22. When chloroplasts are continuously illuminated with a weak white light, the

Question

22. When chloroplasts are continuously illuminated with a weak white light, the intensity of fluorescence from the antenna chlorophylls is quite weak, with only about 2% of the absorbed photons being re-emitted as fluorescence. If the potential of the medium is chang V (low enough to reduce quinones), the fluorescence increases markedly. The explanation for this result is: a) the proton gradient is disrupted b) rubiso is quenching the fluorescence c) the electron transfer through the Z scheme is blocked d) P700 cannot be reduced by NADIH

Explanation / Answer

C) The electron transfer through Z scheme will be blocked.

Upon absorption of photon, P680 is excited to P680* which transfers an electron to pheophytin. Pheophytin inturn transfers electron to plastoquinone QA, which then transfers to plastoquinone QB. After two reductions of plastoquinone, it forms plastoquinol. Plastoquinol diffuses in the membrane and transfers electron to cytochrome b6f complex then to plastocyanin; which then transfers electron to P700 that reduces NADP+ to NADPH.

If the quinone is not reduced to plastoquinol, the electron transfer from plastoquinol to P700 will be inhibited and thus prevents the non cyclic photophosphorylation or Z scheme will be inhibited

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