The first acceptor of carbon dioxide in $C_4$ plant is:
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Distinguish between the primary CO2 acceptor and the first stable product in $C_4$ photosynthesis:
- Primary Acceptor: Phosphoenolpyruvate (PEP) (3C)
- First Stable Product: Oxaloacetic Acid (OAA) (4C).
Step 1: Understanding the Concept:
In $C_4$ plants (such as maize, sorghum, and sugarcane), the carbon assimilation pathway is adapted to minimize photorespiration.
This adaptation involves a spatial separation of carbon dioxide capture (in mesophyll cells) and carbon dioxide reduction (in bundle sheath cells). Step 2: Detailed Explanation:
In the mesophyll cells of $C_4$ plants, atmospheric carbon dioxide is first converted into bicarbonate ions.
The initial molecule that reacts with and accepts this carbon dioxide is Phosphoenolpyruvate (PEP, written as Phosphenol pyruvic acid in the question), which is a 3-carbon compound.
This carboxylation reaction is catalyzed by the enzyme Phosphoenolpyruvate carboxylase (PEP carboxylase), which has a very high affinity for carbon dioxide and is not inhibited by oxygen.
The reaction produces Oxaloacetic acid (OAA), a 4-carbon compound, which is the first stable product of this pathway.
This OAA is subsequently reduced to malic acid or aspartic acid.
Therefore, the first acceptor of carbon dioxide is Phosphoenol pyruvic acid (PEP), while Oxaloacetic acid is the first stable product formed. Step 3: Final Answer
The correct option is (D).
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