RuBP: The True CO₂ Acceptor — NEET Biology

Correct answer: D — S1, S2, S3, and S4

This question evaluates understanding of the distinct characteristics and physiological differences between C₃ and C₄ plants, particularly concerning their anatomy, carbon fixation mechanisms, and photorespiration. Let's analyze each statement: S1: "C₄ plants exhibit Kranz anatomy, characterized by large bundle sheath cells having numerous chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces." This statement is correct. NCERT states, "The particularly large cells around the vascular bundles of the C4 plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy... they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces." (Page 145, Para 4). S2: "In C₃ plants, the primary CO₂ acceptor is RuBP, and the first stable fixation product is a 3-carbon organic acid, 3-PGA, formed in the mesophyll cells." This statement is correct. NCERT mentions, "the acceptor molecule was a 5-carbon ketose sugar – ribulose bisphosphate (RuBP)... The first product identified was 3-phosphoglyceric acid or in short PGA." (Page 143, Para 3,5). Also, "the Calvin pathway occurs in all the mesophyll cells of the C3 plants" (Page 147, Para 3). S3: "C₄ plants have a mechanism that increases the concentration of CO₂ at the RuBisCO site in bundle sheath cells, thereby minimizing its oxygenase activity and preventing photorespiration." This statement is correct. NCERT states, "In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site... this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity." (Page 147, Para 6). S4: "The initial carboxylation reaction in C₄ plants is catalyzed by PEP carboxylase in the mesophyll cells, an enzyme that completely lacks oxygenase activity." This statement is correct. NCERT states, "The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase." (Page 146, Para 3). PEPcase has no affinity for oxygen, thus lacking oxygenase activity, unlike RuBisCO. S5: "Under high light intensities, C₃ plants show saturation at about 360 µlL⁻¹ CO₂ concentration, whereas C₄ plants respond to increased CO₂ concentration and show saturation only beyond 450 µlL⁻¹." This statement is incorrect. The CO₂ saturation levels are reversed for C₃ and C₄ plants. NCERT states, "C₄ plants show saturation at about 360 µlL⁻¹ while C₃ responds to increased CO₂ concentration and saturation is seen only beyond 450 µlL⁻¹." (Page 150, Para 2). Therefore, statements S1, S2, S3, and S4 are correct, while S5 is incorrect. The correct option is D