The photosystem that makes use of light to transfer electron particualrly from plastocyanin to ferredoxin, and whose reaction center chlorophyll is P700.
Photosystem I is named I because it is first discovered before photosystem II. In fact, in plants and algae where two photosystems exist, the flow of electron begins at photosystem II.
Compared with photosystem II, the photosystem I is more complex in structure. It has a more complex antenna system, has more subunits in its overall structure, and may exhibit cyclic or non-cyclic electron transfer from the excited P700 to the electron acceptor.
Abbreviation: PS I
Compare: photosystem II
See also: P700