The core of the water-oxidizing complex. (A) The deduced core structure of the water-oxidizing complex (WOC), including four manganese ions and one calcium ion, is shown. The valence states of the individual manganese ions are not indicated because of uncertainty about the charge on the individual ions. The center is oxidized, one electron at a time, until two H₂O molecules are oxidized to form a molecule of O₂, which is then released from the complex. (B) The absorption of photons by the reaction center generates a tyrosine radical (red arrows), which then extracts electrons from the manganese ions. The structures are designated S₀ to S₄ to indicate the number of electrons that have been removed.