Fotosystémy

Fotosystémy are the fundamental units of light-dependent reactions in photosynthesis. They are large protein-pigment complexes embedded within the thylakoid membranes of chloroplasts and cyanobacteria. Each fotosystém contains an antenna complex and a reaction center. The antenna complex consists of hundreds of chlorophyll and carotenoid molecules that capture light energy and transfer it to the reaction center. The reaction center contains a special pair of chlorophyll molecules that, upon receiving the energy, become excited and initiate the electron transport chain.

There are two main types of fotosystémy: Fotosystém I (PSI) and Fotosystém II (PSII). PSII is responsible for the splitting of water molecules (photolysis), releasing oxygen as a byproduct and producing electrons and protons. These electrons then travel through an electron transport chain to PSI. PSI absorbs light energy, re-energizes the electrons, and uses them to reduce NADP+ to NADPH, a high-energy electron carrier. The energy generated during this electron flow is also used to pump protons into the thylakoid lumen, creating a proton gradient that drives ATP synthesis. The combined action of PSII and PSI is essential for converting light energy into chemical energy in the form of ATP and NADPH, which are then used in the Calvin cycle for carbon fixation.