Question

In C4 plants, CO2 combines with

• A. phosphoenol pyruvate
• B. phosphoglyceraldehyde
• C. phosphoglyceric acid
• D. ribulose diphosphate.

Answer 1

The Correct Option is A

To solve the question "In C4 plants, CO₂ combines with", we need to understand the process of carbon fixation in C4 plants.

C4 photosynthesis is a complex process that evolves to combat the inefficiency of the enzyme Rubisco in high-temperature and low-CO₂ environments. This process involves an additional step of carbon fixation in the mesophyll cells before entering the Calvin cycle in the bundle-sheath cells.

In C4 plants, the first step is the fixation of CO₂ into a 3-carbon compound phosphoenol pyruvate (PEP) to form a 4-carbon compound oxaloacetate. This reaction is catalyzed by the enzyme PEP carboxylase, which has a higher affinity for CO₂ and does not react with O₂. This adaptation allows C4 plants to efficiently fix carbon in hot, arid conditions.

Let's evaluate the options:

• Phosphoenol pyruvate (Correct): CO₂ combines with phosphoenol pyruvate in C4 plants to form oxaloacetate.
• Phosphoglyceraldehyde: This compound is part of the Calvin cycle and not directly involved in the initial fixation of CO₂ in C4 photosynthesis.
• Phosphoglyceric acid: This compound is also part of the Calvin cycle, formed after the reaction involving ribulose bisphosphate in C3 plants.
• Ribulose diphosphate: Known as ribulose bisphosphate, this is the compound that rubisco works with in C3 photosynthesis, not the initial acceptor molecule in C4 photosynthesis.

Hence, the correct answer is phosphoenol pyruvate.