photosystem

Photosystems are protein–pigment complexes embedded in the thylakoid membranes of chloroplasts, cyanobacteria, and some algae. They capture light energy and convert it into chemical energy during the light-dependent reactions of photosynthesis. There are two main complexes, photosystem II (PSII) and photosystem I (PSI), each with a core reaction-center complex surrounded by light-harvesting antenna pigments that funnel energy to the reaction center.

PSII is the first complex in the oxygen-evolving chain. It contains the oxygen-evolving complex, which uses a Mn4CaO5 cluster to split water into electrons, protons, and molecular oxygen. The PSII reaction center, P680, absorbs light and donates an electron to the primary acceptor. Electrons pass through the plastoquinone pool (QA and QB) and the cytochrome b6f complex, driving proton translocation and contributing to the proton motive force used to synthesize ATP. Electrons are then transferred to PSI via plastocyanin.

PSI contains the P700 reaction center. After excitation, electrons are transferred through iron–sulfur clusters and ferredoxin, ultimately reducing NADP+ to NADPH via ferredoxin–NADP+ reductase. The two photosystems are connected by the electron transport chain, enabling noncyclic electron flow that yields both ATP and NADPH. Cyclic electron flow around PSI can occur to increase ATP production without producing NADPH.

In plants, antenna complexes such as light-harvesting complexes I and II increase light capture efficiency and transfer energy to the respective reaction centers. Photosystems thus convert light energy into chemical energy used for carbon fixation and other biosynthetic processes.