(c) : 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 and the first product formed after fixation of CO2 is 3 -phosphoglyceric acid.