Question

The first acceptor of \(CO_2\) _______________ and first product of \(C_4\) plant is ________

• A. Oxaloacetic acid and phosphoenolpyruvate
• B. Malic acid and oxaloacetate
• C. Phosphoenolpyruvate and oxaloacetate
• D. Oxaloacetic acid and malate

Hint:

To remember this:
- \(C_3\) plants: First Acceptor = RuBP (5C), First Product = 3-PGA (3C).
- \(C_4\) plants: First Acceptor = PEP (3C), First Product = OAA (4C).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The \(C_4\) photosynthetic pathway (also known as the Hatch-Slack cycle) is an evolutionary adaptation that allows plants to fix carbon dioxide efficiently in hot, dry environments.
Unlike \(C_3\) plants, which use Rubisco for initial carbon fixation, \(C_4\) plants use a two-step process occurring in two different cell types (mesophyll and bundle sheath cells).

Step 2: Detailed Explanation:
Let us trace the initial carbon fixation steps in the mesophyll cells of a \(C_4\) plant:
Atmospheric carbon dioxide (\(CO_2\)) dissolves into the cytoplasm of mesophyll cells, where it is converted into bicarbonate ions (\(HCO_3^-\)).
The first acceptor of \(CO_2\) (in the form of \(HCO_3^-\)) is a three-carbon compound called phosphoenolpyruvate (PEP).
This carboxylation reaction is catalyzed by the highly efficient enzyme phosphoenolpyruvate carboxylase (PEP carboxylase):
\[ \text{PEP (3C)} + \text{HCO}_3^- \xrightarrow{\text{PEP Carboxylase}} \text{Oxaloacetate (4C)} + \text{P}_i \] The first stable product formed is oxaloacetate (OAA), also known as oxaloacetic acid, which is a four-carbon compound.
Because the first stable product is a four-carbon dicarboxylic acid, this pathway is referred to as the \(C_4\) pathway.
Analyzing the options:
- Option (C) correctly lists phosphoenolpyruvate as the first acceptor and oxaloacetate as the first product.

Step 3: Final Answer:
The correct option is (C).