Which statement best describes what a thylakoid does during the light reactions of photosynthesis?

• A. It houses the enzymes of the Calvin cycle that fix CO2 into sugars using ATP and NADPH.
• B. It breaks down water to release oxygen in the stroma, producing ATP by substrate-level phosphorylation.
• C. It is the site where glucose is directly synthesized from CO2 during the light reactions.
• D. It contains the photosystems and electron transport chain that use light energy to generate a proton gradient used to make ATP and to reduce NADP+ to NADPH.

Answer: (D)

Verified step by step guidance

1. Understand the role of the thylakoid membrane in the chloroplast during the light reactions of photosynthesis.
2. Recall that the light reactions involve capturing light energy to produce ATP and NADPH, which are energy carriers used later in the Calvin cycle.
3. Identify that the thylakoid membrane contains photosystems (protein complexes that absorb light) and an electron transport chain that transfers electrons through a series of carriers.
4. Recognize that as electrons move through the electron transport chain, protons (H+) are pumped into the thylakoid lumen, creating a proton gradient across the membrane.
5. Know that this proton gradient drives ATP synthase to produce ATP, and that NADP+ is reduced to NADPH, both of which are essential outputs of the light reactions occurring at the thylakoid.