photosystems

Photosystems are light-harvesting protein complexes embedded in the thylakoid membranes of chloroplasts in plants and algae, and in the photosynthetic membranes of cyanobacteria. Each photosystem consists of a core reaction center surrounded by a peripheral antenna complex that captures light energy and funnels it to the reaction center. The two main types are Photosystem II (PSII) and Photosystem I (PSI).

Photosystem II absorbs light primarily at 680 nm (P680) and catalyzes the splitting of water at the oxygen-evolving complex, producing oxygen, electrons, and protons. The excited P680 transfers an electron through the primary electron acceptor and plastoquinone to the cytochrome b6f complex, which pumps protons across the thylakoid membrane and passes electrons to plastocyanin.

Photosystem I absorbs light primarily at 700 nm (P700) and uses its excited electrons to reduce ferredoxin, which in turn reduces NADP+ to NADPH via ferredoxin-NADP+ reductase. The antennal and core complexes of both photosystems collaborate to convert light energy into chemical energy, delivering electrons from water to NADP+.

Together, the two photosystems drive the light-dependent reactions in a sequence known as the Z-scheme. The electron flow creates a proton-m gradient that powers ATP synthase to produce ATP, while NADPH provides reducing power for carbon fixation. In addition to linear (noncyclic) electron flow, cyclic electron flow around PSI can generate extra ATP without producing NADPH.

Variations exist among organisms. Cyanobacteria employ phycobilisomes as antennae, whereas plants rely on LHC proteins. Photosystems are ancient, essential components of oxygenic photosynthesis, and central to energy capture in many photosynthetic organisms.