The energy used to produce ATP in the light reactions of photosynthesis comes from __________.

• A. the direct absorption of light by ATP synthase
• B. the movement of protons (H⁺) across the thylakoid membrane (proton gradient)
• C. the breakdown of glucose molecules
• D. the flow of electrons down the electron transport chain

Answer: (B)

Verified step by step guidance

1. Understand that photosynthesis consists of two main stages: the light reactions and the Calvin cycle. The light reactions occur in the thylakoid membranes of chloroplasts and are responsible for producing ATP and NADPH.
2. Recognize that ATP production in the light reactions is driven by chemiosmosis, a process that relies on the movement of protons (H⁺) across the thylakoid membrane to generate a proton gradient.
3. Learn that the proton gradient is established by the electron transport chain (ETC), where electrons are passed through protein complexes embedded in the thylakoid membrane. This process pumps protons into the thylakoid lumen, creating a high concentration of H⁺ inside the lumen compared to the stroma.
4. Understand that ATP synthase, an enzyme embedded in the thylakoid membrane, uses the energy from the flow of protons (H⁺) down their concentration gradient (from the lumen to the stroma) to catalyze the conversion of ADP and inorganic phosphate (Pi) into ATP.
5. Note that the direct absorption of light by ATP synthase or the breakdown of glucose molecules are not involved in ATP production during the light reactions. Instead, the flow of electrons down the electron transport chain and the resulting proton gradient are the key processes driving ATP synthesis.