12 photosynthesis - part 08 - C4 pathway

 


12 photosynthesis - part 08 - C4 pathway


C4 pathway or Hatch-Slack pathway :

  • M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compound (dicarboxylic acid) as the first stable product of photosynthesis. 
  • It has been found to occur in tropical and sub-tropical grasses and some dicotyledons. 
  • Some of the important plants are sugarcane, maize, Sorghum etc.
  • The plants in which CO2 fixation takes place by Calvin cycle are called C3 plants, because first product of CO2 fixation is a 3-carbon phosphoglyceric acid. 
  • But in Hatch- Slack pathway, first product of CO2 fixation is a 4-carbon compound, oxaloacetic acid. Hence such plants are called C4 plants.
  • Anatomy of leaves of C4 plants is different from leaves of C3 plants. 
  • C4 plants show Kranz anatomy. 
  • In the leaves of such plants, palisade tissue is absent. 
  • There is a bundle sheath around the vascular bundles.
  • The chloroplasts in the bundle - sheath cells are large and without or less developed grana.
  • The chloroplasts in the mesophyll cells the chloroplasts are small but with well-developed grana.
In mesophyll cell :
  • CO2 taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme pepco.
  • It is converted to another 4-C compound, the malic acid. 
  • It is transported to the chloroplasts of bundle sheath cells. 
In bundle sheath cell :
  • Here, malic acid (4-C) is converted to pyruvic acid (3-C) with the release of CO2 in the cytoplasm. 
  • Thus concentration of CO2 increases in the bundle sheath cells.
  • Chloroplasts of these cells contain enzymes of Calvin cycle. 
  • Because of high concentration of CO2, RuBP carboxylase participates in Calvin cycle and not photorespiration. 
  • Sugar formed in Calvin cycle is transported into the phloem.
In mesophyll cell again :
  • Pyruvic acid generated in the bundle sheath cells re-enters mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP.
  • Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. 
  • Thus C4 pathway needs 12 additional ATP. 
  • The C3 pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C4 pathway requires 30 ATP.
  • Thus C4 plants are better photosynthesizers and there is no photorespiration in these plants.




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