Photosystem

Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems 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. There are two kinds of photosystems: II and I, respectively.

At the heart of a photosystem lies the reaction center, which is an enzyme that uses light to reduce molecules (provide with electrons). This reaction center is surrounded by light-harvesting complexes that enhance the absorption of light.

Two families of reaction centers in photosystems exist: type I reaction centers (such as photosystem I (P700) in chloroplasts and in green-sulphur bacteria) and type II reaction centers (such as 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), the amount and type of light-harvesting complexes present and the type of terminal electron acceptor used.

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