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Q.
In which plants, Phosphoenol pyruvate (PEP) is the primary $CO_{2}$ acceptor ?
NTA AbhyasNTA Abhyas 2020
Solution:
This question is about the C4 cycle. The leaves of the C4 plants show "Kranz" (German term meaning halo or wreath) type of leaf anatomy. The vascular bundles, in C4 leaves, are surrounded by a layer of bundle sheath cells that contain a large number of chloroplasts. The chloroplasts in C4 leaves are dimorphic. The chloroplasts of bundle sheath cells are larger in size and arranged centripetally. They contain starch grains but lack grana. On the other hand, the mesophyll cells contain standard types of chloroplasts. The mesophyll cells perform the C4 cycle, and the cells of the bundle sheath performs C3 cycle. Mechanism of C4 Cycle: CO2 taken from the atmosphere is accepted by phosphoenolpyruvic acid (PEP) present in the chloroplasts of mesophyll cells of these leaves, leading to the formation of a 4-C compound, oxaloacetic acid (OAA), which, in turn, is converted to another 4-C acid, the malic acid which enters into the chloroplasts of bundle sheath cells, and there undergoes oxidative decarboxylation yielding pyruvic acid (a 3-C compound) and CO2. CO2 released in bundle sheath cells reacts with Ribulose-1,5-biphosphate (RuBP). This is already present in the chloroplasts of bundle sheath cells and thus Calvin cycle starts from here. Pyruvic acid then re-enters mesophyll cells and regenerates PEP (phosphoenol pyruvic acid). CO2 after reacting with RuBP gives rise to sugars and other carbohydrates. Mesophyll cells have PEP carboxylase and pyruvate orthophosphate dikinase enzyme, while the bundle sheath cells have decarboxylase, and complete enzymes of the Calvin cycle. In C4 plants, there are 2 carboxylation reactions, primarily in mesophyll chloroplasts followed by in-bundle sheath chloroplast.