energy released during cellular respiration isconserved in other chemical compounds for use in energy-requiring processes; the remaining energy flows to the envi-ronment as heat.

Sun(fusion)

Nuclear energyNuclear energy

Radiantenergy

Radiantenergy

H2O

CO2CO2

O2

Chlorophyll6CO2 6O26H2O Stored energy

GlucoseLipidProtein

O2

Figure 5.4 • The endergonic process of photosynthesis in plants, algae, and some bacteria serves as the mechanism tosynthesize carbohydrates, lipids, and proteins. In this example, a glucose molecule forms when carbon dioxide binds withwater with a positive free energy (useful energy) change (��G).

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CHAPTER 5 Introduction to Energy Transfer 123


From the perspective of human bioenergetics,discuss the significance of a bumper stickerthat reads: Have you thanked a green planttoday?

BIOLOGIC WORK IN HUMANSFigure 5.5 also illustrates that biologic work takes one ofthree forms:

1. Mechanical work of muscle contraction2. Chemical work that synthesizes cellular molecules3. Transport work that concentrates substances in the

intracellular and extracellular fluids

Mechanical WorkMechanical work generated by muscle contraction and subse-quent movement provides the most obvious example of en-ergy transformation. The molecular motors in a muscle fiber’sprotein filaments directly convert chemical energy into me-chanical energy. This does not represent the body’s only formof mechanical work. In the cell nucleus, contractile elementsliterally tug at chromosomes to facilitate cell division.Specialized structures (such as cilia) also perform mechanical

work in many cells. “In a Practical Sense,” see p. 125, showsthe method for quantifying work (and power) for three com-mon exercises.

Chemical WorkAll cells perform chemical work for maintenance and growth.Continuous synthesis of cellular components takes place asother components break down. The muscle tissue synthesisthat occurs in response to chronic overload in resistance train-ing vividly illustrates chemical work.

Transport WorkThe biologic work of concentrating substances in the body(transport work) progresses much less conspicuously thanmechanical or chemical work. Cellular materials normallyflow from an area of high concentration to one of lower con-centration. This passive process of diffusion does not requireenergy. Under normal physiologic conditions, some chemicalsrequire transport “uphill” from an area of lower to higher con-centration. Active transport describes this energy-requiringprocess. Secretion and reabsorption in the kidney tubules relyon active transport mechanisms, as does neural tissue to es-tablish the proper electrochemical gradients about its plasmamembranes. These “quiet” forms of biologic work require acontinual expenditure of stored chemical energy.

Cellular respiration(reverse of photosynthesis)

Chemical work

6 6 6Glucose

Glucose Glycogen

Glycerol +fatty acids Triacylglycerol

Amino acids Protein

O2 H2OCO2 ATP

ATP

ATP

ATP

Mechanicalwork

ATP

Na+ Na+

Na+K+

K+

K+

Transport workExtracellular fluid

Cytoplasm

ATPADP

P

Figure 5.5 • The exergonic process of cellular respiration. Exergonic reactions, such as the burning of gasoline or theoxidation of glucose, release potential energy. This produces a negative standard free energy change (i.e., reduction in totalenergy available for work or ��G). In this illustration, cellular respiration harvests the potential energy in food to form ATP.Subsequently, the energy in ATP powers all forms of biologic work.

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FACTORS THAT AFFECT THE RATE OF BIOENERGETICSThe upper limits of exercise intensity ultimately depend on therate that cells extract, conserve, and transfer chemical energy infood nutrients to the contractile filaments of skeletal muscle.The sustained pace of a marathon runner at close to 90% ofaerobic capacity, or the sprinter’s rapid speed in all-out exer-cise, directly reflects the body’s capacity to transfer chemicalenergy to mechanical work. Enzymes and coenzymes greatlyalter the rate of energy release during chemical reactions.

What Is in a Name?

Because of confusion in the naming of enzymes, theInternational Union of Biochemistry and MolecularBiology (www.chem.qmul.ac.uk/iubmb/) devel-oped a systematic system that classified and namedenzymes according to their specific functions. Eachenzyme class has a general name as well as a rec-ommended name. Except for older enzyme namessuch as renin, trypsin, and pepsin, the suffix –aseappends to the enzyme based on its mode of opera-tion or substance with which it interacts.

The six classifications of enzymes are as follows:

1. Oxidoreductases—Catalyze oxidation-reduction reactions where the substrate oxidizedis regarded as hydrogen or electron donor;includes dehydrogenases, oxidates, oxygenases,reductases, peroxidases, and hydroxylases.(Example � lactate dehydrogenase)

2. Transferases—Catalyze the transfer of a group(for example, the methyl group or a glycosylgroup) from one compound (generally regarded asdonor) to another compound (generally regardedas acceptor) and include kinases, transcarboxy-lases, and transaminases. (Example � hexokinase)

3. Hydrolases—Catalyze reactions that add waterand include esterases, phosphatases, and pepti-dases. (Example � lipase)

4. Lyases—Catalyze reactions that cleave C–C,C–O, C–N, and other bonds by other means thanby hydrolysis or oxidation. They differ from otherenzymes in that two substrates are involved inone reaction direction, but only one in the otherdirection. Include synthases, deaminases, and de-carboxylases. (Example � carbonic anhydrase)

5. Isomerases—Catalyze reactions that rearrangemolecular structure and include isomerases andepimerases. These enzymes catalyze changeswithin one molecule. (Example � phospho-glycerate mutase)

6. Ligases—Catalyze bond formation between twosubstrate molecules with concomitant hydrolysisof the diphosphate bond in ATP or a similartriphosphate. (Example � pyruvate carboxylase)

Enzymes as Biologic Catalysts

Enzymes are highly specific and large protein catalysts thataccelerate the forward and reverse rates of chemical reac-tions without being consumed or changed in the reaction.Enzymes only govern reactions that normally take place, butat a much slower rate. In a way, enzymes reduce requiredactivation energy—the energy input to initiate a reaction—so its rate changes. Enzyme action takes place without alter-ing equilibrium constants and total energy released (freeenergy change, or �G) in the reaction. FIGURE 5.6 contraststhe effectiveness of a catalyst in initiating a chemical reac-tion with initiation in the uncatalyzed state. The vertical axisrepresents energy required to activate each reaction; the

Waves cannot clear barrier

Waves readily clear barrier

Free

ene

rgy

Course of reaction

Reactants

Catalyzedreaction

Uncatalyzedreaction

Products

G

Figure 5.6 • The presence of a catalyst greatly reduces theactivation energy to initiate a chemical reaction comparedwith the energy for an uncatalyzed reaction. For theuncatalyzed reaction to proceed, the reactant must have ahigher free energy level than the product.

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An ergometer is an exercise apparatus that quantifies and standard-izes physical exercise in terms of work and/or power output. Themost common ergometers include treadmills, cycle and arm-crankergometers, stair steppers, and rowers.

Work (W) represents application of force (F) through a distance (D):

W � F � D

For example, for a body mass of 70 kg and vertical jump score of0.5 m, work accomplished equals 35 kilogram-meters (70 kg � 0.5 m).The most common units of measurement to express work includekilogram-meters (kg-m), foot-pounds (ft-lb), joules (J), Newton-meters (Nm), and kilocalories (kCal).

Power (P) represents W performed per unit time (T):

P � F � D � T

CALCULATION OF TREADMILL WORKConsider the treadmill as a moving conveyor belt with variable angleof incline and speed. Work performed on a treadmill equals the prod-uct of the weight (mass) of the person (F) and the vertical distance(vert dist) the person achieves walking or running up the incline. Vertdist equals the sine of the treadmill angle (theta, or ) multiplied bythe distance traveled (D) along the incline (treadmill speed � time).

W � body mass (force) � vertical distance

EXAMPLEFor an angle of 8° (measured with an inclinometer or determinedby knowing the percent grade of the treadmill), the sine of angle equals 0.1392 (see table). The vert dist represents treadmill speedmultiplied by exercise duration multiplied by sine . For example, vert dist on the incline while walking at 5000 m � h�1 for 1 hour equals696 m (5000 � 0.1392). If a person with a body mass of 50 kgwalked on a treadmill at an incline of 8° (grade approximately 14%)for 60 minutes at 5000 m � h�1, work accomplished computes as:

W � F � vert dist (sine � � D)

� 50 kg � (0.1392 � 5000 m)

� 34,800 kg-m

The value for power equals 34,800 kg-m � 60 minutes, or 580 kg-m � min�1.

CALCULATION OF CYCLE ERGOMETER WORKThe mechanically braked cycle ergometer contains a flywheel witha belt around it connected by a small spring at one end and an

adjustable tension lever at the other end. A pendulum balance indi-cates the resistance against the flywheel as it turns. Increasing thetension on the belt increases flywheel friction, which increases re-sistance to pedaling. The force (flywheel friction) represents brakingload in kg or kilopounds (kp � force acting on 1-kg mass at the nor-mal acceleration of gravity). The distance traveled equals numberof pedal revolutions times flywheel circumference.

EXAMPLEA person pedaling a bicycle ergometer with a 6-m flywheel circum-ference at 60 rpm for 1 minute covers a distance (D) of 360 m eachminute (6 m � 60). If the frictional resistance on the flywheelequals 2.5 kg, total work computes as:

W � F � D

� frictional resistance � distance traveled

� 2.5 kg � 360 m

� 900 kg-m

I N A P R A C T I C A L S E N S E

Measurement of Work on a Treadmill, Cycle Ergometer, and Step Bench

D�

Angle (°) Sine () Grade (%)

1 0.0175 1.752 0.0349 3.493 0.0523 5.234 0.0698 6.985 0.0872 8.726 0.1045 10.517 0.1219 12.288 0.1392 14.059 0.1564 15.84

10 0.1736 17.6315 0.2588 26.8020 0.3420 36.40

Adjustable tension knob

Spring

Tension belt

Chain

Flywheel

Pedal

Pendulum

Continued on page 126


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Power generated by the effort equals 900 kg-m in 1 minute or 900kg-m � min�1 (900 kg-m � 1 min).

CALCULATION OF WORK DURING BENCH STEPPINGOnly the vertical (positive) work can be calculated in bench stepping.Distance (D) computes as bench height times the number of timesthe person steps; force (F ) equals the person’s body mass (kg).

EXAMPLEIf a 70-kg person steps on a bench 0.375-m high at a rate of 30steps per minute for 10 minutes, total work computes as

W � F � D

� body mass, kg � (vertical distance [m] �

steps per min � 10 min)

� 70 kg � (0.375 m � 30 � 10)

� 7875 kg-m

Power generated during stepping equals 787 kg-m � min�1

(7875 kg-m � 10 min).

I N A P R A C T I C A L S E N S E Continued

horizontal axis plots the reaction’s progress. Clearly, initia-tion (activation) of an uncatalyzed reaction requires consid-erably more energy than a catalyzed one. Without enzymeaction, the complete digestion of a breakfast meal mighttake 50 years!

Enzymes possess the unique property of not being read-ily altered by reactions they affect. Consequently, enzymeturnover in the body remains relatively slow, and the specificenzymes are continually reused. A typical mitochondrion maycontain up to 10 billion enzyme molecules, each carrying outmillions of operations within a brief time. During all-outexercise, enzyme activity increases tremendously as energydemands rise about 100 times above the resting level. A singlecell can contain thousands of different enzymes, each with aspecific function that catalyzes a distinct cellular reaction. Forexample, glucose breakdown to carbon dioxide and water re-quires 19 different chemical reactions, each catalyzed by itsown specific enzyme. Many enzymes operate outside thecell—in the bloodstream, digestive mixture, or intestinalfluids.

Reaction Rates

Enzymes do not all operate at the same rate; some operateslowly, others more rapidly. Consider the enzyme carbonic an-hydrase, which catalyzes the hydration of carbon dioxide toform carbonic acid. Its maximum turnover number—numberof moles of substrate that react to form product per mole of en-zyme per unit time—is 800,000. In contrast, the turnover num-ber is only 2 for tryptophan synthetase, which catalyzes thefinal step in tryptophan synthesis. Enzymes also act alongsmall regions of substrate, each time working at a different ratethan previously. Some enzymes delay initiating their work.The precursor digestive enzyme trypsinogen, manufactured bythe pancreas in inactive form, serves as a good example.Trypsinogen enters the small intestine where upon activationby intestinal enzyme action it becomes the active enzymetrypsin, which digests complex proteins into simple aminoacids. Proteolytic action describes this catabolic process.Without the delay in activity, trypsinogen would literally di-gest the pancreatic tissue that produced it.

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FIGURE 5.7 shows that pH and temperature dramaticallyalter enzyme activity. For some enzymes, peak activity re-quires relatively high acidity, whereas others function opti-mally on the alkaline side of neutrality. Note that the twoenzymes pepsin and trypsin exhibit different pH profiles thatmodify their activity rates and determine optimal function.Pepsin operates optimally at a pH between 2.4 and 2.6,whereas trypsin’s optimum range approximates that of salivaand milk (6.2 to 6.6). This pH effect on enzyme dynamicstakes place because changing a fluid’s hydrogen ion concen-tration alters the balance between positively and negativelycharged complexes in the enzyme’s amino acids. Increases intemperature generally accelerate enzyme reactivity. As tem-perature rises above 40 to 50°C, the protein enzymes perma-nently denature and their activity ceases.

Mode of Action

Interaction with its specific substrate represents a uniquecharacteristic of an enzyme’s 3-dimensional globular proteinstructure. Interaction works like a key fitting a lock, as illus-trated in FIGURE 5.8. The enzyme turns on when its active site(usually a groove, cleft, or cavity on the protein’s surface) joinsin a “perfect fit” with the substrate’s active site. Upon formingan enzyme–substrate complex, the splitting of chemical bondsforms a new product with new bonds. This frees the enzyme toact on additional substrate. The example depicts the interactionsequence of the enzyme maltase as it disassembles (hydrolyzes)maltose into its component two glucose building blocks:

Step 1: The active site of the enzyme and substrate lineup to achieve a perfect fit, forming an enzyme–substratecomplex.

Step 2: The enzyme catalyzes (greatly speeds up) thechemical reaction with the substrate. Note that thehydrolysis reaction adds a water molecule.

Step 3: An end product forms (two glucose molecules),releasing the enzyme to act on another substrate.

First proposed in the early 1890s by the German chemistand Nobel laureate Emil Fischer (1852–1919), a “lock-and-keymechanism” describes the enzyme–substrate interaction. Thisprocess ensures that the correct enzyme “mates” with its spe-cific substrate to perform a particular function. Once the en-zyme and substrate join, a conformational change in enzymeshape takes place as it molds to the substrate. Even if an en-zyme links with a substrate, unless the specific conformationalchange occurs in the enzyme’s shape, it will not interact chem-ically with the substrate. A more contemporary hypothesis con-siders the lock and key more of an “induced fit” because of therequired conformational characteristics of enzymes.

Optimum Temperature

Temperature of reaction pH of reaction

Rate

of

reac

tion

Rate

of

reac

tion

A

Optimum pH

Pepsin Trypsin

B

Figure 5.7 • Effects of (A) temperature and (B) pH on the enzyme action turnover rate.

Substrate

Enzyme-substratecomplex

Active site

H2O

Substrate matches active site of enzyme

1

Enzyme-substrate complex splits to yield product

2Enzyme now available for interaction with other substrate

3

Enzyme

Glucosemolecules

Maltase Maltase

Maltase

Maltose

Figure 5.8 • Sequence of steps in the “lock-and-keymechanism” of an enzyme with its substrate. The exampleshows how two monosaccharide glucose molecules formwhen maltase interacts with its disaccharide substratemaltose.

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The lock-and-key mechanism serves a protective func-tion so only the correct enzyme activates a given substrate.Consider the enzyme hexokinase, which accelerates a chemi-cal reaction by linking with a glucose molecule. When thisoccurs, a phosphate molecule transfers from ATP to a specificbinding site on one of glucose’s carbon atoms. Once the twobinding sites join to form a glucose–hexokinase complex, thesubstrate begins its stepwise degradation (controlled by otherspecific enzymes) to form less complex molecules during en-ergy metabolism.

CoenzymesSome enzymes remain totally dormant unless activated byadditional substances termed coenzymes. These nonproteinorganic substances facilitate enzyme action by binding thesubstrate with its specific enzyme. Coenzymes then regener-ate to assist in further similar reactions. The metallic ions ironand zinc play coenzyme roles, as do the B vitamins or theirderivatives. Oxidation–reduction reactions use the B vitaminsriboflavin and niacin, while other vitamins serve as transferagents for groups of compounds in different metabolicprocesses (see Table 2.1).

Vitamins Serve As Coenzymes But Do NotProvide Energy

Some advertisements for vitamins imply that tak-ing vitamin supplements provides immediate us-able energy for exercise. This simply does notoccur. Vitamins often serve as coenzymes to “makereactions go,” but they contain no chemical energyfor biologic work.

A coenzyme requires less specificity in its action than anenzyme because the coenzyme affects a number of differentreactions. It either acts as a “cobinder” or serves as a tempo-rary carrier of intermediary products in the reaction. For ex-ample, the coenzyme nicotinamide adenine dinucleotide(NAD�) forms NADH in transporting hydrogen atoms andelectrons released from food fragments during energy metab-olism. The electrons then pass to other special transportermolecules in another series of chemical reactions that ulti-mately deliver the electrons to oxygen.

Enzyme Inhibition. A variety of substances inhibitenzyme activity to slow the rate of a reaction. Competitiveinhibitors closely resemble the structure of the normal sub-strate for an enzyme. They bind to the enzyme’s active sitebut the enzyme cannot change them. The inhibitor repetitivelyoccupies the active site and blunts the enzyme’s interactionwith its substrate. Noncompetitive inhibitors do not resem-ble the enzyme’s substrate and do not bind to its active site.Instead, they bind to the enzyme at a site other than the activesite. This changes the enzyme’s structure and ability to catalyze

the reaction because of the presence of the bound inhibitor.Many drugs act as noncompetitive enzyme inhibitors.

HYDROLYSIS AND CONDENSATION:THE BASIS FOR DIGESTION ANDSYNTHESISIn general, hydrolysis reactions digest or break down com-plex molecules into simpler subunits; condensation reactionsbuild larger molecules by bonding their subunits together.

Hydrolysis ReactionsHydrolysis catabolizes carbohydrates, lipids, and proteinsinto simpler forms the body easily absorbs and assimilates.This basic decomposition process splits chemical bonds byadding H+ and OH– (constituents of water) to the reactionbyproducts. Examples of hydrolytic reactions include diges-tion of starches and disaccharides to monosaccharides, pro-teins to amino acids, and lipids to their glycerol and fatty acidconstituents. Specific enzymes catalyze each step of thebreakdown process. For disaccharides, the enzymes are lac-tase (lactose), sucrase (sucrose), and maltase (maltose). Thelipid enzymes (lipases) degrade the triacylglycerol moleculeby adding water. This cleaves the fatty acids from their glyc-erol backbone. During protein digestion, protease enzymesaccelerate amino acid release when the addition of watersplits the peptide linkages. The following represents the gen-eral form for all hydrolysis reactions:

AB � HOH : A-H � B-OH

Water added to the substance AB causes the chemical bondthat joins AB to decompose to produce the breakdown productsA-H (H refers to a hydrogen atom from water) and B-OH (OHrefers to the hydroxyl group from water). FIGURE 5.9A illustratesthe hydrolysis reaction for the disaccharide sucrose to its end-product molecules, glucose and fructose. The figure alsoshows the hydrolysis of a dipeptide (protein) into its two con-stituent amino acid units. Intestinal absorption occurs quicklyfollowing hydrolysis of the carbohydrate, lipid, and proteinmacronutrients.

Condensation ReactionsThe reactions of hydrolysis can occur in the opposite direc-tion as the compound AB synthesizes from A-H and B-OH. Awater molecule also forms in this building process of conden-sation (also termed dehydration synthesis). The structuralcomponents of the nutrients bind together in condensation re-actions to form more complex molecules and compounds.Figure 5.9B shows the condensation reactions for maltosesynthesis from two glucose units and the synthesis of a morecomplex protein from two amino acid units. During proteinsynthesis, a hydroxyl removed from one amino acid and a hy-drogen from the other amino acid join to create a water mole-cule. Peptide bond describes the new bond that forms for the

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protein. Water also forms in the synthesis of more complexcarbohydrates from simple sugars; for lipids, water formswhen glycerol and fatty acid components combine to form atriacylglycerol molecule.

Oxidation and Reduction Reactions

Literally thousands of simultaneous chemical reactions occurin the body that involve the transfer of electrons from onesubstance to another. Oxidation reactions transfer oxygenatoms, hydrogen atoms, or electrons. A loss of electronsalways occurs in oxidation reactions, with a correspondingnet gain in valence. For example, removing hydrogen from asubstance yields a net gain of valence electrons. Reduction

involves any process in which the atoms in an element gainelectrons, with a corresponding net decrease in valence.

The term reducing agent describes the substance thatdonates or loses electrons as it oxidizes. The substance beingreduced or gaining electrons is called the electron acceptor, oroxidizing agent. Electron transfer requires both oxidizingand reducing agents. Oxidation and reduction reactions be-come characteristically coupled. Whenever oxidation occurs,the reverse reduction also takes place; when one substanceloses electrons, the other substance gains them. The termredox reaction commonly describes a coupled oxidation–reduction reaction.

An excellent example of a redox reaction involves thetransfer of electrons within the mitochondria. Here, special

A Hydrolysis

B Condensation

H

OH2CH

CH2OH

H

H

HO

OH

R

C

H

N

H

H20

H20

H20

H20

H

CH2OH

H

OHHO

OH

O

H

OH

H

Sucrose

Dipeptide

Peptidebond

O

C

O

C

H

N

H HO

R

C

H

H

OH2CH

CH2OH

H

H

HO

OH

H

CH2OH

H

OHHO

OH H

OH

H

Glucose Fructose

OH H

OH H

O

New bondcreated

R

C

H

N

H

Amino acid Amino acid

O

C

O

C

H

N

H HO

R

C

H

C

H

N

H

H

CH2OH

H

OHHO OH

OH

O

H

OH

H

H

CH2OH

H

OH

OH H

OH

H

Maltose

Dipeptide

Peptidebond

O

C

O

C

H

N

H HO

C

H

H

CH2OH

H

OHHO

OH H

OH

H

Glucose

OH

H

CH2OH

H

OHO

OH H

OH

H

Glucose

OHH

OH H

R R R R

C

H

N

H

Amino acid Amino acid

O

C

O

C

H

N

H HO

C

H

Figure 5.9 • A. Hydrolysis of the disaccharide sucrose to the end-product molecules glucose and fructose and the hydrolysis ofa dipeptide (protein) into two amino acid constituents. B. A condensation chemical reaction for synthesizing maltose from twoglucose units and creation of a protein dipeptide from two amino acid subunits. Note that the reactions in B illustrate thereverse of the hydrolysis reaction for the dipeptide. The symbol R represents the remainder of the molecule.

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Cell Mitochondrion

Matrix Inner membrane

Outermembrane

Outer membrane

Chemical events

Monoamine oxidasePhospholipid synthesisFatty acid desaturationFatty acid elongation

Fatty acid transportElectron transportOxidative phosphoralyationTranshydrogenaseTransport systems

Matrix

Citric acid cyclePryuvate dehydrogenase complexGlutamate dehydrogenaseFatty acid oxidationUrea cycleReplicationTranscriptionTranslation

H+

H+

H+

H+

H+

H2O

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+H+ H+

H+

H+

H+

H+

H+

e-

ATP

ADP Pi

Pyruvate, fatty acids,and amino acids

from cytosol

CitricAcidCycle

FAD

FADH2

NAD+

NADH

NADH

Complex I

Complex II

CoenzymeQ

Cytochromec

Complex III

Complex IV

Complex V

Amino acids

Fumarate

Succinate

Acetyl-CoA

O2

from cytosol

Matrix

NADH

Inner membrane

Inner membrane

Figure 5.10 • The mitochondrion, its intramitochondrial structures, and primary chemical reactions. The inset tablesummarizes the different chemical events in relation to mitochondrial structures.

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down the chain and reduce one oxygen atom. The processends when oxygen accepts two hydrogens and forms water.This coupled redox process constitutes hydrogen oxidationand subsequent oxygen reduction. Chemical energy trapped(conserved) during cellular oxidation–reduction forms ATP,the energy-rich molecule that powers all biologic work.

FIGURE 5.11 illustrates a redox reaction during vigorousphysical activity. As exercise intensifies, hydrogen atoms arestripped from the carbohydrate substrate faster than their oxi-dation in the respiratory chain. To continue energy metabo-lism, a substance other than oxygen must “accept” thenonoxidized excess hydrogens. This occurs when a pyruvatemolecule, an intermediate compound formed in the initialphase of carbohydrate catabolism, temporarily accepts a pairof hydrogens (electrons). A new compound, lactate (ionizedlactic acid in the body), forms when reduced pyruvate acceptsadditional hydrogens. Fig. 5.11 illustrates that as more intenseexercise produces a greater flow of excess hydrogens to pyru-vate, lactate concentration rises rapidly within the blood andactive muscle. During recovery, the excess hydrogens inlactate oxidize (electrons removed and passed to NAD�) tore-form a pyruvate molecule. The enzyme lactate dehydroge-nase (LDH) accelerates this reversal. Chapter 6 more fullydiscusses oxidation–reduction reactions in human energymetabolism.

Measuring Energy Release in HumansThe gain or loss of heat in a biologic system provides a sim-ple way to determine the energy dynamics of any chemicalprocess. In food catabolism within the body, a humancalorimeter (see Fig. 8.1), similar to the bomb calorimeter

CHAPTER 5 Introduction to Energy Transfer 131carrier molecules transfer oxidized hydrogen atoms and theirremoved electrons for delivery to oxygen, which becomesreduced. The carbohydrate, fat, and protein substrates providea ready source of hydrogen atoms. Dehydrogenase (oxidase)enzymes speed up the redox reactions. Two hydrogen-accept-ing dehydrogenase coenzymes are the vitamin B–containingNAD� and flavin adenine dinucleotide (FAD). Transferringelectrons from NADH and FADH2 harnesses energy in theform of ATP.

Energy release in glucose oxidation occurs when elec-trons reposition (shift) as they move closer to oxygenatoms—their final destination. The close-up illustration of amitochondrion in FIGURE 5.10 shows the various chemicalevents that take place on the outer and inner mitochondrialmembranes and matrix. The inset table summarizes the mito-chondrion’s molecular reactions related to its structures. Mostof the energy-generating “action,” including the redox reac-tions, takes place within the mitochondrial matrix. The innermembrane is rich in protein (70%) and lipid (30%), two keymacromolecules whose configurations encourage transfer ofchemicals through membranes.


What biologic benefit comes from the coupling ofoxidation and reduction reactions?

The transport of electrons by specific carrier moleculesconstitutes the respiratory chain. Electron transport repre-sents the final common pathway in aerobic (oxidative) metab-olism. For each pair of hydrogen atoms, two electrons flow

Reduction reaction

2C3H4O3 + 2H 2C3H6O3Pyruvate Lactate (gains 2 hydrogens)

LDH

Level of exercise

Exercise Recovery

Bloo

d la

ctat

e le

vel

Ligh

t

Mod

erat

e

Stre

nuou

s

Very

str

enuo

us

Oxidation reaction

2C3H6O3 2H 2C3H4O3Lactate Pyruvate (loses 2 hydrogens)

LDH

Time

Bloo

d la

ctat

e le

vel

Figure 5.11 • Example of a redox (oxidation–reduction) reaction. During progressively more strenuous exercise when oxygensupply (or use) becomes inadequate, some pyruvate formed in energy metabolism gains two hydrogens (two electrons) andbecomes reduced to a new compound, lactate. In recovery, when oxygen supply (or use) becomes adequate, lactate loses twohydrogens (two electrons) and oxidizes back to pyruvate. This example shows how a redox reaction continues energymetabolism, despite limited oxygen availability (or use) in relation to exercise energy demands.

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Wilmore JH, Costill DL. Adequacy of the Haldanetransformation in the computation of exercise V·O2 inman. J Appl Physiol 1973;35:85.

➤ Oxygen consumption using open-circuit spirometry repre-sents a fundamental measurement in exercise physiology.This methodology assumes no nitrogen production or reten-tion by the body, so the nitrogen volume remains equal in theinspired and expired air. Because of this intrinsic relation-ship, no need exists to collect and analyze both inspired andexpired air volumes during measurement of oxygen con-sumption and carbon dioxide production. The followingmathematical relationship, known as the Haldane transfor-mation, exists between inspired and expired air volumes:

VI � VE � FEN2 � FIN2

where VI equals air volume inspired, VE equals air volumeexpired, and FEN2 and FIN2 equal the fractional concentra-tions of nitrogen in the expired and inspired air. Becausethe fractional concentrations for inspired oxygen, carbondioxide, and nitrogen are known, only VE (or VI) and theconcentrations in expired air of CO2 (FECO2) and O2 (FEO2)are required to calculate the oxygen consumed eachminute (V·O2):

V·O2 � V· E � FEN2/FIN2 � FIO2 � V· E � FEO2

In this formula, FEN2 usually equals 1.00 � (FEO2 �FECO2).

The Wilmore and Costill study determined any nitro-gen retention or production and how it influenced the accu-racy of oxygen consumption computations using thetraditional Haldane transformation during light-to-intenseexercise. Six subjects completed treadmill exercise bywalking on the level at 4 mph; a 5-minute jog followed at6.0 mph, followed again by a 5-minute run at 7.5 mph.Oxygen consumption, continuously monitored using open-circuit spirometry, included measurement of inspired andexpired ventilation volumes. Measurements also includedbarometric pressure, inspired and expired gas temperatures,relative humidity, and FEO2, FECO2, FIO2, and FICO2.

The figure shows V·O2 calculated from the inspired andexpired air volumes (actual) for all subjects compared withthe values estimated from the Haldane transformation. Theslope of the regression line deviates only 0.003 units fromunity (the intercept equals nearly zero), demonstrating thecloseness between the actual oxygen consumption and thatpredicted by the Haldane transformation. The largest differ-ence between the 68 actual and estimated V·O2 values was230 mL, an error of 7.3%. The average difference of 0.8%for all subjects fell within the measurement error of the

F O C U S O N R E S E A R C H

Valid Determination of Oxygen Consumption

instruments. For the nitrogen data, a difference of 1.6%occurred between the minute volume of nitrogen inspiredand expired for any subject at any exercise intensity; 11 of17 subjects’ work rates exhibited less than 1% difference.The largest difference, 1099 mL of N2 � min–1, occurred dur-ing intense exercise (2.1% difference).

The major sources of variation in assessing V·O2 in-cluded the measurement of ventilation volume, gas metercalibration, and determination of the inspired air’s watervapor pressure (PH2o). Ventilation volume posed a problembecause accuracy depended on the subject being “switchedin” and “switched out” at the same phase of the tidal volumeat the beginning and end of the collection period. This re-mains difficult (if not impossible) to achieve, so an inspired-to-expired volume differential nearly always occurs. Also, a10 percentage point difference in inspired PH2o (e.g., from50 to 60% relative humidity) produces more than a 100-mLdifference between the inspired and expired N2 volumes.

This study supported the continued use of the Haldanetransformation to calculate exercise V·O2. Although produc-tion and/or retention of N2 can occur during exercise, it ex-erts little or no effect on the V·O2 computation.

3.5

VO2 Actual (L min–1)

3.0

1.0 1.5 2.0 2.5 3.0 3.5

2.5

2.0

1.5VO2

Esti

mat

ed (

L m

in–1

)

Subject 2Subject 1 Subject 3 Subject 4Subject 5 Subject 6

Y=0.997X - 0.024r=0.998

Actual versus estimated exercise oxygen consumption for six subjects. Thesolid line represents the line of identity, and the dashed line represents the regression line that predicts oxygen consumption estimated from theHaldane transformation (y axis) from the actual oxygen consumption (x axis).Note the slope of nearly 1.00 and intercept of 0. Colored data points indicatethe same subjects measured under each condition.

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CHAPTER 5 Introduction to Energy Transfer 133described in Chapter 4 (Fig. 4.1), measures the energychange directly as heat (kCal) liberated from the chemical re-actions.

The complete combustion of food takes place at the ex-pense of molecular oxygen, so the heat generated in these ex-ergonic reactions can be inferred readily from oxygenconsumption measurements. Oxygen consumption measure-ment forms the basis of indirect calorimetry to determine theenergy expended by humans during rest and diverse physicalactivities (see “Focus on Research,” p. 132). Chapter 8 dis-cusses how direct calorimetry and indirect calorimetry deter-mine heat production (energy metabolism) in humans.


Discuss the implications of the second law ofthermodynamics for measuring energyexpenditure.

Summary

1. Energy, defined as the ability to perform work,emerges only when a change takes place.

2. Energy exists in either potential or kinetic form.Potential energy refers to energy associated witha substance’s structure or position; kinetic energyrefers to energy of motion. Potential energy canbe measured when it transforms into kineticenergy.

3. The six forms of energy are chemical, mechanical,heat, light, electrical, and nuclear. Each energyform can convert or transform to another form.

4. Exergonic energy reactions release energy to thesurroundings. Endergonic energy reactions store,conserve, or increase free energy. All potential en-ergy ultimately degrades into kinetic (heat) energy.

5. Living organisms temporarily conserve a portion ofpotential energy within the structure of new com-pounds, some of which power biologic work.

6. Entropy describes the tendency of potential energyto degrade to kinetic energy with a lower capacityfor work.

7. Plants transfer the energy of sunlight to the poten-tial energy bound within carbohydrates, lipids, andproteins through the endergonic process of photo-synthesis.

8. Respiration, an exergonic process, releases storedenergy in plants for coupling to other chemicalcompounds for biologic work.

9. Energy transfer in humans supports three forms ofbiologic work: chemical (biosynthesis of cellularmolecules), mechanical (muscle contraction), ortransport (transfer of substances among cells).

10. Enzymes represent highly specific protein catalyststhat accelerate chemical reaction rates withoutbeing consumed or changed in the reaction.

11. Coenzymes consist of nonprotein organic sub-stances that facilitate enzyme action by binding asubstrate to its specific enzyme.

12. Hydrolysis (catabolism) of complex organic mole-cules performs critical functions in macronutrientdigestion and energy metabolism. Condensation(anabolism) reactions synthesize complex biomole-cules for tissue maintenance and growth.

13. The linking (coupling) of oxidation–reduction(redox) reactions enables oxidation (a substanceloses electrons) to coincide with the reverse reac-tion of reduction (a substance gains electrons).Redox reactions provide the basis for the body’senergy-transfer processes.

14. The transport of electrons by specific carrier mole-cules constitutes the respiratory chain. Electrontransport represents the final common pathway inaerobic metabolism.

Suggested Readings are available online athttp://thepoint.lww.com/mkk7e.

On the InternetInternational Union of Biochemistry and Molecular BiologyRecommendations on Biochemical and Organic Nomenclature,Symbols & Terminology

www.chem.qmul.ac.uk/iubmb/

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