Photosynthesis In Higher Plants

The dual nature of RuBisCO is a key idea in NCERT Class 11, Chapter 13 (Photosynthesis in Higher Plants). RuBisCO, the enzyme that fixes carbon in the Calvin cycle, has active sites that can accept either carbon dioxide or oxygen, and the outcome depends on their relative concentrations. When oxygen levels are high relative to carbon dioxide, RuBisCO catalyses the oxygenation of ribulose-1,5-bisphosphate instead of its carboxylation, channelling the substrate into the photorespiratory pathway. Photorespiration consumes energy and releases carbon dioxide without producing sugar or ATP, lowering net photosynthetic gain in C3 plants. Cyclic photophosphorylation is a light-reaction process generating ATP at photosystem I and is unrelated to RuBisCO's substrate choice. Chemiosmosis is the mechanism of ATP synthesis driven by a proton gradient and again does not involve RuBisCO binding oxygen. Reductive amination is a step in amino acid synthesis, not a consequence of RuBisCO's affinity. A plausibility check supports the answer: because RuBisCO competitively accepts oxygen, raising oxygen relative to carbon dioxide must trigger the wasteful oxygen-fixing route, which is photorespiration, and this is precisely why C4 plants, which concentrate carbon dioxide around RuBisCO, largely avoid the problem.