12 phtotosynthesis - part 03 - Mechanism of Photosynthesis

 

12 phtotosynthesis - part 03 - Mechanism of Photosynthesis


Mechanism of Photosynthesis :

  • In 1931, Van Neil proved that bacteriaused H2S and CO2 to synthesize carbohydrates as follows :
  • This led Van Neil to postulate that in green plants, water is utilized in place of H2S and O2 is evolved in place of sulphur. 
  • Ruben (in 1941) confirmed it in Chlorella. 
  • He used water labeled with heavy oxygen (O2 having atomic mass 18 ) i.e. H2O.
  • The oxygen evolved contain 18O2 thereby proving Van Neil’s hypothesis that oxygen evolved in photosynthesis comes from water. 
  • This leads to the currently accepted general equation of photosynthesis -

Hill reaction:
  • In 1937, R. Hill demonstrated that isolated chloroplasts evolved oxygen when they were illuminated in the presence of a suitable electron acceptor such as ferricyanide.
  • Ferricyanide is reduced to ferrocyanide by photolysis of water. This is called Hill reaction.
  • Thus Hill reaction proves that :
  1. In photosynthesis, oxygen is released from water.
  2. Electrons for the reduction of CO2 are obtained from water.

According to Arnon

  • In this process light energy is converted to chemical energy.
  • This energy is stored in ATP and NADPH is formed as hydrogen donor. This ATP formation is known as photophosphorylation.


In modern concept:
  • The process of photosynthesis is an oxidation and reduction process in which water is oxidized (to releaseO2) and CO2 is reduced to form sugar. 
  • It consists of two successive series of reactions.
  1. Light or Hill reaction - photochemical reaction
  2. Dark or Blackman reaction - biochemical reaction.

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