Photosystem II

Photosystem II (PSII) is the protein complex in the thylakoid membranes of plants, algae and cyanobacteria that uses light energy to split water into oxygen, protons and electrons. It is the source of almost all the oxygen in Earth's atmosphere.

Light absorbed by chlorophyll excites a special pair called P680, which passes an electron to a nearby acceptor and becomes a strong oxidant. The oxidant removes electrons, one at a time, from a cluster of four manganese atoms and one calcium atom (Mn₄CaO₅) in the oxygen-evolving complex. After four such steps, the cluster oxidizes two water molecules, releasing O₂ and four protons. The sequence is described by the Kok cycle of S-states.

The electrons pass to plastoquinone, which carries them to the next complex of the electron transport chain in the thylakoid membrane. The protons released in the lumen, together with those pumped by later steps, drive ATP synthesis, and the electrons ultimately reduce NADP⁺ for use in the Calvin cycle. The structure of the complex was solved to 1.9 angstroms in 2011.

The reactive intermediates damage the D1 protein of the core, and plants break down and replace it within an hour or so in strong light. Herbicides such as diuron and atrazine block the plastoquinone site. PSII evolved in cyanobacteria, whose oxygen production caused the Great Oxidation Event, and a cyanobacterial ancestor of the chloroplast carried it into eukaryotes.

• Chlorophyll — the pigment that powers it
• Photosynthesis — the process it begins
• Great Oxidation Event — the planetary result of its oxygen release