Explanation

In non-cyclic photophosphorylation, evolutions of oxygen take place. It becomes possible due to photolysis of water molecules into $H^{+}$ and $OH^{-}$ ions. Later on $OH^{-}$ ion reassociate and form water and oxygen, while $H^{+}$ ions go to $NADP^{+}$ (a hydrogen acceptor). Each water molecule on photolysis yields one $H^{+}$ and $OH^{-}$ ion $H_{2}O\ \rightleftharpoons HO^{-}H^{+}$ $2OH + 2OH \rightleftharpoons 2H_{2}O + O_{2} \uparrow$ So, $12\ H_{2}O \rightleftharpoons 12OH^{-} + 12H^{+}$