The Approximate Stoichiometry of Photophosphorylation Has Been Established

As electrons move from water to NADP+ in plant chloro-plasts, about 12 H+ move from the stroma to the thy-lakoid lumen per four electrons passed (that is, per O2 formed). Four of these protons are moved by the oxygen-evolving complex, and up to eight by the cy-tochrome b6 f complex. The measurable result is a 1,000-fold difference in proton concentration across the thylakoid membrane (ApH = 3). Recall that the energy stored in a proton gradient (the electrochemical potential) has two components: a proton concentration difference (ApH) and an electrical potential (A^) due to charge separation. In chloroplasts, ApH is the dominant component; counterion movement apparently dissipates most of the electrical potential. In illuminated chloro-plasts, the energy stored in the proton gradient per mole of protons is

17 kJ/mol so the movement of 12 mol of protons across the thylakoid membrane represents conservation of about 200 kJ of energy—enough energy to drive the synthesis of several moles of ATP (AG'° = 30.5 kJ/mol). Experimental measurements yield values of about 3 ATP per O2 produced.

At least eight photons must be absorbed to drive four electrons from H2O to NADPH (one photon per electron at each reaction center). The energy in eight photons of visible light is more than enough for the synthesis of three molecules of ATP.

ATP synthesis is not the only energy-conserving reaction of photosynthesis in plants; the NADPH formed in the final electron transfer is (like its close analog NADH) also energetically rich. The overall equation for noncyclic photophosphorylation (a term explained below) is

Cyclic Electron Flow Produces ATP but Not NADPH or O2

Using an alternative path of light-induced electron flow, plants can vary the ratio of NADPH to ATP formed in the light; this path is called cyclic electron flow to differentiate it from the normally unidirectional or non-cyclic electron flow from H2O to NADP+, as discussed thus far. Cyclic electron flow (Fig. 19-49) involves only PSI. Electrons passing from P700 to ferredoxin do not continue to NADP + , but move back through the cytochrome b6 f complex to plastocyanin. The path of electrons matches that in green sulfur bacteria (Fig. 19-47b). Plastocyanin donates electrons to P700, which transfers them to ferredoxin when the plant is illuminated. Thus, in the light, PSI can cause electrons to cycle continuously out of and back into the reaction center of PSI, each electron propelled around the cycle by the energy yielded by the absorption of one photon. Cyclic electron flow is not accompanied by net formation of NADPH or evolution of O2. However, it is accompanied by proton pumping by the cytochrome b6 f complex and by phosphorylation of ADP to ATP, referred to as cyclic photophosphorylation. The overall equation for cyclic electron flow and photophosphorylation is simply

By regulating the partitioning of electrons between NADP+ reduction and cyclic photophosphorylation, a plant adjusts the ratio of ATP to NADPH produced in the light-dependent reactions to match its needs for these products in the carbon-assimilation reactions and other biosynthetic processes. As we shall see in Chapter 20, the carbon-assimilation reactions require ATP and NADPH in the ratio 3:2.

The ATP Synthase of Chloroplasts Is Like That of Mitochondria

The enzyme responsible for ATP synthesis in chloro-plasts is a large complex with two functional components, CFo and CF1 (C denoting its location in chloroplasts). CFo is a transmembrane proton pore composed of several integral membrane proteins and is homologous to mitochondrial Fo. CF1 is a peripheral membrane protein complex very similar in subunit composition, structure, and function to mitochondrial F^

Electron microscopy of sectioned chloroplasts shows ATP synthase complexes as knoblike projections on the outside (stromal or N) surface of thylakoid membranes; these complexes correspond to the ATP synthase complexes seen to project on the inside (matrix or N) surface of the inner mitochondrial membrane. Thus the relationship between the orientation of the ATP synthase and the direction of proton pumping is the same in chloroplasts and mitochondria. In both cases, the Fj^ portion of ATP synthase is located on the more alkaline (N) side of the membrane through which protons flow down their concentration gradient; the direction of proton flow relative to Fj^ is the same in both cases: P to N (Fig. 19-58).

The mechanism of chloroplast ATP synthase is also believed to be essentially identical to that of its mito-chondrial analog; ADP and Pj readily condense to form ATP on the enzyme surface, and the release of this enzyme-bound ATP requires a proton-motive force. Rotational catalysis sequentially engages each of the three 3 subunits of the ATP synthase in ATP synthesis, ATP release, and ADP + P, binding (Figs 19-24, 19-25).

Chloroplasts Evolved from Endosymbiotic Bacteria

Like mitochondria, chloroplasts contain their own DNA and protein-synthesizing machinery. Some of the polypeptides of chloroplast proteins are encoded by chloroplast genes and synthesized in the chloroplast; others are encoded by nuclear genes, synthesized outside the chloroplast, and imported (Chapter 27). When plant cells grow and divide, chloroplasts give rise to new



Bacterium (E. coli)

Matrix (N side)

Matrix (N side)

Intermembrane space (p side)


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