Which molecule donates electrons and protons to CO2 during the dark stage?

• A. NADP+
• B. Water
• C. NADPH
• D. Oxygen

Answer: (C)

NADPH provides the reducing power for the Calvin cycle. In the dark stage, CO2 is fixed and then reduced to sugars, and the electrons and protons needed for that reduction come from NADPH. This carrier is formed in the light reactions when NADP+ accepts electrons and H+; it donates those electrons and protons to the carbon compounds in the cycle, enabling the conversion of CO2 into triose phosphates. ATP supplies the energy, but NADPH is the actual electron and proton donor in this stage. Water only donates electrons during the light reactions (splitting to release O2), not during this dark-stage reduction.