Question

The first stable product of CO₂ fixation in sorghum is

• A. Phosphoglyceric acid
• B. Pyruvic acid
• C. Oxaloacetic acid
• D. Succinic acid

Answer 1

The Correct Option is C

The question pertains to the process of CO₂ fixation in sorghum, a plant that uses the C₄ carbon fixation pathway. Understanding the biochemical pathways involved in plant photosynthesis is crucial to answering this question correctly.

Key Concepts:

• C₃ Pathway: Also known as the Calvin cycle, it is not the primary pathway in sorghum.
• C₄ Pathway: In C₄ photosynthesis, like in sorghum, carbon dioxide is initially fixed into a four-carbon compound.
• Key Enzyme: Phosphoenolpyruvate carboxylase (PEP carboxylase) fixes CO₂ in the C₄ pathway.

In C₄ plants, the first stable product of CO₂ fixation is oxaloacetic acid, a four-carbon compound. This is different from C₃ plants where the first stable compound is 3-phosphoglyceric acid (a three-carbon compound).

Correct Answer: Oxaloacetic acid

Explanation of Options:

• Phosphoglyceric acid: It is the first stable product in the C₃ pathway, not applicable for C₄ plants like sorghum.
• Pyruvic acid: It is not directly involved in the primary fixation of CO₂ in either the C₃ or C₄ pathways.
• Oxaloacetic acid: Correct answer for C₄ plants, being the first stable product formed from the fixation of CO₂.
• Succinic acid: It is part of the Krebs cycle, and not involved in the initial CO₂ fixation process in photosynthesis.

As such, given the C₄ nature of photosynthesis in sorghum, the correct answer is "Oxaloacetic acid".