RuBisCO (Ribulose bisphosphate carboxylase-oxygenase) catalyses the most crucial step of the Calvin cycle: the carboxylation of Ribulose bisphosphate (RuBP) . Regarding the incorrect options: RuBisCO has a much greater affinity for CO2 than for O2 when the CO2:O2 ratio is nearly equal ; photolysis of water is associated with Photosystem II, not RuBisCO ; and the biosynthetic phase (Calvin cycle) is dependent on the products of light reactions (ATP and NADPH), meaning it does not strictly occur 'only in the dark' .