Photosystem I

Photosystem I (PSI, or water-plastoquinone oxidoreductase) is a large transmembrane protein complex that is embedded in the thylakoid membranes of chloroplasts. It is one of the two photosystems in the light-dependent reactions of oxygenic photosynthesis, the other being photosystem II. PSI uses light energy to do two things: firstly, it transfers electrons across the membrane from plastocyanin to ferredoxin; secondly, it pumps protons (H^+) across the membrane, generating a proton gradient that is used to generate ATP by ATP synthase. The overall reaction catalyzed by PSI is:
nadp^+ + h^+ + 2e^- -> nadph + h^+
This reaction is coupled to the following redox reaction:
2h^+ + 2e^- -> h_2 (this generates the proton gradient)
PSI also has an important role in non-oxygenic photosynthesis, where it acts as an electron donor to ferredoxin instead of plastocyanin. In this case, PSI donates electrons to ferredoxin which then reduces NADP^+ to NADPH. This type of photosynthesis occurs in green sulfur bacteria and cyanobacteria that lack chlorophyll a.
The structure of PSI consists of two main parts: an antenna complex and a core complex. The antenna complex contains pigment molecules that absorb sunlight and transfer its energy to the core complex. The core complex contains most of the proteins involved in electron transfer and proton pumping. It consists of three subunits: PsaA, PsaB, and PsaC. Each subunit has a different function:
PsaA – quinone acceptor
PsaB – iron-sulfur cluster acceptor
PsaC – primary electron donor
PSI also contains several small accessory proteins that are required for its function or stability. These include LipA (lipoyl domain protein), PscB (phycobilisome scaffold protein), PsbO (stabilizes protein complexes), PsbP (binds lipids), PsbQ (unknown function), and PsbR (chaperone).