Splitting of water

(iii) Photolysis of water and replacement of ejected electron of P680. Breakdown of water molecules under the influence of light is known as photolysis.

The positively charged P680 molecule (after having given an electron) exerts a strong pull on the water molecule, splitting it into H+ and OH ions. This is called photolysis of water. It requires presence of OEC (oxygen evolving complex) enzyme, Cl and Mn++ ions as catalysts (Ca+2 ions control their concentrations).

4H2O 4(OH) + 4H+ ...(i)

4(OH) 4e + 4OH ...(ii)

The OH donate their electrons to oxidised P680 so that it once again becomes functional.

The (OH) radical obtained in equation (ii) above eventually forms water and liberates oxygen.

4(OH) 2H2O + O2 ...(iii)

Hydrogen ions (4H+) formed by the photolysis of water {equation (i)}, accumulate within the lumen of thylakoids.

Redox potential is the tendency of an atom or molecule having low redox potential to lose electrons (electron donors), while those with high redox potential to accept electron (gain electrons).

Hence, electrons move from substances having low redox potentials to those having high redox potentials.

Assimilatory power: NADPH + H+ and ATP produced in light reaction constitute assimilatory power, NADPH + H+ produced in light reaction is called as reducing power.