Which of the following statements correctly describes the initial carbon fixation step in C3 plants during photosynthesis?

• A. C3 plants use CO\(_2\) to form a 3-carbon molecule as the first stable product.
• B. C3 plants initially fix CO\(_2\) at night to form malate.
• C. C3 plants use CO\(_2\) to form a 4-carbon molecule as the first stable product.
• D. C3 plants use phosphoenolpyruvate (PEP) carboxylase for the initial fixation of CO\(_2\).

Answer: (A)

Verified step by step guidance

1. Understand the process of carbon fixation in C3 plants: In C3 plants, the Calvin cycle is the primary pathway for carbon fixation. The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the reaction between CO\(_2\) and ribulose-1,5-bisphosphate (RuBP), a 5-carbon molecule.
2. Recognize the first stable product: The reaction between CO\(_2\) and RuBP produces an unstable 6-carbon intermediate, which immediately breaks down into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound. This is why these plants are called C3 plants.
3. Eliminate incorrect options: C3 plants do not fix CO\(_2\) at night to form malate. This is a characteristic of CAM plants, not C3 plants. Similarly, C3 plants do not produce a 4-carbon molecule as the first stable product; this is a feature of C4 plants.
4. Clarify the role of enzymes: C3 plants use RuBisCO for the initial fixation of CO\(_2\), not phosphoenolpyruvate (PEP) carboxylase. PEP carboxylase is used in C4 and CAM plants for initial CO\(_2\) fixation.
5. Conclude the correct statement: The correct description of the initial carbon fixation step in C3 plants is that they use CO\(_2\) to form a 3-carbon molecule (3-PGA) as the first stable product.