Question

Calvin cycle represents the phenomenon of:

• A. Oxidative Carboxylation
• B. Substrate Level Phosphorylation
• C. Dark Respiration
• D. Reductive Carboxylation

Hint:

The Calvin cycle is also called C3 pathway since the first stable product is 3-carbon PGA.

Answer 1

The Correct Option is D

Calvin cycle fixes CO$_2$ through reductive carboxylation
Key steps:
Carboxylation: RuBP + CO$_2$ → 2 PGA
Reduction: PGA → G3P using NADPH and ATP
Regeneration: RuBP reformed
Process is reductive (consumes NADPH) and carboxylating

Answer 2

Alternate approach - eliminate the other named processes:
Oxidative Carboxylation would involve oxidation coupled with CO₂ fixation, which does not describe the Calvin cycle since the cycle consumes reducing power rather than releasing it.
Substrate Level Phosphorylation is the direct transfer of a phosphate group to ADP during glycolysis or the Krebs cycle, unrelated to CO₂ fixation.
Dark Respiration refers to mitochondrial respiration occurring without light, which breaks down sugars rather than building them.
The Calvin cycle instead uses ATP and NADPH generated by the light reactions to reduce and fix atmospheric CO₂ into carbohydrate - it both reduces carbon compounds and adds (carboxylates) CO₂ onto RuBP, which is precisely the definition of Reductive Carboxylation.