CO₂ Fixation & RuBisCO Role — NEET Biology

Correct answer: B — S1, S2 and S4 are correct.

The question tests the understanding of the Calvin cycle, its energy requirements, enzyme localization in C4 plants, photorespiration, and dependence on light. S1: Carboxylation is the initial step of the Calvin cycle where CO₂ combines with Ribulose-1,5-bisphosphate (RuBP), a 5-carbon ketose sugar, catalyzed by the enzyme RuBisCO. This process occurs in the stroma of the chloroplast. Hence, S1 is correct. S2: The Calvin cycle requires energy in the form of ATP and reducing power in the form of NADPH. Specifically, for every molecule of CO₂ entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are utilized for reduction and regeneration of RuBP. Hence, S2 is correct. S3: In C4 plants, RuBisCO is located in the bundle sheath cells, not the mesophyll cells. The primary CO₂ fixation in mesophyll cells of C4 plants is catalyzed by PEP carboxylase (PEPcase). Hence, S3 is incorrect. S4: Photorespiration occurs when RuBisCO binds with O₂ instead of CO₂ due to its oxygenase activity. This leads to the formation of one molecule of 3-phosphoglycerate and one molecule of 2-carbon phosphoglycolate. Photorespiration is a wasteful process as it consumes ATP and releases CO₂ without synthesizing sugars or ATP. Hence, S4 is correct. S5: The biosynthetic phase (Calvin cycle) is not directly light-dependent. It depends on the products of the light reaction (ATP and NADPH). Although it needs these products which are formed in light, the enzymatic reactions of the Calvin cycle themselves are not directly driven by light. Hence, S5 is incorrect. Therefore, statements S1, S2, and S4 are correct. The core concept tested is the mechanism and requirements of the Calvin cycle, along with its variations and associated processes like photorespiration in different plant types.