What happens to ADP and NADP+ at the end of the dark stage of photosynthesis?

• A. They are degraded in the dark stage
• B. Both return to the light stage
• C. They are stored in the chloroplast
• D. They are excreted from the cell

Answer: (B)

In photosynthesis, the energy carriers ATP and NADPH are produced during the light-dependent reactions and then consumed in the dark (Calvin) stage to drive carbon fixation. By the end of the dark stage, these carriers have been used up and are in their spent forms: ADP and NADP+. They don’t stay unused in the chloroplast; instead, they head back to the light-dependent reactions to be reused—ADP to ATP and NADP+ to NADPH—so the cycle can continue when light is available. That recycling is why the best answer is that both return to the light stage.

Other possibilities don’t fit what happens in the cycle: the carriers aren’t degraded, stored, or excreted; they’re regenerated and reused.