The Calvin cycle occurs in the stroma of chloroplast of C3 plants and consists of three main parts i.e., carboxylation, reduction and regeneration. Carboxylation involves addition of CO2 to Ribulose 1−5 bisphosphate in presence of Rubisco enzyme to form 3 PGA (3 phosphoglyceric acid) i.e., single carboxylation occur in Calvin cycle. In C4 plants two carboxylation occurs, the first in mesophyll cell and the second in bundle sheath cells.