Photosystem II (PSII) of plants, algae and cyanobacteria is a specialised protein complex that uses light energy to transfer electrons from water to plastoquinone, producing molecular oxygen and reduced plastoquinone. The PSII complex includes a peripheral antenna containing chlorophyll and other pigments to absorb light, a reaction centre that utilises the excitation energy transferred to it for charge separation, cofactors that stabilise the charge pair via electron transfer reactions, a Mn4CaO5 cluster that oxidises water, and a binding pocket where plastoquinone is reduced. The electrons and protons that PSII extracts from water are employed in the overall photosynthetic process for the reduction of CO2, which provides the chemical energy for most life on Earth. PSII is the only known biological source of O2 produced from water and is responsible for the molecular oxygen in the atmosphere.