12 photosynthesis - part 06 - Dark reaction

 

12 photosynthesis - part 06 - Dark reaction


Dark reaction :

  • Carbon fixation occurs in the stroma by a series of enzyme catalyzed steps. 
  • Molecules of ATP and NADPH produced in the thylakoids (light reaction) come in the stroma where carbohydrates are synthesized.
  • The path of carbon fixation in dark reaction through intermediate compounds leading to the formation of sugar and starch was worked out by Calvin, Benson and their co -workers. 
  • For this, Calvin was awarded Nobel Prize in 1961. 
  • Path of carbon was studied with the help of radioactive tracer technique using Chlorella, a unicellular green alga and radioactive 14CO2. 
  • With the help of radioactive carbon, it becomes possible to trace the intermediate steps of fixation of 14CO2. 
Various steps in the dark reactions / Calvin cycle / C-3 pathway are as follows:


  1. Carboxylation  
  2. Glycolytic Reversal
  3. Regeneration of RuBP


1. Carboxylation  

  • CO2 reduction starts with a 5-carbon sugar, ribulose-1,5-bisphosphate (RuBP). 
  • It is a 5-carbon sugar (pentose) with two phosphate groups attached to it.
  • RuBP reacts with CO2 to produce a short - lived 6-carbon intermediate in the presence of an enzyme RuBP carboxylase or Rubisco and immidiately splits into 3-carbon compound, 3-phosphoglyceric acid (3-PGA). 
  • Rubisco is a large protein molecule and comprises 16% of the chloroplast proteins.

2. Glycolytic Reversal :


  • Molecules of 3-PGA form 1,3-diphosphoglyceric acid utilizing ATP molecules. 
  • These are reduced to glyceraldehyde- 3-phosphate (3-PGAL) by NADPH supplied by the light reactions of photosynthesis.
  • For the Calvin cycle to run continuously, there must be sufficient amount of RuBP which accepts CO2 and a regular supply of ATP and NADPH. 
  • Out of each of 12 molecule of 3-phosphoglyceraldehyde (3-PGAL), 2 molecules are used for synthesis of one glucose molecule. 
  • Remaining 10 molecules are used for regeneration of 6 molecules of RuBP.

3. Regeneration of RuBP : 

  • Through a series of complex reactions, 10 molecules of 3-PGAL are used for regenration of six molecules of RuBP at the cost of 6 ATP. 
  • For this purpose, six turns of Calvin cycle are needed to be operated so that a molecule of glucose can be synthesized.
  • Plants form a variety of organic compounds required for its structure and function through these complex reactions.
  • Thus, for every 6 molecules of CO2 and Ribulose-1, 5-biphosphate used, 12 molecules of 3-phosphoglyceraldehyde are produced. 
  • Out of these 12 molecules, only two are utilized for the formation of a molecule of glucose.
  • The other 10 molecules are converted into ribulose-1, 5-biphosphate which combines with fresh CO2. 
  • Thus, the Calvin cycle regenerates ADP and NADP required for the light reaction.
1.  Light Reaction (in granum) :
2. Dark reaction (in stroma) :



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