In the light reactions of photosynthesis, the primary function of sunlight is to do which of the following?

• A. Provide carbon atoms used to build glucose during the Calvin cycle
• B. Directly convert $C_{O}2$ into carbohydrate without using electron carriers
• C. Excite electrons in chlorophyll, initiating electron transport that drives ATP and NADPH production
• D. Convert ATP into ADP to release energy for water splitting

Answer: (C)

Verified step by step guidance

1. Understand the role of light reactions in photosynthesis: Light reactions occur in the thylakoid membranes of chloroplasts and use sunlight to produce energy-rich molecules ATP and NADPH.
2. Recognize that sunlight provides energy to excite electrons in chlorophyll molecules. This excitation raises electrons to a higher energy state, which is essential for electron transport.
3. Identify that the excited electrons travel through an electron transport chain, which helps generate a proton gradient used to synthesize ATP and reduces NADP+ to NADPH.
4. Note that the ATP and NADPH produced in the light reactions are then used in the Calvin cycle to fix carbon dioxide into glucose, but sunlight itself does not provide carbon atoms or directly convert CO2 into carbohydrates.
5. Conclude that the primary function of sunlight in the light reactions is to excite electrons in chlorophyll, initiating the electron transport chain that drives the production of ATP and NADPH.