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noun, plural: photosystems
A multisubunit complex found mainly in the thylakoid membranes of plants and algae, and in the cytoplasmic membranes of photosynthetic bacteria. It is primarily involved in capturing light to cause a series of redox reactions.
The photosystem is where the light reactions take place. Two photosystems have been found in plants and algae: photosystem I and photosystem II. Each of them consists of chlorophyll molecules, accessory pigments, proteins and other molecules. Both of them are involved in the release and transfer of excited electrons to the electron transport chain. They differ in their reaction system chlorophylls (P700 and P680) and in distinct biochemical reactions which they are involved.
Word origin: ancient Greek: phos = light and systema = assembly
See also: light reactions

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