Question

Describe C₃ Pathway.

Answer 1

The cycle is divided into the following phases:

1. Carboxylation phase: RuBP (Ribulose-1,5-bisphosphate) accepts atmospheric CO₂ in the presence of the enzyme RuBP carboxylase (RuB is CO) and forms a 6-carbon unstable compound. This unstable compound soon splits into two molecules of a 3-carbon compound called PGA (phosphoglyceric acid) in the presence of the same enzyme.
   \[\ce{\underset{(5C)}{RuBP} + \underset{(1C)}{CO2} ->[Mg^{++}][RuBP Carboxylase] \underset{(6C)}{Unstable compound}}\]
   \[\ce{\underset{(6C)}{Unstable compound} + H2O ->[Mg^{++}][RuBP Carboxylase] \underset{(3C)}{2 molecules of 3-PGA}}\]
2. Reduction phase (utilization of assimilatory power):
   1. The phosphoglyceric acid molecules are first phosphorylated by using ATP to produce 1,3 di-phosphoglyceric acid.

   2. It is then reduced by using NADPH₂ to produce phosphoglyceraldehyde (PGAL) and inorganic phosphate is released.
      \[\ce{3PGA + ATP -> 1,3-diphosphoglyceric acid + ADP}\]
      \[\ce{1,3-diphosphoglyceric acid + NADPH2 -> 3PGAL + NADP + Pi}\]

   3. Some molecules of 3 PGAL are converted into its isomer dihydroxyacetone phosphate (DHAP) in the presence of the enzyme triose phosphate isomerase.
3. Synthesis of sugar and Regeneration phase:
   1. Synthesis of sugar:
      1. For the synthesis of one molecule of glucose, six molecules of RuBP and six molecules of CO₂ are required, i.e. six turns of the Calvin cycle are required for the synthesis of one molecule of glucose.
      2. Out of 12 molecules of PGAL, 2 molecules (i.e. 1/6th part) are used for the synthesis of glucose.
      3. A molecule of PGAL and DHAP combine together to form a molecule of fructose 1, 6- diphosphate as shown below:
         \[\ce{\underset{(3C)}{3 PGAL} + \underset{(3C)}{DHAP} -> \underset{(6C)}{Fructose-1,6-diphosphate}}\]
         [Note: Fructose-1,6-diphosphate is the first carbohydrate formed during this process]
      4. Fructose-1,6-diphosphate undergoes dephosphorylation to form fructose-6-phosphate which on isomerization forms glucose-6-phosphate.
      5. Glucose-6-phosphate undergoes dephosphorylation to form glucose. Glucose, thus formed is either utilized or stored as starch.
   2. Regeneration phase:
      1. RuBP gets regenerated through many biochemical reactions called sugar phosphate interconversions.
      2. All the intermediate compounds formed like erythrose-4-phosphate, xylulose-5-phosphate, ribose-5-phosphate, sedoheptulose-7-phosphate, etc. are sugar phosphates.
      3. Out of 12 molecules of PGAL, 10 molecules are utilized for the  regeneration of 6 molecules of RuMP (ribulose monophosphate) which on phosphorylation form RuBP as shown below:
         \[\ce{12 PGAL -> 6 RuMP}\] ....(1)
         \[\ce{6 RuMP + 6 ATP -> 6 RuBP}\] ...(2)
      4. Thus, RuBP, which is necessary for the reduction of CO₂, is regenerated to keep the process going.