In the light-dependent reactions of photosynthesis, which molecule is the terminal hydride/electron acceptor that becomes reduced to a key reductant for the Calvin cycle?

Introduction / Context:
The light reactions produce two currencies: ATP and a powerful reductant for CO2 fixation. Identifying the correct electron acceptor at the end of the linear electron flow is essential for connecting light capture to carbon assimilation.

Given Data / Assumptions:

• Linear electron flow moves electrons from H2O (via PSII) to PSI and ultimately to a soluble carrier.
• The Calvin cycle requires NADPH for reducing 3-phosphoglycerate to G3P.
• Distinct cofactors (NADP+ vs NAD+) serve different pathways.

Concept / Approach:
At the end of the light reactions, ferredoxin–NADP+ reductase transfers electrons from reduced ferredoxin to NADP+, forming NADPH. Thus, NADP+ is the final hydride/electron acceptor in the chloroplast light reactions, not NAD+ or FAD derivatives that are more typical of mitochondrial metabolism.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical assays reveal light-dependent accumulation of NADPH in isolated chloroplasts; inhibitors of ferredoxin–NADP+ reductase block NADPH formation and carbon fixation.

Why Other Options Are Wrong:

• ADP: phosphate acceptor in ATP synthesis, not an electron acceptor.
• NAD+: primarily used in catabolic pathways (glycolysis/TCA) in the cytosol/mitochondria.
• FADH: nonstandard form; FAD/FADH2 participate in mitochondrial ETC, not chloroplasts.
• Ubiquinone: mitochondrial electron carrier; chloroplast uses plastoquinone instead.

Common Pitfalls:
Confusing NADP+ with NAD+ because of similar names; in chloroplasts, NADP+ is the principal terminal acceptor for the light reactions.

Final Answer:
NADP+.