As the electrons travel from Photosystem II down the first electron transport chain during the light reactions of photosynthesis, which of the following occurs?

• A. Oxygen is consumed as electrons are transferred.
• B. NADPH is directly produced at Photosystem II.
• C. Glucose is synthesized from carbon dioxide.
• D. ATP is produced via chemiosmosis as protons are pumped into the thylakoid lumen.

Answer: (D)

Verified step by step guidance

1. Understand the context: The problem is about the light reactions of photosynthesis, specifically the role of Photosystem II and the electron transport chain. Photosynthesis occurs in the chloroplast, and the light reactions take place in the thylakoid membrane.
2. Recall the function of Photosystem II: Photosystem II absorbs light energy, exciting electrons to a higher energy state. These high-energy electrons are passed down the first electron transport chain.
3. Explain the electron transport chain: As electrons move through the electron transport chain, their energy is used to pump protons (H⁺ ions) from the stroma into the thylakoid lumen, creating a proton gradient across the thylakoid membrane.
4. Introduce chemiosmosis: The proton gradient drives ATP synthesis via chemiosmosis. Protons flow back into the stroma through ATP synthase, a protein complex that uses this energy to convert ADP and inorganic phosphate (Pi) into ATP.
5. Clarify why the other options are incorrect: Oxygen is produced, not consumed, during the splitting of water at Photosystem II. NADPH is produced later in the light reactions at Photosystem I, not directly at Photosystem II. Glucose synthesis occurs during the Calvin cycle, not the light reactions.