12 photosynthesis - part 04 - Light reaction

 

12 photosynthesis - part 04 - Light reaction


Light reaction :

  • In light reaction, solar energy is trapped by chlorophyll and stored in the form of chemical energy as ATP and in the form of reducing power as NADPH2. 
  • Oxygen is evolved in the light reaction by splitting of water.
  • When a photon is absorbed by chlorophyll molecule, an electron is boosted to higher energy level. 
  • To boost an electron, a photon must have a certain minimum quantity of energy called quantum energy. 
  • A molecule that has absorbed a photon is in energy rich excited state. 
  • When the light source is turned off, the high energy electrons return rapidly to their normal low energy orbitals as the excited molecule reverts to its original stable condition, called the ground state.

Reaction centre : 

  • The light absorbing pigments are located in the thylakoid membranes. 
  • They are arranged in clusters of chlorophyll and accessory pigments along with special types of chlorophyll molecules P680 and P700 (the letter P stands for Pigment and 680 and 700 for the wavelengths of light at which these molecules show maximum absorbance). 
  • P680 and P700 molecules form the Reaction centers or Photocenters.
  • The accessory pigments and other chlorophyll molecule harvest solar energy and pass it on the reaction centers. These are called Light harvesting or Antenna molecule.
  • They function to absorb light energy, which they transmit at a very high rate to the reaction center where the photochemical act occurs.



Photosystems I and II : 

  • The thylakoid membranes of chloroplasts have two kinds of photosystems, each with its own set of light harvesting chlorophyll and carotenoid molecules. 
  • Chlorophyll and accessory pigments help to capture light over larger area and pass it on to the photocenters. 
  • Thus, a photon absorbed anywhere in the harvesting zone of a P680 center can pass it energy to the P680 molecule. 
  • The cluster of pigment molecules which transfer their energy to P680 absorb at or below the wavelength 680nm. 
  • Together with P680 they form Photosystem-II or PS-II .
  • Likewise, P700 forms Photosystem-I or PS-I along with pigment molecule which absorbs light at or below 700nm.

Photosystem II : 


  • This system brings about photolysis of water and release of oxygen. 
  • In this act, when PS-II absorbs light, electrons are released and chlorophyll molecule is oxidized.
  • The electrons emitted by P680 (PS-II) are ultimately trapped by P700 (PS-I).
  • Oxygen is given out as byproduct by the photosynthesizing plants. 
  • Protons (H+) accumulate inside the thylakoid resulting in a Proton gradient.
  • The energy released by the protons when they defuse across the thylakoid membrane into the stroma against the H+ concentration gradient is used to produce ATP.

Photosystem I : 

  • When light quanta are absorbed by photosystem I (P700), energy rich electrons are emitted from the reaction center.
  • These flow down a chain of electron carriers to NADP along with the proton generated by splitting of water. 
  • This result in the formation of NADPH.
  • Hydrogen attached to NADPH is used for reduction of CO2 in dark reaction. This is also called Reducing power of the cell.



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