The purpose that water serves in the photosynthetic process is as a way to provide the necessary materials (In photosynthesis water is photo-chemically split into oxygen ,hydrogen ions and electrons in the light reation. The electrons formed are used to drive further processes and the oxygen act as the final electrons acceptor.) required for photosynthesis to continue. Photosystem II is the only known biological enzyme that carries out this oxidation of water. The oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four manganese ions.
