Structural model of the OEC and its surroundings

Oxygenic photosynthesis, which provides the biosphere with most of its chemical energy, uses water as its source of electrons. Water is photochemically oxidized by the protein complex photosystem II (PSII), which is found, along with other proteins of the photosynthetic light reactions, in the thylakoid membranes of cyanobacteria and of green plant chloroplasts. Water splitting is catalyzed by the oxygenevolving complex (OEC) of PSII, producing dioxygen gas, protons and electrons. O2 is released into the atmosphere, sustaining all aerobic life on earth; product protons are released into the thylakoid lumen, augmenting a proton concentration gradient across the membrane; and photoenergized electrons pass to the rest of the electron-transfer pathway. The OEC contains four manganese ions, one