Question

CO$_2$ Acceptor in C$_4$ Plants -

• A. PGA
• B. RuBP
• C. PEP
• D. PGAL

Hint:

Remember: C$_3$ plants use RuBP as the carbon dioxide acceptor, whereas C$_4$ plants use PEP as the primary carbon dioxide acceptor.

Answer 1

The Correct Option is C

Concept: Photosynthesis in C$_4$ plants differs from that in C$_3$ plants because the initial fixation of carbon dioxide occurs in mesophyll cells through a specialized pathway known as the Hatch-Slack pathway. This adaptation helps minimize photorespiration and increases photosynthetic efficiency under conditions of high temperature and intense sunlight. In C$_4$ plants, the first stable carbon dioxide acceptor is not RuBP as in the Calvin cycle. Instead, a three-carbon compound called phosphoenol pyruvate (PEP) accepts carbon dioxide to form a four-carbon compound.

Step 1: Understand the first carbon fixation reaction. In the mesophyll cells of C$_4$ plants, carbon dioxide combines with phosphoenol pyruvate (PEP) in the presence of the enzyme PEP carboxylase. \[ PEP + CO_2 \rightarrow OAA \] Thus, PEP acts as the primary acceptor of carbon dioxide.

Step 2: Analyze the given options. PGA is the first stable product of the Calvin cycle in C$_3$ plants and is not the primary CO$_2$ acceptor. RuBP acts as the carbon dioxide acceptor in the Calvin cycle but not during the initial fixation step of the C$_4$ pathway. PEP is the actual initial carbon dioxide acceptor in C$_4$ plants. PGAL is a three-carbon sugar produced during later stages of photosynthesis and does not function as a carbon dioxide acceptor.

Step 3: Draw the conclusion. Therefore, the primary carbon dioxide acceptor in C$_4$ plants is phosphoenol pyruvate. \[ \boxed{\text{PEP}} \] Hence, the correct answer is \[ \boxed{(C)\ \text{PEP}} \]