Two stages of photosynthesis and which require light?

• A. The Calvin cycle requires light; the light-dependent reactions do not.
• B. Light-dependent reactions require light; the Calvin cycle does not require light.
• C. Both stages require light.
• D. Neither stage requires light.

Answer: (B)

The key idea is that photosynthesis has two parts with different light needs. The light-dependent reactions use light directly: chlorophyll captures photons to energize electrons, splits water to release oxygen, and makes ATP and NADPH that power the next stage. The Calvin cycle then uses those ATP and NADPH to fix CO2 and synthesize sugars, but it does not require light itself—it relies on the energy carriers produced by the light-dependent reactions. So the stage that requires light is the light-dependent reactions, while the Calvin cycle does not require light directly.