Question

The Calvin cycle incorporates CO$_2$ molecule, one at a time by attaching it to ribulose bisphosphate. This step is catalyzed by which of the following:

• A. Rubisco
• B. PEP Carboxylase
• C. NADP reductase
• D. ATP synthase

Hint:

RuBisCO is the most abundant enzyme on Earth and is essential for the first step of carbon fixation in the Calvin cycle.

Answer 1

The Correct Option is A

Step 1: Concept
Carbon fixation in the Calvin cycle (C$_3$ pathway).

Step 2: Meaning
The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the first step of carbon fixation.

Step 3: Analysis
Rubisco (A): Catalyses addition of CO$_2$ to RuBP, forming two molecules of 3-phosphoglycerate --- correct.
PEP Carboxylase (B): Fixes CO$_2$ in C$_4$ plants (mesophyll cells), not in the Calvin cycle.
NADP reductase (C): Involved in light reactions, not carbon fixation.
ATP synthase (D): Produces ATP in light reactions, not carbon fixation.

Step 4: Conclusion
RuBisCO catalyses CO$_2$ fixation onto RuBP in the Calvin cycle. Final Answer: (A)