Final answer:
Photosystem I has a reaction center with a chlorophyll a pair known as P700, which absorbs light at 700 nm and transfers the energy to excite an electron that is passed to the primary electron acceptor.
Step-by-step explanation:
Photosystem I contains a specialized pair of chlorophyll a molecules in its reaction center that absorbs light at 700 nm, known as P700. This particular wavelength is part of the light spectrum that is effective for photosynthesis, referred to as photosynthetically active radiation. When light is absorbed by chlorophyll in the light-harvesting complex, its energy is transferred to the chlorophyll a pair in the reaction center. This process excites an electron in the chlorophyll a pair, which is then passed to the primary electron acceptor. The excited electron must be replaced, and in the case of Photosystem I, this electron comes from the chloroplast electron transport chain.