A photosystem consists of one reaction center. The reaction center is located in the bilayer of the thylakoid membrane. Reaction centers contain a small number of proteins. The protein has chlorophyll-a on one side and an electron acceptor on the other. Electrons move from electron acceptors through the electron transport chain (ETC). Absorbed light energy can be converted into chemical energy only if the chlorophyll in the reaction center donates electrons to the electron acceptor.