The two systems of photosynthetic pigments in the thylakoid membranes of chloroplasts that are involved in the light-dependent reactions of photosynthesis. Each photosystem comprises a complex of proteins spanning the thylakoid membrane. Attached to the proteins are multiple arrays of chlorophyll a and accessory pigments forming light-harvesting complexes, or antenna pigments, surrounding the photosystem’s single reaction centre. Photons of light are trapped by the light-harvesting complexes and the excited pigment molecules rapidly transfer their energy via other molecules to a chlorophyll a molecule at the reaction centre, where the absorbed light energy is converted to chemical energy. The photons cause the release of energized electrons, or excitons, which behave in a quantum mechanical manner as both waves and particles. This behaviour contributes to the high efficiency with which light energy is transferred to the reaction centre (see quantum biology). 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:

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