chlorophyll
Figure 8.15 A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the lightharvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In a photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In b photosystem I, the electron comes from the chloroplast electron transport chain discussed below.