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photosystems I and II

photosystems I and II The two systems of photosynthetic pigments in the thylakoid membranes of chloroplasts that are involved in the light-dependent reactions of photosynthesis. Each photosystem contains about 300 chlorophyll molecules that trap light energy, which is then passed to a reaction centre, comprising a chlorophyll a molecule, in each photosystem. In photosystem II a chlorophyll a molecule, known as P680, utilizes light of wavelength 680 nm; in photosystem I the chlorophyll a molecule, known as P700, absorbs light at a wavelength of 700 nm. Light energy is used in each reaction centre to raise electrons to higher energy levels to enable them to be taken up by electron acceptors. This causes P680 and P700 to become positively charged, or oxidized. The chlorophyll in photosystem II replaces its lost electrons with ones supplied by an associated complex of proteins called the oxygen-evolving complex, which is responsible for the photolysis of water: 2H2O → 4H+ + 4e + O2

The oxygen produced is given off as a gas, and the H+ ions, together with the electrons from photosystem I, reduce NADP+ (see photophosphorylation).

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