Where do ADP and NADP+ go after the dark stage?

• A. Back to the light stage
• B. They remain in the dark stage
• C. They are used to synthesize sugars
• D. They return to the light stage

Answer: (D)

The main idea is that the energy carriers created in the light-dependent stage have to be recycled to keep photosynthesis going. Light reactions use light to convert ADP into ATP and NADP+ into NADPH. The dark stage, or Calvin cycle, uses those ATP and NADPH to convert carbon dioxide into sugars. After they’ve been used, the leftover ADP and NADP+ are sent back to the light stage to be re-energized by light. If they stayed in the dark stage, there would be no way to restore the energy and reducing power needed for more sugar synthesis. So they return to the light stage to be recharged by light.