In the light reactions of photosynthesis, what is the primary role of sunlight?

• A. It excites electrons in chlorophyll (photosystems), driving electron transport and the production of ATP and NADPH.
• B. It is used to split glucose into pyruvate to generate energy for the chloroplast.
• C. It provides carbon atoms that are incorporated into glucose during the Calvin cycle.
• D. It directly converts ADP into ATP by acting as an enzyme cofactor in the stroma.

Answer: (A)

Verified step by step guidance

1. Understand that the light reactions of photosynthesis occur in the thylakoid membranes of chloroplasts and require sunlight as an energy source.
2. Recognize that sunlight provides energy to excite electrons in chlorophyll molecules located in photosystems (Photosystem I and II).
3. Know that the excited electrons are transferred through an electron transport chain, which helps generate a proton gradient across the thylakoid membrane.
4. This proton gradient drives ATP synthase to produce ATP, and electrons reduce NADP+ to form NADPH, both of which are energy carriers used in the Calvin cycle.
5. Conclude that the primary role of sunlight in the light reactions is to excite electrons in chlorophyll, initiating the electron transport process that leads to ATP and NADPH production.