Role of water. The photo-oxidation ( photolysis) of water is a key reaction and the primary source of the earth's oxygen. The mechanism is not completely understood, but this much is clear: Electrons lost from the reaction center of PsII are replaced by electrons removed from water in a stepwise process. The P680 molecule removes electrons one at a time from an oxygen-evolving complex. When four have been removed from two water molecules, oxygen (O 2 ) is released together with 4 protons (H + ) and 4 electrons ( e − ). The protons are added to the reservoir in the space between the thylakoid membranes and function in the proton pumps of ATP synthesis. Some of the oxygen is used in plant cell respiration; the rest is released to the atmosphere. Cyclic and noncyclic photophosphorylation. The passage of electrons from water (PsII PsI NADP + ) is
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