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Berg JM, Tymoczko JL, Stryer L. Biochemistry. Fifth edition. Brand-new York: W H Freeman; 2002.

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In 1966, André Jagendorf verified that chloroplast synthesize ATP in the dark when an artificial pH gradient is imposed across the thylakoid membrane. To develop this transient pH gradient, he wet chloroplasts in a pH 4 buffer for numerous hours and then rapidly mixed them through a pH 8 buffer comprise ADP and Pi. The pH the the stroma suddenly increased to 8, whereas the pH of the thylakoid an are remained in ~ 4. A explode of ATP synthesis climate accompanied the disappearance of the pH gradient throughout the thylakoid membrane (Figure 19.24). This incisive experiment was among the an initial to unequivocally assistance the hypothesis placed forth by Peter Mitchell that ATP synthetic is thrust by proton-motive force.


Figure 19.24

Jagendorf"s Demonstration. Chloroplast synthesize ATP ~ the imposition that a pH gradient.

The principles through which ATP synthetic takes ar in chloroplasts are nearly identical through those because that oxidative phosphorylation. We have actually seen just how light induces electron transfer v photosystems II and also I and the cytochrome bf complex. At assorted stages in this process, protons room released right into the optical membrane lumen or taken increase from the stroma, generating a proton gradient. Together a gradient have the right to be maintained because the optical membrane membrane is essentially impermeable come protons. The thylakoid room becomes markedly acidic, through the pH pull close 4. The light-induced transmembrane proton gradient is about 3.5 pH units. As discussed in ar 18.4, power inherent in the proton gradient, referred to as the proton-motive pressure (Δp), is defined as the amount of two components: a fee gradient and also a chemical gradient. In chloroplasts, nearly every one of Δp occurs from the pH gradient, whereas, in mitochondria, the donation from the membrane potential is larger. The reason for this difference is the the optical membrane membrane is fairly permeable come Cl- and Mg2+. The light-induced carry of H+ right into the thylakoid an are is accompanied by the move of either Cl- in the very same direction or Mg2+ (1 Mg2+ per 2 H+) in opposing direction. Consequently, electric neutrality is maintained and also no membrane potential is generated. A pH gradient of 3.5 units throughout the optical membrane membrane corresponds to a proton-motive force of 0.20 V or a ΔG the -4.8 kcal mol-1 (-20.0 kJ mol-1).

19.4.1. The ATP Synthase that Chloroplasts very closely Resembles Those that Mitochondria and Prokaryotes

The proton-motive pressure generated through the irradiate reactions is converted into ATP by the ATP synthase of chloroplasts, additionally called the CF1-CF0complex (C stands for chloroplast and F for factor). CF1-CF0 ATP synthase closely resembles the F1-F0 complicated of mitochondria (Section 18.4.1). CF0 conducts protons throughout the optical membrane membrane, conversely, CF1 catalyzes the development of ATP from ADP and Pi.

CF0 is embedded in the optical membrane membrane. It consists of four different polypeptide chains well-known as i (17 kd), II (16.5 kd), III (8 kd), and also IV (27 kd) having actually an estimated stoichiometry that 1:2:12:1. Subunits I, II, and also III correspond to subunits a, b, and also c, respectively, of the mitochondrial F0 subunit, and also subunit IV is similar in sequence to subunit a. CF1, the website of ATP synthesis, has actually a subunit composition α3β3γδϵ. The β subunits contain the catalytic sites, similar to the F1 subunit that mitochondrial ATP synthase. Remarkably, β subunits that corn chloroplastic ATP synthase are more than 60% the same in amino mountain sequence through those of human ATP synthase, in spite of the i of about 1 exchange rate years due to the fact that the separation that the plant and also animal kingdoms.

Significantly, the membrane orientation that CF1-CF0 is reversed contrasted with the of the mitochondrial ATP synthase (Figure 19.25). Thus, protons flow out that the optical membrane lumen v ATP synthase right into the stroma. Since CF1 is top top the stromal surface ar of the optical membrane membrane, the freshly synthesized ATP is released directly into the stromal space. Recall the NADPH developed through the action of photosystem I and ferredoxin-NADP+ reductase likewise is released right into the stromal space. Thus, ATP and NADPH, the assets of the irradiate reactions that photosynthesis, are as necessary positioned for the succeeding dark reactions, in i beg your pardon CO2is converted right into carbohydrate.


Figure 19.25

Comparison that Photosynthesis and also Oxidative Phosphorylation. The light-induced electron move in photosynthesis drives protons right into the thylakoid lumen. The overabundance protons flow out of the lumen v ATP synthase to generate ATP in the stroma. In (more...)

19.4.2. Cyclic Electron circulation Through Photosystem i Leads to the manufacturing of ATP rather of NADPH

An alternate pathway for electrons occurring from P700, the reaction center of photosystem I, contributes to the flexibility of photosynthesis. The electron in reduced ferredoxin can be transferred to the cytochrome bf complicated rather than to NADP+. This electron climate flows earlier through the cytochrome bf facility to minimize plastocyanin, which deserve to then it is in reoxidized by P700+ to complete a cycle. The network outcome the this cyclic circulation of electron is the pump of proton by the cytochrome bf complex. The resulting proton gradient climate drives the synthetic of ATP. In this process, called cyclic photophosphorylation, ATP is created without the concomitant development of NADPH (Figure 19.26). Photosystem II walk not get involved in cyclic photophosphorylation, and so O2 is not created from H2O. Cyclic photophosphorylation takes location when NADP+ is unavailable to expropriate electrons from reduced ferredoxin, since of a an extremely high ratio of NADPH to NADP+.


Figure 19.26

Cyclic Photophosphorylation. In this pathway, electrons from lessened ferredoxin room transferred to the cytochrome bf complicated rather 보다 to ferredoxin-NADP+ reductase. The circulation of electrons through cytochrome bf pumps protons into the optical membrane lumen. (more...)

19.4.3. The absorption of Eight Photons returns One O2, 2 NADPH, and Three ATP Molecules

We can now estimate the in its entirety stoichiometry because that the irradiate reactions. The absorb of 4 photons through photosystem II generates 1 molecule that O2 and releases 4 protons into the optical membrane lumen. The 2 molecule of plastoquinol are oxidized through the Q cycle of the cytochrome bf complicated to release 8 protons right into the lumen. Finally, the electron from 4 molecule of diminished plastocyanin are propelled to ferredoxin through the absorption of 4 additional photons. The 4 molecules of lessened ferredoxin generate 2 molecules of NADPH. Thus, the in its entirety reaction is:


The 12 protons released in the lumen have the right to then flow through ATP synthase. Offered the noticeable stoichiometry the 12 subunit III components in CF0, we mean that 12 protons need to pass v CF0 to complete one full rotation the CF1 and, hence, generate and release 3 molecules of ATP. Given this ratio, the all at once reaction is

Cyclic photophosphorylation is somewhat much more productive through regard come ATP synthesis. The absorption of 4 photons by photosystem I results in the release of 8 protons into the lumen by the cytochrome bf system. These protons circulation through ATP synthase to productivity 2 molecule of ATP (assuming the same proportion of ATP molecules produced per proton). Thus, every 2 absorbed photons yield 1 molecule that ATP. No NADPH is produced.

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