Hello Abhishek,  

Calvin cycle occurs in all - C3 as well as C4 plants. There are three stages:

Carboxylation - It is the fixation of CO2 into an organic compound. Carboxylation is the most crucial step. CO2 is used for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBisCO. The product formed is Triose phosphate OR 3-PGA OR 3-Phosphoglycerate.

Reduction - It is a series of reactions that lead to the formation of a glucose molecule. In one turn of the cycle, 1 CO2 gets fixed. So to form C6H12O6, 6 turns of the cycle are required so that 6 CO2 get fixed, after which, the fixed 6 CO2 may be reduced to form 1 glucose molecule during reduction. But, for each turn of the cycle, reduction requires 2 molecules of ATP and 2 of NADPH.

Regeneration - The intermediate product formed after reduction if phosphorylised, will form RuBP. To let the cycle continue, this is important. And thus, for the phosphorylation to 1 RuBP, 1 ATP molecule is required.

Total:

Required ATP = 3 per CO2 = 3 per cycle = 18 for 6 turns of cycle

Required NADPH = 2 per CO2 = 2 per cycle = 12 for 6 turns of cycle

Hope it helped you!!

Thank you