fotosystemem

Photosystem is a complex of proteins and pigments embedded in the thylakoid membranes of chloroplasts and cyanobacteria. These complexes are the primary units that capture light energy during photosynthesis. There are two main types of photosystems, Photosystem I (PSI) and Photosystem II (PSII), which work in series.

Photosystem II is responsible for the initial step of light absorption and water splitting. When a photon strikes a pigment molecule within PSII, the energy is transferred to a special pair of chlorophyll molecules called the reaction center chlorophyll P680. This excitation energy leads to the oxidation of P680, which then abstracts an electron from a water molecule, splitting it into oxygen, protons, and electrons. The electrons are then passed along an electron transport chain.

Photosystem I absorbs light energy and transfers it to its reaction center chlorophyll, P700. The excited P700 loses an electron, which is then passed along a second electron transport chain. The electron lost by P700 is eventually replenished by the electron coming from PSII's electron transport chain. The electrons from PSI are ultimately used to reduce NADP+ to NADPH, a crucial energy-carrying molecule for the Calvin cycle. The combined action of PSII and PSI, along with the electron transport chain, drives the synthesis of ATP and NADPH, which are essential for converting carbon dioxide into sugars.