electronic state. Absorption of blue light excites the chlorophyll to higher energy state than absorption of Red light, because the energy of photon is higher when their wavelength is shorter. When the pigment molecule is in an excited state, this excitation energy is utilised for the phosphorylation. Phosphorylation takes place with the help of light generated electron and hence it is known as photophosphorylation .
. . Photosystem and Reaction Centre • Thylakoid membrane contains Photosystem I (PS I) and Photosystem II (PS II). • PS I is in unstacked region of granum facing stroma of chloroplast.
• PS II is found in stacked region of thylakoid membrane facing lumen of thylakoid. • Each Photosystem consists of central core complex (CC) and light harvesting Complex (LHC) or Antenna molecules (Figure . ). • The core complex consists of respective reaction centre associated with proteins, electron donors and acceptors.
• PS I – CC I consists of reaction centre P700 and LHC I. • PS II – CC II consists of reaction centre P680 and LHC II (Table . ). • Light Harvesting Complex consists of several chlorophylls, carotenoids and xanthophyll molecules.
• The main function of LHC is to harvest light energy and transfer it to their respective reaction centre. . 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.
Figure . : Photosystem Photon T ransfer of energy Reaction center Chlorophyll Primary electron acceptor Electron transfer Antenna pigment molecules Reaction center Table . : Differences between Photosystem I and Photosystem II Photosystem I Photosystem II . The reaction centre is P700 .
Reaction centre is P . PS I is involved in both cyclic and non-cyclic. . PS II participates in Non-cyclic pathway .
Not involved in photolysis of water and evolution of oxygen , Photolysis of water and evolution of oxygen take place. . It receives electrons from PS II during non- cyclic photophosphorylation . It receives electrons by photolysis of water .