Thylakoid membrane contains a pigment (a molecule that absorbs light) called chlorophyll. The chlorophyll itself is found within protein complexes called Photosystems.

There are multiple kinds of chlorophyll pigments (chlorophyll a,b,etc.), but chlorophyll a is the most active within chloroplasts. There are also multiple kinds of Photosystems, Photosystem I and Photosystem II.

The two photosystems each have a different form of chlorophyll a: PSI is contains P700, and PSII contains P680, with the number representing the wavelength in nanometers that the molecule absorbs best.