During dark reaction for fixation of carbon, the three carbon atoms of each molecule of 3 phosphoglyceric acid (PGA) are derived from
In carboxylation, ribulose 1,5 bisphosphate (RuBP) (= Ribulose diphosphate) acts as CO2 acceptor and 6 molecules of RuBP react with 6 molecules of CO2 and 6 molecules of water giving rise to 12 molecules of 3 phosphoglyceric acid.