A) RuBP (Ribulose Biphosphate)
B) PEP (Phosphoenol pyruvate)
C) Oxaloacetic acid
D) 3-PGA (3-Phosphoglyceric acid)
Correct Answer: B
Solution :
\[{{C}_{4}}-\]plants chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenol pyruvate (PEP), a reaction catalysed by an enzyme called PEP carboxylase and which creates the four-carbon organic acid, oxaloacetic acid. Oxaloacetic acid or malate synthesised by this process is then translocated to specialised bundle sheath cells where the enzyme, rubisco and other Calvin cycle enzymes are located, and where \[C{{O}_{2}}\] released by decarboxylation of the four-carbon acids is then fixed by rubisco activity to the three-carbon sugar 3-phosphoglyceric acid. The physical separation of rubisco from the oxygen-generating light reactions reduces photorespiration and increases \[C{{O}_{2}}\]fixation and thus, photosynthetic capacity of the leaf. Many important crop plants are \[{{C}_{4}}-\]plants, including maize, sorghum, sugarcane, and millets. Plants that do not use PEP-carboxylase in carbon fixation are called \[{{C}_{3}}-\]plants because the primay carboxylation reaction catalysed by rubisco, produces the three-carbon sugar 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Over 90% of plants use \[{{C}_{3}}-\]carbon fixation, compared to 3% that use \[{{C}_{4}}-\]carbon fixation.
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