Describe the cyclic photophosphorylation associated with photosystem 1. Solution

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Cyclic Photophosphorylation * Each CO2 taken up by the Calvin cycle) requires: -> 2 NADPH molecules and -> 3 ATP molecules * Each molecule of oxygen released by the light reactions supplies the 4 electrons needed to make 2 NADPH molecules. * The chemiosmosis driven by these 4 electrons as they pass through the cytochrome b6/f complex liberates only enough energy to pump 12 protons into the interior of the thylakoid. * But in order to make 3 molecules of ATP, the ATPase in chloroplasts appears to have 14 protons (H+) pass through it. * So there appears to be a deficit of 2 protons. * How is this deficit to be made up? * One likely answer: cyclic photophosphorylation. In cyclic photophosphorylation, * the electrons expelled by the energy of light absorbed by photosystem I pass, as normal, to ferredoxin (Fd). * But instead of going on to make NADPH, * they pass to plastoquinone (PQ) and on back into the cytochrome b6/f complex. * Here the energy each electron liberates pumps 2 protons (H+) into the interior of the thylakoid — enough to make up the deficit left by noncyclic photophosphorylation. This process is truly cyclic because no outside source of electrons is required. Like the photocell in a light meter, photosystem I is simply using light to create a flow of current. The only difference is that instead of using the current to move the needle on a light meter, the chloroplast uses the current to help synthesize ATP.

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