Fotosistemas

Fotosistemas are protein-pigment complexes that capture light energy and convert it into chemical energy during photosynthesis. They are embedded in the thylakoid membranes of chloroplasts in plants and algae and in the photosynthetic membranes of cyanobacteria and some bacteria. They comprise light-harvesting antennae that collect photons and core reaction centers where charge separation occurs.

There are two main photosystems, Photosystem II (PSII) and Photosystem I (PSI). PSII contains core reaction-center proteins D1 and D2 and an antenna system that includes CP43 and CP47, and it uses the oxygen-evolving complex to split water, releasing oxygen. Its reaction center is associated with a chlorophyll a molecule that absorbs light at about 680 nm (P680). PSI contains the PsaA and PsaB core; its reaction center is a chlorophyll a pair that absorbs at about 700 nm (P700).

In the established flow of electrons, light energy excites the pigments and drives electrons from PSII through plastoquinone, the cytochrome b6f complex, and plastocyanin to PSI, then to ferredoxin and NADP+ reductase to form NADPH. The process also pumps protons to generate a proton-m Gradient, which powers ATP synthase to produce ATP. In addition to linear flow, PSI can operate in cyclic mode to produce ATP without NADPH.

Fotosistemas are essential to oxygenic photosynthesis in plants, algae, and cyanobacteria, with similar, though varied, light-harvesting and reaction-center arrangements across different organisms.