RuBP: The True CO₂ Acceptor — NEET Biology
📚 Practice ConceptThe studies very unexpectedly showed that the acceptor molecule was a 5-carbon ketose sugar – ribulose bisphosphate (RuBP). Did any of you think of this possibility? Do not worry; the scientists also took a long time and conducted many experiments to reach this conclusion. They also believed that since the first product was a C₃ acid, the primary acceptor would be a 2-carbon compound; they spent many years trying to identify a 2-carbon compound before they discovered the 5-carbon RuBP.
Which of the following statements regarding C₃ and C₄ plants are correct? 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. 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. 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. S4: The initial carboxylation reaction in C₄ plants is catalyzed by PEP carboxylase in the mesophyll cells, an enzyme that completely lacks oxygenase activity. 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. Choose the correct option:
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