Site of NADP+ reduction in photosynthesis During which stage of photosynthesis is NADP+ reduced to NADPH?

Introduction / Context:
NADPH is the principal reducing agent used by the Calvin cycle to reduce carbon dioxide into triose phosphates. Identifying where NADP+ is reduced to NADPH clarifies the division of labor between the light reactions and carbon assimilation steps of photosynthesis.

Given Data / Assumptions:

• NADP+ accepts electrons at the end of the photosynthetic electron transport chain.
• The light reactions occur at the thylakoid membranes.
• The Calvin cycle occurs in the stroma and consumes ATP and NADPH.

Concept / Approach:

Electrons derived from water via PSII and PSI are ultimately used by ferredoxin NADP+ reductase to reduce NADP+ to NADPH. This is a light driven process because the electron flow depends on photoexcitation at the reaction centers. The Calvin cycle uses, rather than produces, NADPH to reduce 3 phosphoglycerate to glyceraldehyde 3 phosphate.

Step-by-Step Solution:

Verification / Alternative check:

Inhibitors of PSI or ferredoxin NADP+ reductase block NADPH formation, while Calvin cycle blockade increases NADPH levels, confirming that reduction occurs in the light reactions.

Why Other Options Are Wrong:

Photorespiration consumes O2 and releases CO2 but does not generate NADPH. The Calvin cycle uses NADPH; it does not reduce NADP+. The mitochondrial processes listed are unrelated to chloroplast NADPH production.

Common Pitfalls:

Assuming the Calvin cycle creates its own reducing power rather than consuming it.

Final Answer:

Light dependent reactions