Photosystem II

Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme captures photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. By obtaining these electrons from water, photosystem II provides the electrons for all of photosynthesis to occur. The hydrogen ions (protons) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce NADP+ to NADPH or are used in cyclic photophosphorylation.

Read more about Photosystem IIStructure, Oxygen-Evolving Complex (OEC), Water Splitting

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Relationship Between Photosystems I and II
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... The mechanism of water oxidation is still not fully elucidated, but we know many details about this process ... The oxidation of water to molecular oxygen requires extraction of four electrons and four protons from two molecules of water ...
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