Ultimately, the electrons that are transferred by Photosystem I are used to produce the moderate-energy hydrogen carrier NADPH.

[2] The photon energy absorbed by Photosystem I also produces a proton-motive force that is used to generate ATP.

[4] Hill and Bendall's hypothesis was later confirmed in experiments conducted in 1961 by the Duysens and Witt groups.

[4] Two main subunits of PSI, PsaA and PsaB, are closely related proteins involved in the binding of the vital electron transfer cofactors P700, Acc, A0, A1, and Fx.

PsaA and PsaB are both integral membrane proteins of 730 to 750 amino acids that contain 11 transmembrane segments.

The terminal electron acceptors FA and FB, also [4Fe-4S] iron-sulfur clusters, are located in a 9-kDa protein called PsaC that binds to the PsaA/PsaB core near FX.

[12][3] Located within the antenna complex of PSI are molecules of chlorophyll called P700 reaction centers.

[13] The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm.

[19] The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme ferredoxin–NADP+ reductase.

[22] Molecular data show that PSI likely evolved from the photosystems of green sulfur bacteria.

The photosystems of green sulfur bacteria and those of cyanobacteria, algae, and higher plants are not the same, but there are many analogous functions and similar structures.

[23] The photosystem of green sulfur bacteria even contains all of the same cofactors of the electron transport chain in PSI.