What protein complex splits water molecules in the light reactions?

The oxygen-evolving complex is the protein-associated complex that splits water molecules during the light reactions of photosynthesis. It is located on the lumen-facing side of Photosystem II, embedded at the interface between the photosynthetic membrane and the thylakoid lumen.

The complex contains a catalytic cluster of four manganese atoms and one calcium atom, often represented as a Mn₄CaO₅ cluster, along with oxygen atoms and associated cofactors. Powered by successive light-driven oxidations in Photosystem II, it removes electrons and protons from water. The overall reaction is 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons replace those lost by Photosystem II, the protons help build the gradient used to make ATP, and molecular oxygen is released as a by-product.

The OEC is also called the water-splitting complex. Photosystem II is the larger photosynthetic machine that contains it, so the two names should not be treated as interchangeable. ATP synthase uses the proton gradient to produce ATP, while cytochrome b6f transfers electrons and helps create that gradient; neither complex splits water.