Carbon Fixation: Calvin Cycle & C₄ Pathway — NEET Biology

Correct answer: A — S1, S3 and S4

This question assesses the understanding of photorespiration, the characteristics of RuBisCO, the C₄ pathway's role in minimizing photorespiration, and the purpose of cyclic photophosphorylation. Let's analyze each statement: S1: "RuBisCO is unique in that its active site has a much greater affinity for O₂ than for CO₂, leading to inefficient carbon fixation in C₃ plants." This statement is NOT correct. NCERT states, "RuBisCO has a much greater affinity for CO₂ when the CO₂ : O₂ is nearly equal." The binding is competitive, and its affinity for CO₂ is generally higher under normal conditions. Photorespiration occurs when O₂ concentration increases relative to CO₂ due to the competitive nature of binding to the active site. (Page 147, Para 4). S2: "In the photorespiratory pathway, RuBP binds with O₂ to form one molecule of phosphoglycerate and phosphoglycolate, which results in the release of CO₂ and utilization of ATP." This statement is correct. NCERT describes, "Here the RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate (2 Carbon) in a pathway called photorespiration. In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO2 with the utilisation of ATP." (Page 147, Para 5). S3: "The wasteful oxygenation activity of RuBisCO is completely avoided in C₄ plants due to the absence of RuBisCO enzyme in their mesophyll cells." This statement is NOT correct. While C₄ plants minimize photorespiration, RuBisCO is not absent; it is present in the bundle sheath cells. The minimization occurs because CO₂ is concentrated in the bundle sheath cells, where RuBisCO is present. The mesophyll cells lack RuBisCO, but the enzyme itself is crucial in the bundle sheath cells. NCERT states, "The mesophyll cells lack RuBisCO enzyme." and "The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase." (Page 146, Para 3; Page 147, Para 1). S4: "The biological function of photorespiration is currently understood to be an adaptive mechanism that provides an important advantage to C₃ plants in dry and hot conditions." This statement is NOT correct. NCERT explicitly states, "The biological function of photorespiration is not known yet." It is generally considered a wasteful process because it consumes ATP and releases CO₂ without synthesizing sugars or ATP/NADPH. (Page 147, Para 5). S5: "Cyclic photophosphorylation occurs to generate additional ATP required to meet the higher ATP to NADPH ratio needed for the Calvin cycle (3 ATP: 2 NADPH per CO₂ fixed)." This statement is correct. NCERT states, "Hence for every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place." (Page 145, Para 2). Therefore, statements S1, S3, and S4 are NOT correct. The correct option is A.