The reaction that is responsible for the primary fixation of CO2 is catalysed by


Carboxylation is the addition of CO2 to an acceptor. Photosynthetic carboxylation requires ribulose-1,5-bisphosphate or RuBP as acceptor of CO2 and RuBP-carboxylase oxygenase as enzyme in C3 plants. In C4 plants, initial fixation of CO2 occurs in mesophyll cells. The primary acceptor of CO2 is phosphoenol pyruvate which combines with carbon dioxide in presence of PEP carboxylase to form oxaloacetic acid.

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