Photosystem II (PSII) is a huge membrane-protein complex that catalyzes light-induced water-splitting, leading to the generation of protons and molecular oxygen. This reaction converts light-energy from the sun into chemical energy that is required to sustain almost all living activities on Earth. The water-splitting reaction is catalyzed by a Mn4CaO5-cluster embedded within the protein matrix of PSII, and proceeds through five intermediate states called Si-states. The structures of PSII and the Mn4CaO5-cluster have been resolved with atomic resolution, however, mechanisms governing water-splitting are unclear due to the lack of intermediate structures of the enzyme.