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The photosensitizing pigments required for the interception of light energy have the potential to pass that energy to oxygen to create one of the most destructive species in biology, singlet oxygen. Similarly, the photosynthetic requirement for electron carriers with negative electrochemical potentials, which allows electron transport to oxygen, is potentially extremely hazardous. Oxygen is one of the products of photosynthetic electron transport, while on the other hand, it is also reduced by the photosynthetic electron transport chain during pseudocyclic electron flow and assimilated via photorespiration. The regulatory functions of photorespiration and pseudocyclic electron flow may be quite distinct. The regeneration of ascorbate requires the participation of the electron transport chain in the chloroplast and has important implications for the regulation of electron transport. The subsequent production of …