Photosystem II (PSII)

Photosystem II is a multi-subunit pigment-protein complex located in the thylakoid membrane of cyanobacteria, algae, and higher plants. It uses light energy to drive the oxidation of water, releasing molecular oxygen, protons, and electrons—an essential initial step in oxygenic photosynthesis. The core of PSII is composed of proteins D1 and D2, as well as associated cofactors such as chlorophylls, carotenoids, quinones, and the unique Mn_4CaO_5 cluster that catalyzes water splitting. PSII is the only known enzyme complex that oxidizes water to molecular oxygen, providing both the oxygen we breathe and the electrons that power photosynthetic metabolism in plants. It is a major target of herbicides in agriculture, which block electron flow and photosynthetic activity, leading to plant death

Other names

Water-plastoquinone oxidoreductasePSIIPhotosystem II complex

Inhibition of electron transfer from water to plastoquinone by binding the Q_B site (e.g., DCMU, atrazine) Blockage of oxygen evolution and proton gradient generation