[Molecular mechanisms activated during the emission of oxygen in photosynthesis]

Photosynthetic oxygen evolution results from the dehydrogenation of water molecules by the chlorophyll-center cation of photosystem II. The primary charge separation occurs between the chlorophyll center and the primary acceptor. The positive charges are then transferred to the manganese cluster able to accumulate four positive equivalents. Protons are liberated during this process. The oxygen evolution requires the presence of Ca2+ and Cl-. The oxidation of two water molecules, and the transfer of the electrons to the plastoquinone pool, are done by a supra-molecular complex which utilizes photons as substrate, and can therefore transfer electrons against a large redox potential difference.