The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the active site of a water-oxidizing enzyme involved in the photo-oxidation of water during the light reactions of photosynthesis. OEC is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. The mechanism for splitting water involves absorption of three photons before the fourth provides sufficient energy for water oxidation. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S0 to S4, with S0 the most reduced and S4 the most oxidized. Energy from the photons captured by photosystem II moves the system from state S0 to S1 to S2 to S3 and finally to S4. S4 reacts with water producing free oxygen:2 H2O → O2 + 4 H+ + 4 e−