The main difference between Photosystem I and Photosystem II is the absorption peak. According to HyperPhysics from Georgia State University, Photosystem I has an absorption peak of 700 nanometers, while Photosystem II has an absorption peak of 680 nanometers.
Photosystem I and II are major parts of the photosynthesis process because they are the two large proteins, or photosynthetic reaction centers, responsible for capturing the individual light photons used to produce sugar. Photosystem I is a complex of a dozen proteins that supports and positions over 100 cofactors. It uses antenna complexes, which are light harvesting pigments such as chlorophyll and beta carotene, to absorb the pigment surrounding the chlorophyll a molecules in the reaction center. The pigments absorb the photons and transfer the energy from molecule to molecule until it reaches the reaction center of Photosystem I. Once it reaches the reaction center, the energy is used to transfer an electron to an electron acceptor that is later used to power the Calvin cycle.
Photosystem II has a much smaller binding protein than Photosystem I, which has a molecular weight of 110,000. Photosystem II only has a molecular weight of about 47,000, according to HyperPhysics. Additionally, Photosystem II contains chlorophyll a and b molecules and xanthophylls but does not contain any beta carotene pigments like Photosystem I. Finally, whereas Photosystem I collects energy for the Calvin cycle, Photosystem II collects energy for the first stages of the non-cyclic electron transport cycle.