Photosystem I is the photosystem whose reaction center contains chlorophyll absorbing maximally near 700 nanometres. Its special reaction-center chlorophyll pair is called P700, so the 700-nanometre clue points directly to Photosystem I.
Photosystem I is embedded in the thylakoid membranes of plants, algae, and cyanobacteria. Antenna pigments absorb light and transfer excitation energy toward P700. When P700 becomes excited, it donates a high-energy electron to a chain of acceptors, ultimately helping reduce NADP⁺ to NADPH, an energy-rich electron carrier used in carbon fixation.
The number “I” does not mean that Photosystem I acts first in the ordinary flow of electrons. In oxygenic photosynthesis, Photosystem II functions earlier and has the P680 reaction center. Photosystem I then receives replacement electrons through the electron-transport chain. “Reaction center” and “antenna complex” are structural or functional parts, not the name of the photosystem itself. P700 refers to the reaction-center chlorophyll, while Photosystem I refers to the larger pigment-protein complex.