Biochemistry, 4th Edition P67 docx

We can calculate an efficiency for the pathways of gly-colysis, the TCA cycle, electron transport, and oxidative phosphorylation of 1600/2937 100% 54%.. ATP Yield per Glucose Glycerol–

other hand, if these 2 NADH take part in the malate–aspartate shuttle, each yields

2.5 ATP, giving a total (in this case) of 32 ATP formed per glucose oxidized Most of

the ATP—26 out of 30 or 28 out of 32—is produced by oxidative phosphorylation;

only 4 ATP molecules result from direct synthesis during glycolysis and the TCA

cycle.

The situation in bacteria is somewhat different Prokaryotic cells need not carry

out ATP–ADP exchange Thus, bacteria have the potential to produce

approxi-mately 38 ATP per glucose.

3.5 Billion Years of Evolution Have Resulted in a Very Efficient System

Hypothetically speaking, how much energy does a eukaryotic cell extract from the

glucose molecule? Taking a value of 50 kJ/mol for the hydrolysis of ATP under

cel-lular conditions (see Chapter 3), the production of 32 ATPs per glucose oxidized

yields 1600 kJ/mol of glucose The cellular oxidation (combustion) of glucose

yields G  2937 kJ/mol We can calculate an efficiency for the pathways of

gly-colysis, the TCA cycle, electron transport, and oxidative phosphorylation of

1600/2937  100%  54%.

ATP Yield per Glucose Glycerol– Malate–

Phosphate Aspartate

Glycolysis: glucose to pyruvate (cytosol)

Oxidation of 2 molecules of

glyceraldehyde-3-phosphate yields 2 NADH

Pyruvate conversion to acetyl-CoA (mitochondria)

2 NADH

Citric acid cycle (mitochondria)

of succinyl-CoA

Oxidation of 2 molecules each of isocitrate,

-ketoglutarate, and malate yields 6 NADH

Oxidation of 2 molecules of succinate yields 2 [FADH2]

Oxidative phosphorylation (mitochondria)

2 NADH from glycolysis yield 1.5 ATPs each if NADH 3 5

is oxidized by glycerol–phosphate shuttle; 2.5 ATP

by malate–aspartate shuttle

Oxidative decarboxylation of 2 pyruvate to 2 acetyl-CoA:

1.5 ATPs each

6 NADH from citric acid cycle produce 2.5 ATPs each 15 15

Note: These P/O ratios of 2.5 and 1.5 for mitochondrial oxidation of NADH and [FADH 2 ] are “consensus values.” Because

they may not reflect actual values and because these ratios may change depending on metabolic conditions, these

esti-mates of ATP yield from glucose oxidation are approximate.

624 Chapter 20 Electron Transport and Oxidative Phosphorylation

Mitochondria not only are the home of the TCA cycle and oxidative phosphoryla-tion but also are a crossroads for several cell signaling pathways Mitochondria take

up Ca2ions released from the endoplasmic reticulum, thus helping control intra-cellular Ca2signals They produce reactive oxygen species (ROS) that play signal-ing roles in cells, although ROS can also cause cellular damage Mitochondria also

participate in the programmed death of cells, a process known as apoptosis (the

sec-ond “p” is silent in this word)

Apoptosis is a mechanism through which certain cells are eliminated from higher organisms It is central to the development and homeostasis of multicellular organisms, and it is the route by which unwanted or harmful cells are eliminated Under normal circumstances, apoptosis is suppressed through compartmentation

of the involved activators and enzymes Mitochondria play a major role in this sub-cellular partitioning of the apoptotic activator molecules One such activator is

cyto-chrome c, which normally resides in the intermembrane space, bound tightly to a

lipid chain of cardiolipin in the membrane (Figure 20.33) A variety of triggering agents, including Ca2, ROS, certain lipid molecules, and certain protein kinases, can induce the opening of pores in the mitochondrial membrane For example,

us-ing hydrogen peroxide as a substrate, cytochrome c can oxidize its bound

cardio-lipin chain, releasing itself from the membrane When the outer membrane is made

permeable by other apoptotic signals, cytochrome c can enter the cytosol.

Permeabilization events, which occur at points where outer and inner mito-chondrial membranes are in contact, involve association of the ATP–ADP

translo-case in the inner membrane and the voltage-dependent anion channel (VDAC) in

the outer membrane This interaction leads to the opening of protein-permeable

pores Cytochrome c, as well as several other proteins, can pass through these pores.

Cyt c

OOH

(a)

CL CO-POX

H2O2 Cardiolipin

OOH

CO-POX

(b)

C C

C C

FIGURE 20.33(a) Cytochrome c is anchored at the inner

mitochondrial membrane by association with

cardio-lipin (diphosphatidylglycerol) The peroxidase activity of

cytochrome c oxidizes a cardiolipin lipid chain, releasing

cytochrome c from the membrane (b) The opening of

pores in the outer membrane, induced by a variety of

triggering agents, releases cytochrome c to the cytosol,

where it initiates the events of apoptosis

ROS: Reactive oxygen species, such as oxygen

ions, free radicals, and peroxides

Pore formation is carefully regulated by the BCL-2 family of proteins, which

in-cludes both proapoptotic members (proteins known as Bax, Bid, and Bad) and

antiapoptotic members (BCL-2 itself, as well as BCL-XLand BCL-W).

Cytochrome c Triggers Apoptosome Assembly

But how is the release of cytochrome c translated into the activation of the death

cascade, a point of no return for the cell? The answer lies in the assembly, in the

cy-tosol, of a signaling platform called the apoptosome (Figure 20.34) The function

of the apoptosome is to activate a cascade of proteases called caspases (Here, “c” is

for cysteine and “asp” is for aspartic acid Caspases have Cys at the active site and

cleave their peptide substrates after Asp residues.) The apoptosome is a

wheel-shaped, heptameric platform that looks like (and in some ways behaves like) an

earth-orbiting space station It is assembled from seven subunits of the apoptotic

protease-activating factor 1 (Apaf-1), a multidomain protein Apaf-1 contains an

ATPase domain (which prefers dATP over ATP in some organisms), a

caspase-recruitment domain (CARD), and a WD40 repeat domain Normally (before the

death-signaling cytochrome c is released from mitochondria), these three domains

are folded against each other (Figure 20.34b), with dATP tightly bound, and

Apaf-1 is “locked” in an inactive monomeric state Binding of cytochrome c to the WD40

domain, followed by dATP hydrolysis, converts Apaf-1 to an extended

conforma-tion Then, exchange of dADP for a new molecule of dATP prompts assembly of the

heptameric platform (Figure 20.34), which goes on to activate the death-dealing

caspase cascade

CARD

CARD NOD

(a) Apaf-1

WD40

Cytochrome c

Cytochrome c

binding dATP hydrolysis

WD40

autoinhibited form

Apoptosome

dATP-dADP exchange

(c)

FIGURE 20.34 (a) Apaf-1 is a multidomain protein, consisting of an N-terminal CARD, a nucleotide-binding

and oligomerization domain (NOD), and several WD40 domains (b) Binding of cytochrome c to the WD40

domains and ATP hydrolysis unlocks Apaf-1 to form the semiopen conformation Nucleotide exchange leads

to oligomerization and apoptosome formation (c) A model of the apoptosome, a wheellike structure with

molecules of cytochrome c bound to the WD40 domains, which extend outward like spokes.

626 Chapter 20 Electron Transport and Oxidative Phosphorylation

Mitochondria-mediated apoptosis is the mode of cell death for many neurons in the brain during strokes and other brain-trauma injuries When a stroke occurs, the neurons at the site of oxygen deprivation die within minutes by a nonspecific process of necrosis, but cells adjacent to the immediate site of injury die more slowly

by apoptosis These latter cells have been saved by a variety of therapeutic inter-ventions that suppress apoptosis in laboratory studies, raising the hope that strokes and other neurodegenerative conditions may someday be treated clinically in simi-lar ways.

SUMMARY

20.1 Where in the Cell Do Electron Transport and Oxidative

Phospho-rylation Occur? The processes of electron transport and oxidative

phosphorylation are membrane associated In prokaryotes, the

conver-sion of energy from NADH and [FADH2] to the energy of ATP via

elec-tron transport and oxidative phosphorylation is carried out at (and

across) the plasma membrane In eukaryotic cells, electron transport

and oxidative phosphorylation are localized in mitochondria

Mito-chondria are surrounded by a simple outer membrane and a more

com-plex inner membrane The space between the inner and outer

mem-branes is referred to as the intermembrane space

20.2 What Are Reduction Potentials, and How Are They Used to

Ac-count for Free Energy Changes in Redox Reactions? Just as the group

transfer potential is used to quantitate the energy of phosphoryl transfer,

the standard reduction potential, denoted by Ᏹo, quantitates the

ten-dency of chemical species to be reduced or oxidized Standard reduction

potentials are determined by measuring the voltages generated in

reac-tion half-cells A half-cell consists of a solureac-tion containing 1 M

concen-trations of both the oxidized and reduced forms of the substance whose

reduction potential is being measured and a simple electrode

20.3 How Is the Electron-Transport Chain Organized? The

com-ponents of the electron-transport chain can be purified from the

mito-chondrial inner membrane as four distinct protein complexes:

(I) NADH–coenzyme Q reductase, (II) succinate–coenzyme Q

reduc-tase, (III) coenzyme Q–cytochrome c reducreduc-tase, and (IV) cytochrome c

oxidase In complexes I, II, and IV, electron transfer drives the

move-ment of protons from the mitochondrial matrix to the intermembrane

space

Complex I (NADH dehydrogenase) involves more than 45

polypep-tide chains, 1 molecule of flavin mononucleopolypep-tide (FMN), and as many

as nine Fe-S clusters, together containing a total of 20 to 26 iron atoms

The complex transfers electrons from NADH to FMN, then to a series

of FeS proteins, and finally to coenzyme Q

Complex II (succinate dehydrogenase) oxidizes succinate to fumarate,

with concomitant reduction of bound FAD to FADH2 This FADH2

trans-fers its electrons immediately to Fe-S centers, which pass them on to UQ

Electrons flow from succinate to UQ

Complex III drives electron transport from coenzyme Q to cytochrome

c via a unique redox pathway known as the Q cycle UQ–cytochrome

c reductase (UQ–cyt c reductase), as this complex is known, involves three

different cytochromes and an Fe-S protein In the cytochromes of these

and similar complexes, the iron atom at the center of the porphyrin ring

cycles between the reduced Fe2(ferrous) and oxidized Fe3 (ferric)

states

Complex IV transfers electrons from cytochrome c to reduce oxygen

on the matrix side Complex IV (cytochrome c oxidase) accepts

elec-trons from cytochrome c and directs them to the four-electron

reduc-tion of O2to form 2H2O via CuAsites, the heme iron of cytochrome a,

CuB, and the heme iron of a3

20.4 What Are the Thermodynamic Implications of Chemiosmotic

Coupling? Peter Mitchell was the first to propose that electron transport

leads to formation of a proton gradient that drives ATP synthesis The

free energy difference for protons across the inner mitochondrial

mem-brane includes a term for the concentration difference and a term for the electrical potential It is this energy that drives the synthesis of ATP, in ac-cord with Mitchell’s model

20.5 How Does a Proton Gradient Drive the Synthesis of ATP? The mi-tochondrial complex that carries out ATP synthesis is ATP synthase (F1F0–ATPase) ATP synthase consists of two principal complexes, desig-nated F1and F0 Protons taken up from the cytosol by one of the proton

access channels in the a-subunit of F0ride the rotor of c-subunits until they reach the other proton access channel on a, from which they are

re-leased into the matrix Such rotation causes the -subunit of F1to turn relative to the three -subunit nucleotide sites of F1, changing the con-formation of each in sequence, so ADP is first bound, then phosphory-lated, then released, according to Boyer’s binding change mechanism The inhibitors of oxidative phosphorylation include rotenone, a common insecticide that strongly inhibits the NADH–UQ reductase Complex IV is specifically inhibited by cyanide (CN), azide (N3 ), and carbon monoxide (CO) Cyanide and azide bind tightly to the

ferric form of cytochrome a3, whereas carbon monoxide binds only to the ferrous form

Uncouplers disrupt the coupling of electron transport and ATP syn-thase Uncouplers share two common features: hydrophobic character and a dissociable proton They function by carrying protons across the inner membrane, acquiring protons on the outer surface of the mem-brane (where the proton concentration is high) and carrying them to the matrix side Uncouplers destroy the proton gradient that couples electron transport and the ATP synthase

ATP–ADP translocase mediates the movement of ATP and ADP across the mitochondrial membrane The ATP–ADP translocase is an inner membrane protein that tightly couples the exit of ATP with the entry of ADP so that the mitochondrial nucleotide levels remain approximately constant For each ATP transported out, one ADP is transported into the matrix ATP–ADP translocase binds ATP on the matrix side, reorients to face the intermembrane space, and exchanges ATP for ADP, with subse-quent reorientation back to the matrix face of the inner membrane

20.6 What Is the P/O Ratio for Mitochondrial Oxidative Phosphoryla-tion? The P/O ratio is the number of molecules of ATP formed in ox-idative phosphorylation per two electrons flowing through a defined segment of the electron-transport chain The consensus value for the mitochondrial P/O ratio is 10/4, or 2.5, for electrons entering the elec-tron-transport chain as NADH For succinate to O2, the P/O ratio in this case would be 6/4, or 1.5

20.7 How Are the Electrons of Cytosolic NADH Fed into Electron Trans-port? Eukaryotic cells have a number of shuttle systems that collect the electrons of cytosolic NADH for delivery to mitochondria without actu-ally transporting NADH across the inner membrane In the glyc-erophosphate shuttle, two different glycglyc-erophosphate dehydrogenases, one in the cytosol and one on the outer face of the mitochondrial inner membrane, work together to carry electrons into the mitochondrial ma-trix In the malate–aspartate shuttle, oxaloacetate is reduced in the cy-tosol, acquiring the electrons of NADH (which is oxidized to NAD) Malate is transported across the inner membrane, where it is reoxidized

by malate dehydrogenase, converting NADto NADH in the matrix

20.8 How Do Mitochondria Mediate Apoptosis? Mitochondria are a

crossroads for several cell signaling pathways Mitochondria take up

Ca2ions released from the endoplasmic reticulum, helping control

in-tracellular Ca2signals They produce ROS that play signaling roles in

cells They also participate in apoptosis, the programmed death of cells

Triggering agents, including Ca2, ROS, and certain lipid molecules

and protein kinases, can induce the opening of pores in the

mitochon-drial membrane, releasing cytochrome c, which then binds to the WD40

domain of Apaf-1, activating formation of the heptameric apoptosome platform Mitochondria-mediated apoptosis is the mode of cell death of many neurons in the brain during strokes and other bratrauma in-juries, and interventions that suppress apoptosis may eventually be use-ful in clinical settings

PROBLEMS

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1.For the following reaction,

[FAD]  2 cyt c (Fe2) 2 H⎯⎯→ [FADH2] 2 cyt c (Fe3)

determine which of the redox couples is the electron acceptor and

which is the electron donor under standard-state conditions,

calcu-late the value of Ᏹo, and determine the free energy change for the

reaction

2.Calculate the value of Ᏹo for the glyceraldehyde-3-phosphate

dehydrogenase reaction, and calculate the free energy change for

the reaction under standard-state conditions

3.For the following redox reaction,

NAD 2 H 2 e⎯⎯→ NADH  H

suggest an equation (analogous to Equation 20.12) that predicts

the pH dependence of this reaction, and calculate the reduction

potential for this reaction at pH 8

4.Sodium nitrite (NaNO2) is used by emergency medical personnel as

an antidote for cyanide poisoning (for this purpose, it must be

administered immediately) Based on the discussion of cyanide

poisoning in Section 20.5, suggest a mechanism for the lifesaving

ef-fect of sodium nitrite

5.A wealthy investor has come to you for advice She has been

ap-proached by a biochemist who seeks financial backing for a

com-pany that would market dinitrophenol and dicumarol as weight-loss

medications The biochemist has explained to her that these agents

are uncouplers and that they would dissipate metabolic energy as

heat The investor wants to know if you think she should invest in

the biochemist’s company How do you respond?

6.Assuming that 3 Hare transported per ATP synthesized in the

mi-tochondrial matrix, the membrane potential difference is 0.18 V

(negative inside), and the pH difference is 1 unit (acid outside,

basic inside), calculate the largest ratio of [ATP]/[ADP][Pi] under

which synthesis of ATP can occur

7.Of the dehydrogenase reactions in glycolysis and the TCA cycle, all but

one use NADas the electron acceptor The lone exception is the

suc-cinate dehydrogenase reaction, which uses covalently bound FAD of a

flavoprotein as the electron acceptor The standard reduction

poten-tial for this bound FAD is in the range of 0.003 to 0.091 V (see Table

20.1) Compared with the other dehydrogenase reactions of glycolysis

and the TCA cycle, what is unique about succinate dehydrogenase?

Why is bound FAD a more suitable electron acceptor in this case?

8. a What is the standard free energy change (G°) for the reduction

of coenzyme Q by NADH as carried out by Complex I

(NADH–coenzyme Q reductase) of the electron-transport

path-way if Ᏹo (NAD/NADH) 0.320 V and Ᏹo (CoQ/CoQH2)

0.060 V

b What is the equilibrium constant (Keq) for this reaction?

c Assume that (1) the actual free energy release accompanying

the NADH–coenzyme Q reductase reaction is equal to the

amount released under standard conditions (as calculated in

part a), (2) this energy can be converted into the synthesis of

ATP with an efficiency 0.75 (that is, 75% of the energy

re-leased upon NADH oxidation is captured in ATP synthesis), and (3) the oxidation of 1 equivalent of NADH by coenzyme Q leads

to the phosphorylation of 1 equivalent of ATP

Under these conditions, what is the maximum ratio of [ATP]/ [ADP] attainable by oxidative phosphorylation when [Pi] 1 mM ?

(AssumeG° for ATP synthesis  30.5 kJ/mol.)

9. Consider the oxidation of succinate by molecular oxygen as carried out via the electron-transport pathway

Succinate1

2 O2⎯⎯→ fumarate  H2O

a What is the standard free energy change (G°) for this reaction if

Ᏹo (Fum/Succ)  0.031 V and Ᏹo (1

2 O2/H2O) 0.816 V

b What is the equilibrium constant (Keq) for this reaction?

c Assume that (1) the actual free energy release accompanying succinate oxidation by the electron-transport pathway is equal to the amount released under standard conditions (as calculated in part a), (2) this energy can be converted into the synthesis of ATP with an efficiency 0.7 (that is, 70% of the energy released upon succinate oxidation is captured in ATP synthesis), and (3) the oxi-dation of 1 succinate leads to the phosphorylation of 2 equivalents

of ATP

Under these conditions, what is the maximum ratio of [ATP]/ [ADP] attainable by oxidative phosphorylation when [Pi] 1 mM ?

(AssumeG° for ATP synthesis  30.5 kJ/mol.)

10. Consider the oxidation of NADH by molecular oxygen as carried out via the electron-transport pathway

NADH H1

2 O2⎯⎯→ NAD H2O

a What is the standard free energy change (G°) for this reac-tion if Ᏹo (NAD/NADH) 0.320 V and Ᏹo (O2/H2O)

0.816 V

b What is the equilibrium constant (Keq) for this reaction?

c Assume that (1) the actual free energy release accompanying NADH oxidation by the electron-transport pathway is equal to the amount released under standard conditions (as calculated in part a), (2) this energy can be converted into the synthesis of ATP with an efficiency 0.75 (that is, 75% of the energy released upon NADH oxidation is captured in ATP synthesis), and (3) the oxi-dation of 1 NADH leads to the phosphorylation of 3 equivalents

of ATP

Under these conditions, what is the maximum ratio of [ATP]/ [ADP] attainable by oxidative phosphorylation when [Pi] 2 mM ?

(AssumeG° for ATP synthesis  30.5 kJ/mol.)

11. Write a balanced equation for the reduction of molecular oxygen by

reduced cytochrome c as carried out by Complex IV (cytochrome

oxidase) of the electron-transport pathway

a What is the standard free energy change (G°) for this reaction if

Ᏹo cyt c(Fe3)/cyt c(Fe2) 0.254 volts and

Ᏹo (1

2 O2/H2O) 0.816 volts

b What is the equilibrium constant (Keq) for this reaction?

c Assume that (1) the actual free energy release accompanying

cytochrome c oxidation by the electron-transport pathway is equal

to the amount released under standard conditions (as calculated

628 Chapter 20 Electron Transport and Oxidative Phosphorylation

in part a), (2) this energy can be converted into the synthesis of

ATP with an efficiency 0.6 (that is, 60% of the energy released

upon cytochrome c oxidation is captured in ATP synthesis), and

(3) the reduction of 1 molecule of O2by reduced cytochrome c

leads to the phosphorylation of 2 equivalents of ATP

Under these conditions, what is the maximum ratio of [ATP]/

[ADP] attainable by oxidative phosphorylation when [Pi] 3 mM ?

(AssumeG° for ATP synthesis  30.5 kJ/mol.)

12. The standard reduction potential for (NAD/NADH) is 0.320 V,

and the standard reduction potential for (pyruvate/lactate) is

0.185 V

a What is the standard free energy change (G°) for the lactate

de-hydrogenase reaction:

NADH H pyruvate ⎯⎯→ lactate  NAD

b What is the equilibrium constant (Keq) for this reaction?

c If [pyruvate] 0.05 mM and [lactate]  2.9 mM and G for the

lactate dehydrogenase reaction 15 kJ/mol in erythrocytes,

what is the [NAD]/[NADH] ratio under these conditions?

13. Assume that the free energy change (G) associated with the

move-ment of 1 mole of protons from the outside to the inside of a

bac-terial cell is 23 kJ/mol and 3 Hmust cross the bacterial plasma

membrane per ATP formed by the bacterial F1F0–ATP synthase

ATP synthesis thus takes place by the coupled process:

3 Hout ADP  Pi34 3 H

in ATP  H2O

a If the overall free energy change (Goverall) associated with ATP

synthesis in these cells by the coupled process is 21 kJ/mol, what

is the equilibrium constant (Keq) for the process?

b What is Gsynthesis, the free energy change for ATP synthesis, in

these bacteria under these conditions?

c The standard free energy change for ATP hydrolysis (G°hydrolysis)

is30.5 kJ/mol If [Pi] 2 mM in these bacterial cells, what is the

[ATP]/[ADP] ratio in these cells?

14. Describe in your own words the path of electrons through the Q

cycle of Complex III

15.Describe in your own words the path of electrons through the cop-per and iron centers of Complex IV

16.In the course of events triggering apoptosis, a fatty acid chain of car-diolipin undergoes peroxidation to release the associated

cyto-chrome c Lipid peroxidation occurs at a double bond Suggest a

mechanism for the reaction of hydrogen peroxide with an unsatura-tion in a lipid chain, and identify a likely product of the reacunsatura-tion

17.In problem 18 at the end of Chapter 19, you might have calculated the number of molecules of oxaloacetate in a typical mitochon-drion What about protons? A typical mitochondrion can be thought of as a cylinder 1

pH in the matrix is 7.8, how many protons are contained in the mitochondrial matrix?

18.Considering that all other dehydrogenases of glycolysis and the TCA cycle use NADH as the electron donor, why does succinate de-hydrogenase, a component of the TCA cycle and the electron trans-fer chain, use FAD as the electron acceptor from succinate, rather than NAD? Note that there are two justifications for the choice of FAD here—one based on energetics and one based on the mecha-nism of electron transfer for FAD versus NAD

Preparing for the MCAT Exam

19.Based on your reading on the F1F0–ATPase, what would you con-clude about the mechanism of ATP synthesis:

a The reaction proceeds by nucleophilic substitution via the SN2 mechanism

b The reaction proceeds by nucleophilic substitution via the SN1 mechanism

c The reaction proceeds by electrophilic substitution via the E1 mechanism

d The reaction proceeds by electrophilic substitution via the E2 mechanism

20.Imagine that you are working with isolated mitochondria and you manage to double the ratio of protons outside to protons inside In or-der to maintain the overall G at its original value (whatever it is), how would you have to change the mitochondria membrane potential?

FURTHER READING

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ATP–ADP Translocase

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Bioenergetics

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Electron Transfer

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© Richard Hamilton Smith/CORBIS

The vast majority of energy consumed by living organisms stems from solar energy captured by the process of photosynthesis Only chemolithotrophic prokaryotes are independent of this energy source Of the 1.5  1022kJ of energy reaching the earth each day from the sun, 1% is absorbed by photosynthetic organisms and transduced into chemical energy.1This energy, in the form of biomolecules, be-comes available to other members of the biosphere through food chains The transduction of solar, or light, energy into chemical energy is often expressed in

terms of carbon dioxide fixation, in which hexose is formed from carbon dioxide

and oxygen is evolved:

Light

6 CO2 6 H2O ⎯⎯→ C6H12O6 6 O2 (21.1) Estimates indicate that 1011 tons of carbon dioxide are fixed globally per year, of which one-third is fixed in the oceans, primarily by photosynthetic marine micro-organisms.

Although photosynthesis is traditionally equated with CO2fixation, light energy (or rather the chemical energy derived from it) drives all endergonic processes in phototrophic cells The assimilation of inorganic forms of nitrogen and sulfur into organic molecules (see Chapter 25) represents two other metabolic conversions closely coupled to light energy in green plants Our previous considerations of aer-obic metabolism (Chapters 18 through 20) treated cellular respiration (precisely the reverse of Equation 21.1) as the central energy-releasing process in life It nec-essarily follows that the formation of hexose from carbon dioxide and water, the products of cellular respiration, must be endergonic The necessary energy comes from light Note that in the carbon dioxide fixation reaction described, light is used

to drive a chemical reaction against its thermodynamic potential.

Photosynthesis Occurs in Membranes

Organisms capable of photosynthesis are very diverse, ranging from simple

prokaryotic forms to the largest organisms of all, Sequoia gigantea, the giant

red-wood trees of California Despite this diversity, we find certain generalities

regard-ing photosynthesis An important one is that photosynthesis occurs in membranes In

photosynthetic prokaryotes, the photosynthetic membranes fill up the cell interior;

in photosynthetic eukaryotes, the photosynthetic membranes are localized in large

organelles known as chloroplasts (Figures 21.1 and 21.2) Chloroplasts are one

Field of goldenrod

In a sun-flecked lane,

Beside a path where cattle trod,

Blown by wind and rain,

Drawing substance from air and sod;

In ruggedness, it stands aloof,

The ragged grass and puerile leaves,

Lending a hand to fill the woof

In the pattern that beauty makes.

What mystery this, hath been wrought;

Beauty from sunshine, air, and sod!

Could we thus gain the ends we

sought-Tell us thy secret, Goldenrod.

Rosa Staubus

Oklahoma pioneer (1886–1966)

KEY QUESTIONS

21.1 What Are the General Properties of

Photosynthesis?

21.2 How Is Solar Energy Captured by

Chlorophyll?

21.3 What Kinds of Photosystems Are Used

to Capture Light Energy?

21.4 What Is the Molecular Architecture of

Photosynthetic Reaction Centers?

21.5 What Is the Quantum Yield of

Photosynthesis?

21.6 How Does Light Drive the Synthesis of ATP?

21.7 How Is Carbon Dioxide Used to Make

Organic Molecules?

21.8 How Does Photorespiration Limit CO2

Fixation?

ESSENTIAL QUESTIONS

Photosynthesis is the primary source of energy for all life forms (except chemolitho-trophic prokaryotes) Much of the energy of photosynthesis is used to drive the syn-thesis of organic molecules from atmospheric CO2.

How is solar energy captured and transformed into metabolically useful chemi-cal energy? How is the chemichemi-cal energy produced by photosynthesis used to create organic molecules from carbon dioxide?

Create your own study path for

this chapter with tutorials, simulations, animations,

and Active Figures at www.cengage.com/login. 1Of the remaining 99%, two-thirds is absorbed by the earth and oceans, thereby heating the planet;

the remaining one-third is lost as light reflected back into space

member in a family of related plant-specific organelles known as plastids

Chloro-plasts themselves show a range of diversity, from the single, spiral chloroplast that

gives Spirogyra its name to the multitude of ellipsoidal plastids typical of higher

plant cells (Figure 21.3).

Characteristic of all chloroplasts, however, is the organization of the inner

mem-brane system, the so-called thylakoid memmem-brane The thylakoid memmem-brane is

orga-nized into paired folds that extend throughout the organelle, as in Figure 21.2.

These paired folds, or lamellae, give rise to flattened sacs or discs, thylakoid vesicles

(from the Greek thylakos, meaning “sack”), which occur in stacks called grana A

sin-gle stack, or granum, may contain dozens of thylakoid vesicles, and different grana

are joined by lamellae that run through the soluble portion, or stroma, of the

or-ganelle Chloroplasts thus possess three membrane-bound aqueous compartments:

the intermembrane space, the stroma, and the interior of the thylakoid vesicles, the

so-called thylakoid space (also known as the thylakoid lumen) As we shall see, this

third compartment serves an important function in the transduction of light energy

into ATP formation The thylakoid membrane has a highly characteristic lipid

com-position and, like the inner membrane of the mitochondrion, is impermeable to

FIGURE 21.1 Electron micrograph of a representative chloroplast

Intermembrane

space

Granum

(stack of thylakoids)

Stroma

Thylakoid lumen

Lamella

Inner membrane

Outer membrane Thylakoid vesicle

FIGURE 21.2 Schematic diagram of an idealized chloroplast

632 Chapter 21 Photosynthesis

most ions and molecules Chloroplasts, like their mitochondrial counterparts, pos-sess DNA, RNA, and ribosomes and consequently display a considerable amount of autonomy However, many critical chloroplast components are encoded by nuclear genes, so autonomy is far from absolute.

Photosynthesis Consists of Both Light Reactions and Dark Reactions

If a chloroplast suspension is illuminated in the absence of carbon dioxide, oxygen

is evolved Furthermore, if the illuminated chloroplasts are now placed in the dark and supplied with CO2, net hexose synthesis can be observed (Figure 21.4) Thus, the evolution of oxygen can be temporally separated from CO2fixation and also has

a light dependency that CO2fixation lacks The light reactions of photosynthesis, of

which O2evolution is only one part, are associated with the thylakoid membranes.

In contrast, the light-independent reactions, or so-called dark reactions, notably

CO2fixation, are located in the stroma A concise summary of the photosynthetic process is that radiant electromagnetic energy (light) is transformed by a specific photochemical system located in the thylakoids to yield chemical energy in the form

of reducing potential (NADPH) and high-energy phosphate (ATP) NADPH and ATP can then be used to drive the endergonic process of hexose formation from

FIGURE 21.3 (a) Spirogyra—a freshwater green alga (b) A higher plant cell.

© Perennou Nuridsany/Photo Researchers, Inc Biophoto Associates/Science Source

O 2

O 2

2

CO 2

CO 2

CO 2

CO 2

CO 2

Chloroplast suspension

into sugars

O 2 evolved

ANIMATED FIGURE 21.4 The light-dependent and light-independent reactions of

photosyn-thesis See this figure animated at www.cengage.com/login.