In photosynthesis, all the energy gained is handed over to special chlorophyll molecules called P680 and P700.These special chlorophyll molecules are called the photosystems or photo-centers or reaction centers.
There are two photosystems, one working at wavelength 680 nm and the other at about 700 nm. These two photosystems are called Photosystem II (PS II) and Photosystem I (PS I) respectively.
Photosystem I is present in stroma-thylakoids. Photosystem II is present in appressed part of granal thylakoids. PS II is more strongly fluorescent than PS I.
Both the photosystems work in harmony and bring about the light phase of photosynthesis.