The Calvin cycle and light reactions relationship?

• A. The Calvin cycle uses ATP and NADPH from the light reactions to fix carbon dioxide into carbohydrate.
• B. The Calvin cycle generates ATP and NADPH for the light reactions.
• C. The light reactions fix carbon dioxide during the night.
• D. The light reactions produce glucose directly.

Answer: (A)

The energy and reducing power from the light-dependent reactions drive the Calvin cycle. The light reactions harvest light to produce ATP and NADPH, which the Calvin cycle uses to convert carbon dioxide into carbohydrate. ATP provides the energy for phosphorylation steps, while NADPH supplies electrons to reduce CO2 into glyceraldehyde-3-phosphate. A portion of that G3P goes on to form glucose and other sugars, and the rest regenerates the starting molecule to keep the cycle going.

This frame aligns with why the other ideas don’t fit: the Calvin cycle does not generate ATP or NADPH—those are consumed, not produced. The light reactions themselves do not fix CO2, they supply energy and electrons for the fixation process. And the light reactions do not produce glucose directly; glucose is assembled later from the carbohydrates produced by the Calvin cycle.