Figure 1.

Catalytic cycle of water oxidation, driven by solar light absorption. Each cycle is initiated by light absorption by the chlorophylls P₆₈₀, followed by a charge separation. On the donor side, the electron sequentially reduces the special pheophytin and the pair of quinones Q_A and Q_B. After accepting two electrons, Q_BH₂ is replaced by an oxidized quinone from a plastoquinone (PQ) pool. On the donor side, the hole oxidizes tyrosine Y_z, which in turn oxidizes the oxygen-evolving complex. In each turn of the cycle, the OEC of PSII evolves through “storage” states, S_n (n = 0–4), catalyzing water splitting, as follows: 2H₂O → O₂ + 4H⁺ + 4e^–. The water supply and proton release are likely happening through the water channels (labeled Large, Narrow and Broad) surrounding the OEC on the luminal side.