Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses light energy to catalyze the transfer of electrons across the thylakoid membrane from plastocyanin to ferredoxin. Ultimately, the electrons that are transferred by Photosystem I are used to produce the moderate-energy hydrogen carrier NADPH. The photon energy absorbed by Photosystem I also produces a proton-motive force that is used to generate ATP. PSI is composed of more than 110 cofactors, significantly more than Photosystem II. History This photosystem is known as PSI because it was discovered before Photosystem II, although future experiments showed that Photosystem II is actually the first enzyme of the photosynthetic electron transport chain. Aspects of PSI were discovered in the 1950s, but the significance of these discoveries was not yet recognized at the time. 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 coherent theory of serial photosynthetic reactions. Hill and Bendall's hypothesis was later confirmed in experiments conducted in 1961 by the Duysens and Witt groups. Components and action Two main subunits of PSI, PsaA and PsaB, are closely related proteins involved in the binding of the vital electron transfer cofactors P, Acc, A, A, and F. PsaA and PsaB are both integral membrane proteins of 730 to 750 amino acids that contain 11 transmembrane segments. A [4Fe-4S] iron-sulfur cluster called F is coordinated by four cysteines; two cysteines are provided each by PsaA and PsaB. The two cysteines in each 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 F and F, also [4Fe-4S] iron-sulfur clusters, are located in a 9-kDa protein called PsaC that binds to the PsaA/PsaB core near F. Photon Photoexcitation of the pigment molecules in the antenna complex induces electron and energy transfer. Antenna complex The antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins. These pigment molecules transmit the resonance energy from photons when they become photoexcited. Antenna molecules can absorb all wavelengths of light within the visible spectrum. The number of these