Photophosphorylation

FIGURE 19-57 Proton and electron circuits in thylakoids. Electrons (blue arrows) move from H2O through PSII, through the intermediate chain of carriers, through PSI, and finally to NADP+. Protons (red arrows) are pumped into the thylakoid lumen by the flow of electrons through the carriers linking PSII and PSI, and reenter the stroma through proton channels formed by the Fo (designated CFo) of ATP synthase. The F-, subunit (CF-,) catalyzes synthesis of ATP.

FIGURE 19-57 Proton and electron circuits in thylakoids. Electrons (blue arrows) move from H2O through PSII, through the intermediate chain of carriers, through PSI, and finally to NADP+. Protons (red arrows) are pumped into the thylakoid lumen by the flow of electrons through the carriers linking PSII and PSI, and reenter the stroma through proton channels formed by the Fo (designated CFo) of ATP synthase. The F-, subunit (CF-,) catalyzes synthesis of ATP.

Photophosphorylation

Thylakoid membrane

Thylakoid membrane

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

AG = 2.3i?7T ApH + Z f Ai/j = -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 b6f complex to plastocyanin. The path of

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