An enzyme ribulose biphosphate carboxylase catalyses the dark reaction, pertaining to the addition of CO2, to ribulose-1-5-diphosphate. It is found in abundance in leaves and it is believed that it is the single most abundant protein on earth. It is clear that the first acceptor of CO2, is Ribulose 1-5 diphosphate. The first product formed after fixation of CO2, is 3-phosphoglyceric acid.