Explain Calvin cycle.

• Calvin cycle explains about CO₂ fixation in photosynthesis. It is worked out by Calvin and his co-workers.
• It is present in both C3 plants and C4 plants.
• It occurs in 3 stages.
  • i) Carboxylation 
  • ii) Reduction
  • iii) Regenaration

i) Carboxylation: 6RUBP molecules accept 6CO₂ molecules and converting into 12 molecules of 3- PGA (Phosphoglyceric acid) in presence of RuBisco$6\mathrm{RUBP} + 6 {\mathrm{CO}}_2 + {\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{RUBISCO}}{\to }12, 3 - \mathrm{PGA}$

ii) Reduction: This occurs in two steps. 

• In first step 12PGA taking 12 ATP and converting 12 molecules of 1,3 bis PGA (Phosphoglyceric acid) in presence of phosphoglycerokinase.
  • 12 molecules of PGA + 12 ATP $\stackrel{\mathrm{Kinase}}{\to }$12 molecules of 1,3 bis PGA + 12 ADP
• In 2^nd step 12 mol of 1,3 bis phosphoglyceric acid is reduced into12 molecules of G3P.
  • 12PGA + 12NADPH + H⁺ $\stackrel{\mathrm{Dehydrogenase}}{\to }$12G - 3 - P + 12 NADP⁺ + 12 H₂PO₄^-

iii) Regeneration: 2 molecules of G-3p used to synthesise hexose in cytoplasam. So
10 G-3-P are used for RUBP regeneration.

• 4 molecules of - G - 3 - P $\stackrel{\mathrm{Isomerase}}{\to }$ 4 DHAP(dihydroxy acetone phosphate)
• 2 molecules of - G - 3 - P + 2 molecules of DHAP $\stackrel{\mathrm{Aldolase}}{\to }$2 Fructose,1,6 bis phosphate
• 2 mol of Fructose -1,6 - bis phosphate $\stackrel{\mathrm{Phosphatase}}{\to }$2 mol of Fructose 6 phosphate 2Pi
• 2 mol of Fructose - 6phosphate + 2 G - 3 - P $\stackrel{\mathrm{Transketolase}}{\to }$2mol of Xylulose - 5 - phosphate + 2 mol Erythrose - 4 - Phosphate.
• 2 E - 4 - p + 2 DHAP $\stackrel{\mathrm{Aldolase}}{\to }$2 mol of sedoheptulose - 1,7 - bisphosphate
• 2 mol of Sedoheptulose - 1,7 - bis phosphate $\stackrel{\mathrm{phosphatase}}{\to }$2 mol of Sedoheptulose - 7 - phosphate + 2 mol G - 3 - P $\stackrel{\mathrm{Transketolase}}{\to }$2 mol of Xyluose - 5 - P + 2 mol of Ribose - 5 - Phosphate
• 4 mol of xylusose - 5 - p $\stackrel{\mathrm{epimerase}}{\to }$4 mol of Ribulose - 5 - P
• 2mol of Ribose - 5 - p $\stackrel{\mathrm{isomerase}}{\to }$2 mol of Ribulose - 5 - P
• 6mol of Ribulose - 5 - p + 6 ATP $\stackrel{\mathrm{kinase}}{\to }$6 mol of Ribulose 1, - 5 - P + 6 ADP

Note : 6 times of cycle gives 1 glucose molecule. 18 ATP, 12 NADPH required for 6CO₂ molecule
fixation.