《生物化学原理》（英文版）chapter 19 Oxidative phosphorylation

8885dc19690-7503/1/0411:32 AM Page720mac76mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation RNA NDI ND5 OATP synthase O Transfer rna bosomal rna O Control region of DNA (b) LHON (11,77 FIGURE 19-32 Mitochondrial genes and mutations. ND4 (a) Map of human mitochondrial DNA, showing the genes that encode proteins of Complex I, the NADH dehydroge- nase(NDI to ND6); the cytochrome b of Complex Ill( Cyt NDLY b); the subunits of cytochrome oxidase(Complex IV)(COI to ATPase8 COlIn; and two subunits of ATP synthase (ATPase6 and ATPase8 The colors of the genes correspond to those of the complexes shown in Figure 19-7. Also included here are the genes for ribosomal RNAs COlI TPase (rRNA)and for a number of mitochondrion-specific transfer RNAs; tRNA specificity is indicated by the one-letter codes for amino acids. Arrows indicate the positions of mutations that cause Leber's hered tary optic neuropathy(LHON) and myoclonic epilepsy and ragged red fiber disease(MERRF). Numbers in parentheses indicate the posi ion of the altered nucleotides(nucleotide 1 is at the top of the circle and numbering proceeds counterclockwise).(b)Electron micrograph of an abnormal mitochondrion from the muscle of an individual with MERRE, showing the paracrystalline protein inclusions sometimes pres- ent in the mutant mitochondria CH2CH=C一CH2)-H Cvt b Q 4Fe-4S (b) Bacterial inner FIGURE 19-33 Bacterial respiratory chain. (a) Shown here are the respiratory carriers of the inner membrane of E. coli. Eubacteria con- membrane tain a minimal form of Complex L, containing all the prosthetic groups NADH su normally associated with the mitochondrial complex but only 1 polypeptides. This plasma membrane complex transfers electrons from NADH to ubiquinone or to(b)menaquinone, the bacterial equivalent Cytosol (N side) of ubiquinone, while pumping protons outward and creating an elec trochemical potential that drives ATP synthesi

720 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Menaquinone n  7–9 CH3 (CH2CH C CH2)n CH3 H O O (b) Cytosol (N side) Bacterial inner (plasma) membrane Periplasmic space (P side) Cyt b Cyt o Cu 2Fe-2S FAD NADH, succinate, or glycerol 4Fe-4S Q H O2 + H+ (a) FIGURE 19–33 Bacterial respiratory chain. (a) Shown here are the respiratory carriers of the inner membrane of E. coli. Eubacteria con￾tain a minimal form of Complex I, containing all the prosthetic groups normally associated with the mitochondrial complex but only 14 polypeptides. This plasma membrane complex transfers electrons from NADH to ubiquinone or to (b) menaquinone, the bacterial equivalent of ubiquinone, while pumping protons outward and creating an elec￾trochemical potential that drives ATP synthesis. FIGURE 19–32 Mitochondrial genes and mutations. (a) Map of human mitochondrial DNA, showing the genes that encode proteins of Complex I, the NADH dehydroge￾nase (ND1 to ND6); the cytochrome b of Complex III (Cyt b); the subunits of cytochrome oxidase (Complex IV) (COI to COIII); and two subunits of ATP synthase (ATPase6 and ATPase8). The colors of the genes correspond to those of the complexes shown in Figure 19–7. Also included here are the genes for ribosomal RNAs (rRNA) and for a number of mitochondrion-specific transfer RNAs; tRNA specificity is indicated by the one-letter codes for amino acids. Arrows indicate the positions of mutations that cause Leber’s heredi￾tary optic neuropathy (LHON) and myoclonic epilepsy and ragged￾red fiber disease (MERRF). Numbers in parentheses indicate the posi￾tion of the altered nucleotides (nucleotide 1 is at the top of the circle and numbering proceeds counterclockwise). (b) Electron micrograph of an abnormal mitochondrion from the muscle of an individual with MERRF, showing the paracrystalline protein inclusions sometimes pres￾ent in the mutant mitochondria. 0/16,569 12S rRNA 16S rRNA ND1 ND2 COI COII COIII ATPase6 ND3 ND4L ND4 ND5 ND6 Cyt b F V L I M WA N C Y S D K G (a) R H L S E PT LHON (15,257) Q LHON (3,460) LHON (4,160) LHON (11,778) MERRF (8,344) ATPase8 Complex I Complex III Complex IV ATP synthase Transfer RNA Ribosomal RNA Control region of DNA (b) 8885d_c19_690-750 3/1/04 11:32 AM Page 720 mac76 mac76:385_reb:

8885dc19690-7503/1/0411:32 AM Page721mac76mac76:385 19.5 The Role of Mitochondria in Apoptosis and Oxidative Stress 721 Mitochondria Evolved from Endosymbiotic Bacteria The respiration-linked extrusion of protons across The existence of mitochondrial DNA, ribosomes, and the bacterial plasma membrane also provides the driv- tRNAs supports the hypothesis of the endosymbiotic ing force for other processes. Certain bacterial trans origin of mitochondria(see Fig 1-36), which holds that port systems bring about uptake of extracellular nutri- ents (lactose, for example) against a concentration the first organisms capable of aerobic metabolism, in- gradient, in symport with protons(see Fig. 11-42)And cluding respiration-linked ATP production, were prokar- yotes Primitive eukaryotes that lived anaerobically (by the rotary motion of bacterial flagella is provided by fermentation) acquired the ability to carry out oxidative proton turbines, molecular rotary motors driven not phosphorylation when they established a symbiotic re- by atP but directly by the transmembrane electro- lationship with bacteria living in their cytosol. After much chemical potential generated by respiration-linked pro- evolution and the movement of many bacterial genes into ton pumping (Fig. 19-34). It appears likely that the chemiosmotic mechanism evolved early, before the the nucleus of the "host" eukaryote, the endosymbiotic bacteria eventually became mitochondria mergence of eukaryotes This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative SUMMARY 19.4 Mitochondrial Genes: Their Origin phosphorylation and predicts that their modern and the Effects of Mutations prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do I A small proportion of human mitochondrial Aerobic bacteria carry out NAD-linked electron trans- proteins(13 proteins) are encoded in the ter from substrates to O2, coupled to the phosphoryla- mitochondrial genome and synthesized within tion of cytosolic ADP. The dehydrogenases are located mitochondria. about 900 mitochondrial in the bacterial cytosol and the respiratory chain in the proteins are encoded in nuclear genes and plasma membrane. The electron carriers are similar te imported into mitochondria after their some mitochondrial electron carriers(Fig. 19-33).They translocate protons outward across the plasma mem- a Mutations in the genes that encode brane as electrons are transferred to o bacteria such components of the respiratory chain, whether as Escherichia coli have FoFI complexes in their n the mitochondrial genes or in the nuclear plasma membranes, the FI portion protrudes into the genes that encode mitochondrial proteins cytosol and catalyzes ATP synthesis from ADP and P cause a variety of human diseases, which often as protons flow back into the cell through the proton affect muscle and brain most severely. channel of F Mitochondria most likely arose from aerobic prokaryotes that entered into an endosymbiotic relationship with ancestral eukaryotes 19. 5 The Role of Mitochondria in Apoptosis Outer membrane and oxidative Stress Inner(plasma) /membrane Besides their central role in ATP synthesis, mitochon dria also participate in processes associated with cellu- lar damage and death. Apoptosis is a controlled process Rotary moto by which cells die for the good of the organism, while the organism conserves the molecular components Peptidoglycan (amino acids, nucleotides, and so forth) of the dead Electron-transfer and periplasmic cells. Apoptosis may be triggered by an external signal, acting at a receptor in the plasma membrane, or by in- ternal events such as a viral infection when a cell re- FIGURE 19-34 Rotation of bacterial flagella by proton-motive force. ceives a signal for apoptosis, one consequence is an in- The shaft and rings at the base of the flagellum make up a rotary crease in the permeability of the outer mitochondrial tor that has been called a"proton turbine. " Protons ejected by elec membrane, allowing escape of the cytochrome c nor- tron transfer flow back into the cell through the turbine, causing ro- mally confined in the intermembrane space(see Fig tation of the shaft of the flagellum. This motion differs fundamentally 12-50). The released cytochrome c activates one of the from the motion of muscle and of eukaryotic flagella and cilia, for proteolytic enzymes(caspase 9)responsible for protein which ATP hydrolysis is the energy source degradation during apoptosis. This is a dramatic case of

Mitochondria Evolved from Endosymbiotic Bacteria The existence of mitochondrial DNA, ribosomes, and tRNAs supports the hypothesis of the endosymbiotic origin of mitochondria (see Fig. 1–36), which holds that the first organisms capable of aerobic metabolism, in￾cluding respiration-linked ATP production, were prokar￾yotes. Primitive eukaryotes that lived anaerobically (by fermentation) acquired the ability to carry out oxidative phosphorylation when they established a symbiotic re￾lationship with bacteria living in their cytosol. After much evolution and the movement of many bacterial genes into the nucleus of the “host” eukaryote, the endosymbiotic bacteria eventually became mitochondria. This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron trans￾fer from substrates to O2, coupled to the phosphoryla￾tion of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19–33). They translocate protons outward across the plasma mem￾brane as electrons are transferred to O2. Bacteria such as Escherichia coli have FoF1 complexes in their plasma membranes; the F1 portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and Pi as protons flow back into the cell through the proton channel of Fo. The respiration-linked extrusion of protons across the bacterial plasma membrane also provides the driv￾ing force for other processes. Certain bacterial trans￾port systems bring about uptake of extracellular nutri￾ents (lactose, for example) against a concentration gradient, in symport with protons (see Fig. 11–42). And the rotary motion of bacterial flagella is provided by “proton turbines,” molecular rotary motors driven not by ATP but directly by the transmembrane electro￾chemical potential generated by respiration-linked pro￾ton pumping (Fig. 19–34). It appears likely that the chemiosmotic mechanism evolved early, before the emergence of eukaryotes. SUMMARY 19.4 Mitochondrial Genes: Their Origin and the Effects of Mutations ■ A small proportion of human mitochondrial proteins (13 proteins) are encoded in the mitochondrial genome and synthesized within mitochondria. About 900 mitochondrial proteins are encoded in nuclear genes and imported into mitochondria after their synthesis. ■ Mutations in the genes that encode components of the respiratory chain, whether in the mitochondrial genes or in the nuclear genes that encode mitochondrial proteins, cause a variety of human diseases, which often affect muscle and brain most severely. ■ Mitochondria most likely arose from aerobic prokaryotes that entered into an endosymbiotic relationship with ancestral eukaryotes. 19.5 The Role of Mitochondria in Apoptosis and Oxidative Stress Besides their central role in ATP synthesis, mitochon￾dria also participate in processes associated with cellu￾lar damage and death. Apoptosis is a controlled process by which cells die for the good of the organism, while the organism conserves the molecular components (amino acids, nucleotides, and so forth) of the dead cells. Apoptosis may be triggered by an external signal, acting at a receptor in the plasma membrane, or by in￾ternal events such as a viral infection. When a cell re￾ceives a signal for apoptosis, one consequence is an in￾crease in the permeability of the outer mitochondrial membrane, allowing escape of the cytochrome c nor￾mally confined in the intermembrane space (see Fig. 12–50). The released cytochrome c activates one of the proteolytic enzymes (caspase 9) responsible for protein degradation during apoptosis. This is a dramatic case of 19.5 The Role of Mitochondria in Apoptosis and Oxidative Stress 721 Flagellum Outer membrane Inner (plasma) membrane Rotary motor Peptidoglycan and periplasmic space Electron-transfer chain H
H
FIGURE 19–34 Rotation of bacterial flagella by proton-motive force. The shaft and rings at the base of the flagellum make up a rotary mo￾tor that has been called a “proton turbine.” Protons ejected by elec￾tron transfer flow back into the cell through the turbine, causing ro￾tation of the shaft of the flagellum. This motion differs fundamentally from the motion of muscle and of eukaryotic flagella and cilia, for which ATP hydrolysis is the energy source. 8885d_c19_690-750 3/1/04 11:32 AM Page 721 mac76 mac76:385_reb:

8885dc19690-7503/1/0411:32 AM Page722mac76mac76:385 722 Chapter 19 Oxidative Phosphorylation and Photophosphorylation mitochondrial transhydrogenase membrane IV NADH NAI mutase NADP NADPH 2 GSH 少H2O dEnz oxidative ) active FIGURE 19-35 Mitochondrial production and disposal of super-(GSH; see Fig. 22-27) donates electrons for the reduction of hydrogen oxide. Superoxide radical, O2, is formed in side reactions at peroxide(H202)and of oxidized Cys residues(--S--S-)in proteins, Complexes I and Ill, as the partially reduced ubiquinone radical(Q) and GSH is regenerated from the oxidized form(GSSG) by reduction oates an electron to O2. The reactions shown in blue defend the with NADPH cell against the damaging effects of superoxide. Reduced glutathione one protein(cytochrome c) playing two very different The hydrogen peroxide (h.o. generated by this reac roles in the cell tion is rendered harmless by the action of glutathione Mitochondria are also involved in the cells response peroxidase(Fig. 19-35). This enzyme is remarkable for to oxidative stress. As we have seen, several steps in the presence of a selenocysteine residue(see Fig. 3-sa) the path of oxygen reduction in mitochondria have the in which an atom of selenium replaces the sulfur atom potential to produce highly reactive free radicals that normally present in the thiol of the side chain. The se- can damage cells. The passage of electrons from QH2 to lenol group (Seh) is more acidic than the thiol ( -Sh) cytochrome bl through Complex Ill, and passage of elec its pKa is about 5, so at neutral ph, the selenocysteine trons from Complex I to QH2, involve the radical Q side chain is essentially fully ionized (CHoSe). Gluta- an intermediate. The Q can, with a low probability, thione reductase recycles oxidized glutathione to its re pass an electron to O2 in the reaction duced form, using electrons from the NADPH formed by nicotinamide nucleotide transhydrogenase or by the pentose phosphate pathway(see Fig. 14-20) Reduced The superoxide free radical thus generated, O2, is very glutathione also serves in keeping protein sulfhydryl reactive and can damage enzymes, membrane lipids, groups in their reduced state, preventing some of the and nucleic acids. Antimycin A, an inhibitor of Complex deleterious effects of oxidative stress(Fig. 19-35) Ill, may act by occupying the Q site (Fig. 19-11), thus blocking the Q cycle and prolonging the binding of Q to the Qp site; this would increase the likelihood of su SUMMARY 19.5 The role of mitochondria in peroxide radical formation and cellular damage From Apoptosis and Oxidative Stress 0. 1% to as much as 4% of the Oe used by actively respir ing mitochondria forms O2 -more than enough to have a Mitochondrial cytochrome c, released into the lethal effects on a cell unless the free radical is quickly cytosol, participates in activation of one of the disposed of. proteases(caspase 9) involved in apoptos To prevent oxidative damage by 02, cells hav eral forms of the enzyme superoxide dismutase a Reactive oxygen species produced in mitochondria are inactivated by a set of which catalyzes the reaction protective enzymes, including superoxide 202+2H+—H2O2+O2 dismutase and glutathione peroxidase

one protein (cytochrome c) playing two very different roles in the cell. Mitochondria are also involved in the cell’s response to oxidative stress. As we have seen, several steps in the path of oxygen reduction in mitochondria have the potential to produce highly reactive free radicals that can damage cells. The passage of electrons from QH2 to cytochrome bL through Complex III, and passage of elec￾trons from Complex I to QH2, involve the radical Q as an intermediate. The Q can, with a low probability, pass an electron to O2 in the reaction O2
e On O2  The superoxide free radical thus generated, O2 , is very reactive and can damage enzymes, membrane lipids, and nucleic acids. Antimycin A, an inhibitor of Complex III, may act by occupying the QN site (Fig. 19–11), thus blocking the Q cycle and prolonging the binding of Q to the QP site; this would increase the likelihood of su￾peroxide radical formation and cellular damage. From 0.1% to as much as 4% of the O2 used by actively respir￾ing mitochondria forms O2 —more than enough to have lethal effects on a cell unless the free radical is quickly disposed of. To prevent oxidative damage by O2 , cells have sev￾eral forms of the enzyme superoxide dismutase, which catalyzes the reaction 2 O2 
2H
88n H2O2
O2 The hydrogen peroxide (H2O2) generated by this reac￾tion is rendered harmless by the action of glutathione peroxidase (Fig. 19–35). This enzyme is remarkable for the presence of a selenocysteine residue (see Fig. 3–8a), in which an atom of selenium replaces the sulfur atom normally present in the thiol of the side chain. The se￾lenol group (OSeH) is more acidic than the thiol (OSH); its pKa is about 5, so at neutral pH, the selenocysteine side chain is essentially fully ionized (OCH2Se). Gluta￾thione reductase recycles oxidized glutathione to its re￾duced form, using electrons from the NADPH formed by nicotinamide nucleotide transhydrogenase or by the pentose phosphate pathway (see Fig. 14–20). Reduced glutathione also serves in keeping protein sulfhydryl groups in their reduced state, preventing some of the deleterious effects of oxidative stress (Fig. 19–35). SUMMARY 19.5 The Role of Mitochondria in Apoptosis and Oxidative Stress ■ Mitochondrial cytochrome c, released into the cytosol, participates in activation of one of the proteases (caspase 9) involved in apoptosis. ■ Reactive oxygen species produced in mitochondria are inactivated by a set of protective enzymes, including superoxide dismutase and glutathione peroxidase. 722 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Nicotinamide nucleotide transhydrogenase NADH NADPH GSSG H2O2 H2O 2 GSH S S 2 GSH Enz inactive active oxidative stress protein thiol reduction GSSG NADP+ glutathione reductase glutathione peroxidase superoxide NAD+ dismutase O2 O2 NAD+ Inner mitochondrial membrane Q III IV I Cyt c . – SH SH FIGURE 19–35 Mitochondrial production and disposal of super￾oxide. Superoxide radical, ?O2 , is formed in side reactions at Complexes I and III, as the partially reduced ubiquinone radical (?Q) donates an electron to O2. The reactions shown in blue defend the cell against the damaging effects of superoxide. Reduced glutathione (GSH; see Fig. 22–27) donates electrons for the reduction of hydrogen peroxide (H2O2) and of oxidized Cys residues (OSOSO) in proteins, and GSH is regenerated from the oxidized form (GSSG) by reduction with NADPH. 8885d_c19_690-750 3/1/04 11:32 AM Page 722 mac76 mac76:385_reb:

8885dc19690-7503/1/0411:32 AM Page723mac76mac76:385 19.6 General Features of Photophosphorylation PHOTOSYNTHESIS 19.6 General Features HARVESTING LIGHT ENERGY of Photophosphorylation We now turn to another reaction sequence in which the Unlike NADH (the major el noL flow of electrons is coupled to the synthesis of ATP: phosphorylation), H2O is a poor donor of electrons; its light-driven phosphorylation. The capture of solar en- standard reduction potential is 0.816 V, compared with rgy by photosynthetic organisms and its conversion to -0.320 V for NADH Photophosphorylation differs from the chemical energy of reduced organic compounds is oxidative phosphorylation in requiring the input of en- the ultimate source of nearly all biological energy Pho- ergy in the form of light to create a good electron donor tosynthetic and heterotrophic organisms live in a bal- and a good electron acceptor. In photophosphorylation anced steady state in the biosphere(Fig. 19-36). Pho- electrons flow through a series of membrane-bound car- tosynthetic organisms trap solar energy and form ATP riers including cytochromes, quinones, and iron-sulfur and NADPH, which they use as energy sources to make proteins, while protons are pumped across a membrane carbohydrates and other organic compounds from CO o create an electrochemical potential. Electron trans- and H2O; simultaneously, they release O2 into the at- fer and proton pumping are catalyzed by membrane mosphere Aerobic heterotrophs(humans, for example, complexes homologous in structure and function to as well as plants during dark periods) use the O, so Complex Ill of mitochondria. The electrochemical po formed to degrade the energy-rich organic products of tential they produce is the driving force for ATP syn- photosynthesis to COe and HO, generating ATP. The thesis from ADP and Pi, catalyzed by a membrane-bound CO2 returns to the atmosphere, to be used again by pho- ATP synthase complex closely similar to that of oxida- tosynthetic organisms. Solar energy thus provides the tive phosphorylation. riving force for the continuous cycling of CO2 and o Photosynthesis in plants encompasses two pro- through the biosphere and provides the reduced cesses: the light-dependent reactions, or light re- substrates--fuels, such as glucose--on which nonpho- actions, which occur only when plants are illuminated tosynthetic organisms depend. and the carbon-assimilation reactions (or carbon- Photosynthesis occurs in a variety of bacteria and fixation reactions), sometimes misleadingly called the in unicellular eukaryotes (algae) as well as in vascular dark reactions, which are driven by products of the light plants. Although the process in these organisms differs reactions(Fig. 19-37). In the light reactions, chlorophyll in detail, the underlying mechanisms are remarkably and other pigments of photosynthetic cells absorb light similar, and much of our understanding of photosyn- energy and conserve it as ATP and NADPH; simultane- thesis in vascular plants is derived from studies of sim- ously, O2 is evolved. In the carbon-assimilation reac pler organisms. The overall equation for photosynthesis tions, ATP and NADPH are used to reduce CO2 to form in vascular plants describes an oxidation-reduction re- triose phosphates, starch, and sucrose, and other prod ction in which H,O donates electrons (as hydrogen) ucts derived from them. In this chapter we are con for the reduction of CO2 to carbohydrate(CH.O) cerned only with the light-dependent reactions that lead to the synthesis of ATP and NADPH. The reduction of CO2+H2o-O2+(CH,O) CO2 is described in Chapter 20. .O FIGURE 19-36 Solar Photosynthetic reactions FIGURE 19-37 The light reactions gy as the ultimate of photosynthesis generate energy. all biologi NADP+ NADPH ch nadPh and atp at the energy. Photosynthetic ADP+ P ATP expense of solar energy. These organisms use the energy CO2 H products are used in the carbon- of sunlight to manufactur assimilation reactions, which occur glucose and other Carbonassimilate products, which hetero- reactions in light or darkness, to reduce CO trophic cells use as energ Heterotrophic to form trioses and more complex compounds(such as glt and carbon sources Carbohydrate derived from trioses

FIGURE 19–37 The light reactions of photosynthesis generate energy￾rich NADPH and ATP at the expense of solar energy. These products are used in the carbon￾assimilation reactions, which occur in light or darkness, to reduce CO2 to form trioses and more complex compounds (such as glucose) derived from trioses. Light reactions NADP+ Carbon-assimilation reactions ADP + Pi NADPH ATP H2O O2 Carbohydrate CO2 Photosynthetic cells O2 Carbohydrate CO2 H2O Heterotrophic cells FIGURE 19–36 Solar energy as the ultimate source of all biological energy. Photosynthetic organisms use the energy of sunlight to manufacture glucose and other organic products, which hetero￾trophic cells use as energy and carbon sources. PHOTOSYNTHESIS: HARVESTING LIGHT ENERGY We now turn to another reaction sequence in which the flow of electrons is coupled to the synthesis of ATP: light-driven phosphorylation. The capture of solar en￾ergy by photosynthetic organisms and its conversion to the chemical energy of reduced organic compounds is the ultimate source of nearly all biological energy. Pho￾tosynthetic and heterotrophic organisms live in a bal￾anced steady state in the biosphere (Fig. 19–36). Pho￾tosynthetic organisms trap solar energy and form ATP and NADPH, which they use as energy sources to make carbohydrates and other organic compounds from CO2 and H2O; simultaneously, they release O2 into the at￾mosphere. Aerobic heterotrophs (humans, for example, as well as plants during dark periods) use the O2 so formed to degrade the energy-rich organic products of photosynthesis to CO2 and H2O, generating ATP. The CO2 returns to the atmosphere, to be used again by pho￾tosynthetic organisms. Solar energy thus provides the driving force for the continuous cycling of CO2 and O2 through the biosphere and provides the reduced substrates—fuels, such as glucose—on which nonpho￾tosynthetic organisms depend. Photosynthesis occurs in a variety of bacteria and in unicellular eukaryotes (algae) as well as in vascular plants. Although the process in these organisms differs in detail, the underlying mechanisms are remarkably similar, and much of our understanding of photosyn￾thesis in vascular plants is derived from studies of sim￾pler organisms. The overall equation for photosynthesis in vascular plants describes an oxidation-reduction re￾action in which H2O donates electrons (as hydrogen) for the reduction of CO2 to carbohydrate (CH2O): light CO2
H2O 888n O2
(CH2O) 19.6 General Features of Photophosphorylation Unlike NADH (the major electron donor in oxidative phosphorylation), H2O is a poor donor of electrons; its standard reduction potential is 0.816 V, compared with 0.320 V for NADH. Photophosphorylation differs from oxidative phosphorylation in requiring the input of en￾ergy in the form of light to create a good electron donor and a good electron acceptor. In photophosphorylation, electrons flow through a series of membrane-bound car￾riers including cytochromes, quinones, and iron-sulfur proteins, while protons are pumped across a membrane to create an electrochemical potential. Electron trans￾fer and proton pumping are catalyzed by membrane complexes homologous in structure and function to Complex III of mitochondria. The electrochemical po￾tential they produce is the driving force for ATP syn￾thesis from ADP and Pi , catalyzed by a membrane-bound ATP synthase complex closely similar to that of oxida￾tive phosphorylation. Photosynthesis in plants encompasses two pro￾cesses: the light-dependent reactions, or light re￾actions, which occur only when plants are illuminated, and the carbon-assimilation reactions (or carbon￾fixation reactions), sometimes misleadingly called the dark reactions, which are driven by products of the light reactions (Fig. 19–37). In the light reactions, chlorophyll and other pigments of photosynthetic cells absorb light energy and conserve it as ATP and NADPH; simultane￾ously, O2 is evolved. In the carbon-assimilation reac￾tions, ATP and NADPH are used to reduce CO2 to form triose phosphates, starch, and sucrose, and other prod￾ucts derived from them. In this chapter we are con￾cerned only with the light-dependent reactions that lead to the synthesis of ATP and NADPH. The reduction of CO2 is described in Chapter 20. 19.6 General Features of Photophosphorylation 723 8885d_c19_690-750 3/1/04 11:32 AM Page 723 mac76 mac76:385_reb:

8885dc19690-7503/1/0411:32 AM Page724 6mac76:385 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Outer membr Grana(thylakoids) Inner membrane Thylakoid FIGURE 19-38 Chloroplast.(a)Schematic diagram.(b) Electron mi- crograph at high magnification showing grana, stacks of thylakoid Photosynthesis in Plants Takes Place in Chloroplasts was neither required nor reduced to a stable form un- der these conditions: O evolution could be dissociated In photosynthetic eukaryotic cells, both the light-de- from CO2 reduction. Several years later Severo Ochoa showed that NADP is the biological electron acceptor place in the chloroplasts(Fig. 19-38), membrane- in chloroplasts, according to the equation bounded intracellular organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, they are surrounded by two membranes 2H20+2NADP+ light an outer membrane that is permeable to small molecules To understand this photochemical process, we must first and ions and an inner membrane that encloses the consider the more general topic of the effects of light ternal compartment. This compartment contains many absorption on molecular structure. flattened. membrane-surrounded vesicles or sacs. the thylakoids, usually arranged in stacks called grana (Fig. 19-38b). Embedded in the thylakoid membranes (commonly called lamellae) are the photosynthetic pigments and the enzyme complexes that carry out the light reactions and ATP synthesis. The stroma (the queous phase enclosed by the inner membrane) con- tains most of the enzymes required for the carbon- assimilation reactions Light Drives Electron Flow in Chloroplasts Oxidized form Reduced form In 1937 robert hill found that when leaf extracts con- taining chloroplasts were illuminated, they(1) evolved Dichlorophenolindophenol O2 and(2) reduced a nonbiological electron acceptor added to the medium, according to the Hill reaction: SUMMARY 19.6 General Features 2H20 2A light 2AH2+ O2 of Photophosphorylation where A is the artificial electron acceptor, or Hill reagent. One Hill reagent, the dye 2, 6-dichlorophenol- a The light reactions of photosynthesis are those indophenol, is blue when oxidized(A)and colorless directly dependent on the absorption of light when reduced (AH2), making the reaction easy to fol- the resulting photochemistry takes electrons low. When a leaf extract supplemented with the dye was from H2O and drives them through a series of illuminated the blue dye became colorless and O2 was membrane-bound carriers, producing NADPH evolved. In the dark, neither O2 evolution nor dye re and atP duction took place. This was the first evidence that a The carbon-assimilation reactions absorbed light energy causes electrons to flow from H2O photosynthesis reduce CO2 with electrons from to an electron acceptor. Moreover, Hill found that CO NADPH and energy from ATP

Photosynthesis in Plants Takes Place in Chloroplasts In photosynthetic eukaryotic cells, both the light-de￾pendent and the carbon-assimilation reactions take place in the chloroplasts (Fig. 19–38), membrane￾bounded intracellular organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, they are surrounded by two membranes, an outer membrane that is permeable to small molecules and ions, and an inner membrane that encloses the in￾ternal compartment. This compartment contains many flattened, membrane-surrounded vesicles or sacs, the thylakoids, usually arranged in stacks called grana (Fig. 19–38b). Embedded in the thylakoid membranes (commonly called lamellae) are the photosynthetic pigments and the enzyme complexes that carry out the light reactions and ATP synthesis. The stroma (the aqueous phase enclosed by the inner membrane) con￾tains most of the enzymes required for the carbon￾assimilation reactions. Light Drives Electron Flow in Chloroplasts In 1937 Robert Hill found that when leaf extracts con￾taining chloroplasts were illuminated, they (1) evolved O2 and (2) reduced a nonbiological electron acceptor added to the medium, according to the Hill reaction: light 2H2O
2A 888n 2AH2
O2 where A is the artificial electron acceptor, or Hill reagent. One Hill reagent, the dye 2,6-dichlorophenol￾indophenol, is blue when oxidized (A) and colorless when reduced (AH2), making the reaction easy to fol￾low. When a leaf extract supplemented with the dye was illuminated, the blue dye became colorless and O2 was evolved. In the dark, neither O2 evolution nor dye re￾duction took place. This was the first evidence that absorbed light energy causes electrons to flow from H2O to an electron acceptor. Moreover, Hill found that CO2 was neither required nor reduced to a stable form un￾der these conditions; O2 evolution could be dissociated from CO2 reduction. Several years later Severo Ochoa showed that NADP
is the biological electron acceptor in chloroplasts, according to the equation light 2H2O
2NADP
888n 2NADPH
2H

O2 To understand this photochemical process, we must first consider the more general topic of the effects of light absorption on molecular structure. SUMMARY 19.6 General Features of Photophosphorylation ■ The light reactions of photosynthesis are those directly dependent on the absorption of light; the resulting photochemistry takes electrons from H2O and drives them through a series of membrane-bound carriers, producing NADPH and ATP. ■ The carbon-assimilation reactions of photosynthesis reduce CO2 with electrons from NADPH and energy from ATP. OH Reduced form (colorless) Oxidized form (blue) Dichlorophenolindophenol OH O OH N Cl Cl Cl Cl NH 724 Chapter 19 Oxidative Phosphorylation and Photophosphorylation Grana (thylakoids) Stroma (b) Outer membrane Inner membrane Thylakoids (a) FIGURE 19–38 Chloroplast. (a) Schematic diagram. (b) Electron mi￾crograph at high magnification showing grana, stacks of thylakoid membranes. 8885d_c19_690-750 3/1/04 11:32 AM Page 724 mac76 mac76:385_reb: