photosystems 1

Photosystems protein complexes involved in photosynthesis. They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the chloroplasts), or in the cytoplasmic membrane of photosynthetic bacteria. A photosystem (or Reaction Center) is an enzyme which uses light to reduce molecules. This membrane protein complex is made of several subunits and contains numerous cofactors. In the photosynthetic membranes, reaction centers provide the driving force for the bioenergetic electron and proton transfer chain. When light is absorbed by a reaction center (either directly or passed by neighbouring pigment-antennae), a series of oxido-reduction reactions is initiated, leading to the reduction of a terminal acceptor. Two families of photosystems exist: type I reaction centers (like photosystem I (P700) in chloroplasts and in green-sulphur bacteria) and type II reaction centers (like photosystem II (P680) in chloroplasts and in non-sulphur purple bacteria). Each photosystem can be identified by the wavelength of light to which it is most reactive (700 and 680 nanometers, respectively for PSI and PSII in chloroplasts), and the type of terminal electron acceptor. Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. One has to note that both reaction center types are present in chloroplasts and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct.