fotosystem

Photosystems are highly organized complexes in the thylakoid membranes of chloroplasts and cyanobacterial membranes that capture light energy and drive the primary photochemical reactions of oxygenic photosynthesis. They consist of antenna pigment–protein complexes that harvest light and a reaction-center core where electron transfer occurs.

Two major photosystems participate in the light reactions: photosystem II (PSII) and photosystem I (PSI). PSII contains the P680 reaction center and initiates photosynthesis by using absorbed light to oxidize water, releasing oxygen, protons, and electrons. The electrons are transferred from the oxidized PSII through plastoquinone and the cytochrome b6f complex to plastocyanin and then to PSI.

PSI contains the P700 reaction center and further energizes electrons to reduce ferredoxin, which in turn reduces NADP+ via ferredoxin-NADP+ reductase, forming NADPH. The two systems are connected by the electron-transport chain and operate together in what is known as the Z-scheme. The energy that accompanies electron transport across the membrane is used by ATP synthase to generate ATP from ADP and inorganic phosphate, providing energy and reducing power for carbon fixation in the Calvin cycle.

Antennas differ among organisms: higher plants and green algae use chlorophyll a/b–containing light-harvesting complexes, while cyanobacteria use phycobilisomes. Besides their primary role in light harvesting, photosystems are subject to regulation and photoprotection mechanisms to prevent damage under excess light, and to repair processes for the reaction-center pigments.