A photosystem is made up of many different proteins: the light harvesting complex is made up of antenna chlorophyll (around 150 individual proteins), two reaction centre ‘special pair’ chlorophyll, and a scaffold of cofactors. These cofactors vary between the two photosystems, with photosystem II containing a manganese cluster, chlorophyll a, pheophytin, and plastoquinone. Photosystem I contains phylloquinone, chlorophyll, and an iron-sulfur cluster.
The antenna complex exploits Forster resonance energy transfer (FRET) to transfer the excitation energy to the special-pair chlorophyll in the reaction centre.