Answer: because it was discovered first
Photosystem II occurs before Photosystem I
Photosystem I uses the high-energy electrons to reduce NADP+ to NADPH whereas photosystem II uses them to convert ADP into ATP. Because the reactions are not cyclic over time photosystem II would lose all its electrons. This does not happen because energy from photosystem II is used to split water molecules releasing electrons.
Photosystem I was discovered first. Later photosystem II was discovered and found to be earlier in the electron transport chain. But it was too late the name stuck. Electrons first travel through photosystem II and then photosystem I. The Electron Transport Chain. While at photosystem II and I the electrons gather energy from sunlight.
During noncyclic photophosphorylation water is oxidized and the electrons from water pass through photosystem II and photosystem I before reducing: A carbon dioxide. B NADP+. Each step in this pathway is a coupled oxidation-reduction reaction. Water is oxidized as a result of the light reaction of photosystem II. From photosystem II electrons pass to plastoquinone to cytochromes to plastocyanin.
Photosystem II was discovered later. But if we see their function photosystem II comes before photosystem I. Photosystem II is located in at the inner surface of grana of thylakoid. Photosystem II has a smaller binding protein as compare to photosystem I that has MW of 110 000. It absorbs light of about 680nm.
Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thy...