oxygen-evolving complex (OEC) A small complex of proteins and manganese ions associated with the reaction centre of photosystem II (PSII) that splits (oxidizes) water molecules to yield oxygen during photosynthesis. Essentially, this oxidation provides the low-energy electrons from water that are excited by PSII using the light energy trapped by chlorophyll pigments. The resultant high-energy electrons can then flow through the electron transport chain. PSII draws electrons singly from a cluster of four manganese ions held by each OEC. When this cluster has accumulated four positive charges, the OEC oxidizes two molecules of water, producing one molecule of oxygen and four protons. The OEC is located on the lumen side of the thylakoid membrane in the chloroplast and requires chloride ions to function. See photosystems I and II.
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