Chloroplast antioxidative systems

Fig. 4. Photosynthetic transport of electrons from PSII water oxidation is normally used to reduce NADP+ to NADPH for use in Calvin cycle carbon fixation. But, under excess light, when the electron transport chain is over-reduced, electron flow may be diverted at PSI to the Mehler reaction, reducing O2 to superoxide (O2 ). Chloroplast antioxidant systems can further reduce O2 back to water, allowing the water-water cycle to function as a protective alternative electron transport path.

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