Question

Which enzyme is responsible for primary CO2 fixation in C4 plants?

• A. RuBisCO
• B. PEP carboxylase (PEP)
• C. Carbonic anhydrase
• D. Pyruvate kinase

Hint:

$C_{4}$ plants use PEP Case first; $C_{3}$ plants only use RuBisCO.

Answer 1

The Correct Option is B

Step 1: Concept
$C_{4}$ plants have evolved a mechanism to concentrate $CO_{2}$ around RuBisCO to minimize photorespiration.

Step 2: Meaning
Primary fixation refers to the first step where atmospheric $CO_{2}$ is captured into an organic molecule.

Step 3: Analysis
In $C_{4}$ plants, the initial $CO_{2}$ fixation occurs in the mesophyll cells. The enzyme Phosphoenolpyruvate (PEP) carboxylase catalyzes the reaction between $CO_{2}$ (as $HCO_{3}^{-}$) and PEP to form a 4-carbon acid. RuBisCO is involved later in the bundle sheath cells during the secondary fixation.

Step 4: Conclusion
Therefore, PEP carboxylase is the specific enzyme for the primary fixation step in the $C_{4}$ pathway. Final Answer: (B)