Photosynthesis overview: In which part of photosynthesis is carbon dioxide actually reduced to carbohydrate equivalents?

Introduction / Context:
Photosynthesis has two broad phases: light reactions that capture solar energy to form ATP and NADPH, and the Calvin cycle that uses those energy carriers to reduce carbon dioxide into organic molecules. This question asks where the actual chemical reduction of CO2 occurs.

Given Data / Assumptions:

• Light reactions generate ATP and NADPH in the thylakoid membranes.
• The Calvin cycle operates in the stroma, fixing and reducing CO2.
• Reduction means addition of electrons and protons to carbon to form more reduced carbon compounds.

Concept / Approach:
The Calvin cycle comprises carboxylation, reduction, and regeneration phases. CO2 is first fixed to ribulose 1,5-bisphosphate, then reduced at the 1,3-bisphosphoglycerate → glyceraldehyde-3-phosphate step, using NADPH and ATP supplied by the light reactions. Thus, reduction of CO2 is a hallmark of the Calvin cycle, not of the photochemical charge-separation events themselves.

Step-by-Step Solution:

Verification / Alternative check:
Isotopic labeling (14CO2) and stoichiometric measurements show that electron equivalents from NADPH are consumed during the reduction phase in the stroma, confirming the Calvin cycle as the site of CO2 reduction.

Why Other Options Are Wrong:

• Light reactions and noncyclic photophosphorylation produce ATP/NADPH but do not reduce CO2.
• Photorespiration consumes oxygen and releases CO2; it is not a CO2-reducing pathway.
• There is no single step where light and dark reactions reduce CO2 together; they are temporally and spatially linked but distinct.

Common Pitfalls:
Equating energy generation (light reactions) with carbon reduction; the former supplies energy, the latter uses it in the Calvin cycle.

Final Answer:
The Calvin cycle (dark reactions / light-independent reactions)