Question

Why electron excite only on 680 and 700 nm during light reaction?

Answer 1

There is a special molecule called chlorophyll a P700, located in photosystem I, which absorbs light best at 700 nanometers (nm). It also contains other accessory pigments. Another special chlorophyll a molecule in photosystem II is called chlorophyll a P680 because it absorbs best at 680 nm.