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Photosystem II and the Splitting of Water

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Photosystem II's reaction center loses a high-energy electron to the electron transport chain and replaces it by splitting water, releasing oxygen as a byproduct.

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Splitting two water molecules at photosystem II releases one molecule of oxygen gas, the oxygen every aerobic organism breathes.

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Water splitting exists to replace the lost electron — oxygen release is a byproduct, not the reaction's purpose.