Primary carboxylation occurs in ${\text{C}}_{\text{3}}$ and ${\text{C}}_4$ plants with the help of

Carboxylation in ${\text{C}}_{\text{3}}$plants takes place in mesophyll cells by RuBP carboxylase.

Carboxylation occurs twice in C₄ plants , primary carboxylation takes place in mesophyll cells by PEP carboxylase and secondary carboxylation takes place in bundle sheath cells by RuBP carboxylase.