Chloroplasts generate a proton gradient across the thylakoid membrane that is us

Question

Chloroplasts generate a proton gradient across the thylakoid membrane that is used to synthesize ATP. The process is called photophosphorylation. There are three points during photosynthesis where protons are released into the lumen. Identify these points. When ferredoxin is oxidized by ferredoxin-NADP^+ reductase. When P700 transfers an electron to an acceptor, A_0. When water is oxidized at the manganese centers of the oxygen-evolving complex. When plastoquinol is oxidized in the second half of the Q cycle at the cytochrome bf complex. When plastoquinol is oxidized at the cytochrome bf complex. When P680 transfers an electron upon excitation.

Explanation / Answer

During photosynthesis, there are three points where protons are released into the lumen.

1. When water is oxidized at the manganese centers of the oxygen-evolving complex. When the manganese center splits water molecules, it binds two water molecules at once, extracting four electrons, releasing 4 H+ ions, and producing a molecule of O2.

2. When plastoquinol is oxidized at the cytochrome bf complex. During oxidation of plastoquinol, protons are released into the lumen that were initially taken up from the stromal side during the formation of PQH2.

3. When plastoquinol is oxidized in the second half of the Q cycle at the cytochrome bf complex.

The accumulation of these protons in the lumen generates the proton motive force (that includes membrane potential and proton gradient) across the membrane.

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