Photosystems I and II are the two systems of photosynthetic pigments in the thylakoid membranes of chloroplasts that are involved in the light-dependent reactions of photosynthesis. In photosystem II a chlorophyll a molecule, known as $P_{680}$, utilizes light of wavelength 680 nm; in photosystem I the chlorophyll a molecule, known as $P_{700}$, absorbs light at a wavelength of $700\, nm$