Correct option is D
Explanation-
The Calvin-Benson cycle, also known as the C3 cycle, is the dark reaction of photosynthesis and occurs in the stroma of chloroplasts. It uses ATP and NADPH (produced during the light-dependent reactions) to fix CO₂ and synthesize carbohydrates.
The cycle has three main phases:
1. Carboxylation (No ATP used)
Enzyme: RuBisCO
Reaction:
CO₂ + RuBP (Ribulose-1,5-bisphosphate) → 2 molecules of 3-PGA (3-phosphoglycerate)
No ATP or NADPH is used in this step.
2. Reduction (ATP and NADPH used)
Each 3-PGA is converted into G3P (Glyceraldehyde-3-phosphate).
This requires 2 ATP per CO₂ and 2 NADPH per CO₂
ATP is used to phosphorylate 3-PGA, and NADPH reduces it.
3. Regeneration (ATP used)
Some G3P goes to make glucose, the rest regenerates RuBP. This complex step involves multiple reactions and consumes ATP.
Incorrect Options-
Option a: Only carboxylation → No ATP is used here.
Option b: Only regeneration → ATP is used, but it's also used in reduction.
Option c: Carboxylation and reduction → Carboxylation doesn't use ATP.
Final Answer: Option d – Reduction and regeneration
