Photolysis of Water

The PS-II reaction centre (P₆₈₀) by transferring electron to primary acceptor becomes oxidised.

The overall process is called photo-oxidation of water. It requires the presence of Mn⁺⁺, Ca⁺ and Cl^- , a water oxidising enzyme and an unknown substance Z. It is believed that oxygen evolves as oxygen gas. Electrons are accepted by PS-II reaction centre through unknown substance Z and H⁺ temporarily stay in the thylakoid space (loculus).The high energy electrons that leave PS-II are captured by Q which sends them to an electron transport system consisting of PQ, cytochrome complex, PC. Every time electron passes from donor to acceptor, the reduced donor id oxidised and the acceptor is reduced. The electrons of plastocyanin are picked by PS-I.

Simultaneously, the pigment molecules of PS-I complex absorb solar radiation and transfer their absorbed electronic excitation (energy) to PS-I reaction centre - P₇₀₀. P₇₀₀ gets excited and exudes an electron, which goes to reduce an electrons acceptor A. The oxidised reaction centre of PS-I takes electron from plastocyanin and comes to ground state.

The electron emitted from the P₇₀₀ is accepted by an unknown acceptor A which transfers its electron to ferredoxin an iron containing protein positioned at the outer surface of thylakoid membrane. The reduced ferredoxin donates its electrons to NADP⁺ (Nicotinamide Adenine Dinucleotide Phosphate). The NADP⁺ takes electrons from ferredoxin, protons from the medium and gets reduced to NADPH₂ in presence of enzymes, Ferredoxin - NADP - reductase.