PSII is the membrane protein complex found in oxygenic photosynthetic organisms (higher plants, green algae and cyanobacteria), which harnesses light energy to split H2O into O2, protons and electrons. It drives one of the most oxidising reactions known to occur in nature and is responsible for the production of atmospheric oxygen, essential for aerobic life on this planet. Isn addition, by catalysing the first step of the photosynthetic electron transport chain, PSII is also involved in the production of a substantial proportion of the global biomass. A more detailed description may be found here.

View of the higher plant PSII-LHCII complex (based on a single reaction centre)
[See also subunit text descriptions here]