1. Tardigrade
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  3. Biology
  4. Consider the following compounds: 1. Phosphoenolpyruvate 2. Malic acid 3. Pyruvic acid 4. Oxaloacetic acid Which is the correct sequence of these intermediate compounds as they form in C4 dicarboxylic acid pathway flowing the PEP- carboxylation reaction?

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Q. Consider the following compounds:
1. Phosphoenolpyruvate
2. Malic acid
3. Pyruvic acid
4. Oxaloacetic acid
Which is the correct sequence of these intermediate compounds as they form in $C_{4}$ dicarboxylic acid pathway flowing the $PEP-$ carboxylation reaction?

NTA AbhyasNTA Abhyas 2020

Solution:

↵ The C​​​​​​​​​​​4 pathway is also known as Hatch and Slack pathway, occurs in the plant that are adapted to dry tropical regions, e.g., Sugarcane, Maize. It is a cyclic process in which the primary CO​​​​​​​​2 acceptor is three-carbon molecule phosphoenolpyruvate which is present in mesophylls cells. PEP carboxylase or PEPcase is the enzyme that catalyzes the CO​​​​​​​​2 fixation. In the C​​​​​​​​​​​4 plants, the mesophyll cells lack the enzyme RuBisCO.
Solution
The 4-Carbon oxaloacetic acid is formed in the mesophyll cell, which is the first product of this pathway. In the mesophyll cells, the oxaloacetic acid is then converted to other 4-carbon compounds like malic acid or aspartic acid and then it is transported to the bundle sheath cells. To release CO​​​​​​​​2 and a 3-carbon molecule in the bundle sheath cells, these C​​​4 acids are broken down, and the CO​​​​​​​​2 released in the bundle sheath cells enter the Calvin pathway. The bundle sheath cells lack PEPcase enzyme but have RuBisCO enzyme. The three-carbon molecule is transported back to the mesophyll cell where with the help of a cold sensitive enzyme called PEP synthase, it is converted to PEP again and thus completing the cycle.