(i) Initial fixation of CO2 occurs in mesophyll cells.
(ii) The primary acceptor of CO2 is phosphoenol pyruvate or PEP. It combines with CO2 in the presence of PEP carboxylase to form oxaloacetic acid or oxaloacetate.
PEP + CO2 + H2O → Oxalo-acetic acid + H3PO4
(iii) Oxaloacetic acid is reduced to malic acid or transaminated to form aspartic acid.
Oxaloacetic acid + NADPH \(\overset{Dehydrogenase}{\rightarrow}\) Malic acid+ H3PO4
Oxaloacetic acid + Alanine \(\overset{Transaminase}{\rightarrow}\) Aspartic acid + Pyruvic acid.
