Photo chemical phase of light reaction

Photo chemical phase of light reaction

In this phase electrons pass through electron carrier molecules and generate assimilatory powers ATP and NADPH + H+. Splitting of water molecule generates electrons replacing electrons produced by the light.

1. Photolysis of Water

The process of Photolysis is associated with Oxygen Evolving Complex (OEC) or water splitting complex in pigment system II and is catalysed by the presence of Mn⁺⁺ and Cl^-_.

When the pigment system II is active it receives light and the water molecule splits into OH^– ions and H⁺ ions. The OH^– ions unite to form water molecules again and release O₂  and electrons. Photolysis of water is due to strong oxidant which is yet unknown and designated as Z or Yz. Widely accepted theory proposed by Kok et al.,(1970) explaining photo-oxidation of water is water oxidising clock (or) S’ State Mechanism. It consists of a series of 5 states called as S₀, S₁, S₂, S₃ and S₄. Each state acquires positive charge by a photon (hv) and after the S4 state it acquires 4 positive charges, four electrons and evolution of oxygen. Two molecules of water go back to the S0. At the end of photolysis 4 H⁺, 4 e^- and O₂ are evolved from water (Figure 13.13).

2. Electron Transport Chain of Chloroplast

Electron transport chain in each photosystem involves four complexes:

•               Core Complex (CC): CC I in PS I the reaction centre is P700, CC II in PS II the reaction centre is P680

•               Light Harvesting Complex or Antenna complex (LHC):

•               Two types: LHC I in PS I and LHC II in PS II.

•               Cytochrome b6 f complex: It is the non-pigmented protein complex connecting PS I and PS II. Plastoquinone (PQ) and Plastocyanin (PC) are intermediate complexes acting as mobile or shuttle electron carriers of Electron Transport Chain.

PQ acts as shuttle between PS II and Cytochrome b₆- f complex and PC connects

•         Cytochrome b₆-f and PS I complex.

•         ATPase complex or Coupling factor: It is found in the surface of thylakoid membrane. This complex is made up of CF₁ and CF₀ factors. This complex utilizes energy from ETC and converts ADP and inorganic phosphate (P_i) into ATP (Figure 13.14).