What protein complex splits water in photosystem II?

The oxygen-evolving complex splits water in photosystem II. Embedded on the lumen-facing side of photosystem II, this catalytic cluster uses light energy to remove electrons from water during the light-dependent reactions of photosynthesis.

Its active center contains four manganese atoms and one calcium atom, commonly described as a Mn₄CaO₅ cluster. As photosystem II absorbs light, the complex moves through a series of oxidation states known as S-states. After enough energy has accumulated, it extracts electrons and protons from water and releases molecular oxygen.

The oxygen-evolving complex is also called the water-splitting complex, so those names are interchangeable in a quiz context. A common mix-up is ATP synthase: ATP synthase uses the proton gradient produced by the light reactions to make ATP, but it does not split water. Cytochrome b6f transfers electrons and helps build that gradient, while ferredoxin-NADP+ reductase helps produce NADPH.

The oxygen released by this reaction comes from water molecules, not directly from carbon dioxide. This detail was established through isotope-tracing experiments and is central to understanding oxygenic photosynthesis.