Photosystem I (PS I, or plastocyanin: ferredoxin oxidoreductase) is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I (PSI) is an integral membrane protein complex that uses light energy to produce the high energy carriers ATP and NADPH. More than 110 co-factors, significantly more than photosystem II, comprise the PS I system.

Photosystem I is named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known. Louis Duysens first proposed the concepts of photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a cohesive theory of serial photosynthetic reactions. Hill and Bendall's hypothesis was later justified in experiments conducted in 1961 by Duysens and Witt groups.

Two main subunits of PS I, PsaA and PsaB, are closely related proteins involved in the binding of P700, A0, A1, and Fx. PsaA and PsaB are both integral membrane proteins of 730 to 750 amino acids that seem to contain 11 transmembrane segments. The Fx 4Fe-4S iron-sulphur centre is bound by four cysteines; two of these cysteines are provided by the PsaA protein and the two others by PsaB. The two cysteines in both proteins are proximal and located in a loop between the ninth and tenth transmembrane segments. A leucine zipper motif seems to be present downstream of the cysteines and could contribute to dimerisation of psaA/psaB. The terminal electron acceptors, FA and FB, are located in a 9 kDa protein called PsaC.

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