Question

Consider the following compounds.
a. Phosphoenol pyruvate
b. Malic acid
c. Pyruvic acid
d. Oxaloacetic acid
What is the correct sequence of these intermediate compounds as they form in $C_4$-dicarboxylic acid pathway following the PEP-carboxylation reaction?

• A. $1,2,3,4$
• B. $2,4,3,1$
• C. $4,1,2,3$
• D. $4,2,3,1$

Answer 1

The correct option is D $4,2,3,1$

The $C_4$ pathway is designed to efficiently fix $C{O}_2$ at low concentrations and plants that use this pathway are known as $C_4$ plants.

In $C_4$ plants, initial fixation of carbon dioxide occurs in mesophyll cells. The primary acceptor of $CO_2$ is phosphoenolpyruvate or PEP. It combines with carbon dioxide in the presence of PEP carboxylase or PEPcase to form oxaloacetic acid or oxaloacetate.

The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH.

The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce $CO_2$, a three-carbon compound called pyruvate, and NADPH. The $CO_2$ combines with ribulose bisphosphate and goes through the Calvin cycle while the pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the $C_4$ cycle.

So the correct sequence should be 4,2,3, 1.

Hence the correct answer is option D.