PSIIs

PSIIs, short for photosystem II complexes, are the protein–pigment assemblies that drive the initial step of oxygenic photosynthesis in plants, algae, and cyanobacteria. They reside in the thylakoid membranes and use light energy to extract electrons from water, releasing molecular oxygen as a byproduct. The electrons then enter the photosynthetic electron transport chain, ultimately contributing to ATP and NADPH production.

The PSII core is a multi-subunit complex with a central reaction center formed by the D1 and D2 proteins. This D1/D2 core binds the primary pigments and the catalytic site for water oxidation. The oxygen-evolving complex, which contains a Mn4CaO5 cluster and extrinsic subunits PsbO, PsbP, and PsbQ, drives the splitting of water into electrons, protons, and oxygen. On the electron-acceptor side, plastoquinone molecules (QA and QB) accept electrons, which are then passed to the plastoquinone pool and onward to the cytochrome b6f complex before reaching photosystem I.

PSII is often associated with light-harvesting antenna complexes, especially LHCII in plants, forming PSII–LHCII supercomplexes that optimize light capture. The activity of PSII is closely linked to regulatory processes such as state transitions and non-photochemical quenching. D1, a core protein, is particularly susceptible to light-induced damage and is continuously replaced through a repair cycle to maintain photosynthetic efficiency.

PSIIs are essential for life on Earth by supplying the electrons that enable oxygen evolution and by feeding energy into the broader photosynthetic electron transport chain.