Photosystems I and II are both involved in the light dependent stage of photosynthesis, ultimately allowing the transport of protons to form a potential energy gradient that can be exploited for the synthesis of ATP.
Photosystem II transports H+ across the membrane, through the splitting of H2O to O2 and H+.
Photosystem I oxidises plastocyanin to ferredoxin, by transferring electrons. This also leads to protons accumulating in the lumen, providing the driving force for ATP synthase.
Photosystem II and I also contain different reaction cofactors. Photosystem II contains: a manganese cluster, chlorophyll, pheophytin, and plastoquinone. Photosystem I contains: chlorophyll, phylloquinone, and an iron-sulfur cluster.
Due to the two photosystems being electrically linked, cytochrome b6f is used to transfer the electrons from reduced plastoquinone (plastoquinol) to ferredoxin. This ferredoxin is then able to interact with the iron-sulfur cluster of photosystem I.