What is the primary role of sunlight in the light reactions of photosynthesis?

• A. To directly convert carbon dioxide into glucose
• B. To transport protons across the thylakoid membrane
• C. To provide energy that excites electrons in chlorophyll molecules
• D. To supply water molecules for the Calvin cycle

Answer: (C)

Verified step by step guidance

1. Understand the context of photosynthesis: Photosynthesis occurs in two main stages, the light reactions and the Calvin cycle. The light reactions take place in the thylakoid membranes of chloroplasts and require sunlight.
2. Recognize the role of sunlight in the light reactions: Sunlight provides energy in the form of photons, which are absorbed by chlorophyll molecules in the photosystems (Photosystem II and Photosystem I).
3. Explain the excitation of electrons: When chlorophyll absorbs sunlight, its electrons become excited and move to a higher energy state. This is a critical step in initiating the electron transport chain.
4. Describe the electron transport chain: The excited electrons are transferred through a series of proteins embedded in the thylakoid membrane, leading to the generation of ATP and NADPH, which are used in the Calvin cycle.
5. Clarify why sunlight does not directly convert carbon dioxide into glucose or supply water for the Calvin cycle: These processes occur in the Calvin cycle, which uses ATP and NADPH produced during the light reactions to convert carbon dioxide into glucose. Water is split during the light reactions to provide electrons, not directly for the Calvin cycle.