Photosystem I

Similar to photosystem II, photosystem I also has the core domain called the P700 reaction center and a peripheral domain called the light harvesting complex. The electrons are transferred from PSII to PSI through a series of electron acceptor molecules, starting from an electron carrier– plastoquinone, followed by a cytochrome complex, and a mobile electron carrier– plastocyanin. Once the plastocyanin is reduced, it transfers an electron to the oxidized PSI reaction center. The reduced P700 then absorbs light and relays an excited electron to PSI's unique electron acceptor– ferredoxin. Finally, the electrons are accepted by ferredoxin-NADP+ reductase, ultimately forming the high energy product, NADPH. The transfer of electrons through the electron transport chain on the thylakoid membrane releases energy that is used to pump protons into the thylakoid space through the cytochrome complex, creating a proton gradient. ATP synthase, a specialized proton channel, utilizes this proton gradient to attach a third phosphate group to an ADP and produce ATP via chemiosmosis.