Question

Which one of the following molecules captures \(CO_2\) in the \(CO_4\) cycle?

• A. 1,3-Bisphosphoglycerate.
• B. Oxaloacetate.
• C. Phosphoenolpyruvate.
• D. Ribulose-1,5-bisphosphate.

Hint:

In the \(C_4\) cycle, phosphoenolpyruvate (PEP) is the molecule that captures \(CO_2\), a step that is catalyzed by PEP carboxylase. This is an important adaptation in plants for efficiently fixing \(CO_2\) in hot climates.

Answer 1

The Correct Option is C

In the \(CO_4\) cycle, the molecule that captures \(CO_2\) is phosphoenolpyruvate (PEP). PEP, a three-carbon compound, reacts with \(CO_3\) in the presence of the enzyme PEP carboxylase to form oxaloacetate, a four-carbon compound. This reaction is the first step in the \(C_4\) pathway of photosynthesis, which helps in concentrating \(CO_2\) in plants, particularly in hot and arid climates. Therefore, statement (C) is correct.
- (A): 1,3-Bisphosphoglycerate is an intermediate in the Calvin cycle, but it does not capture \(CO_2\). Therefore, statement (A) is incorrect.
- (B): Oxaloacetate is a product in the \(CO_4\) cycle, but it does not capture \(CO_2\). It is formed after phosphoenolpyruvate captures \(CO_2\). Hence, statement (B) is incorrect.
- (D): Ribulose-1,5-bisphosphate is involved in the Calvin cycle, specifically in the fixation of \(CO_2\) in \(C_3\) plants, but not in the \(C_4\) cycle. Thus, statement (D) is incorrect.
Thus, the correct answer is (C).