The electrochemical gradient generated in the light reactions of photosynthesis is used primarily to:

• A. Split water molecules into oxygen and protons
• B. Drive the synthesis of ATP via ATP synthase
• C. Fix carbon dioxide into glucose
• D. Reduce NADP\(^+\) to NADPH directly

Answer: (B)

Verified step by step guidance

1. Understand the context: The light reactions of photosynthesis occur in the thylakoid membranes of chloroplasts. During these reactions, light energy is used to generate an electrochemical gradient of protons (H⁺) across the thylakoid membrane.
2. Recognize the role of the electrochemical gradient: The gradient is created by the movement of protons into the thylakoid lumen, driven by the electron transport chain (ETC). This creates a high concentration of protons inside the lumen compared to the stroma.
3. Identify the function of ATP synthase: ATP synthase is an enzyme embedded in the thylakoid membrane. It uses the energy stored in the proton gradient to catalyze the synthesis of ATP from ADP and inorganic phosphate (Pi). This process is called chemiosmosis.
4. Clarify why the other options are incorrect: Splitting water molecules (photolysis) into oxygen, protons, and electrons occurs earlier in the light reactions and is not directly driven by the electrochemical gradient. Fixing carbon dioxide into glucose occurs in the Calvin cycle, which is separate from the light reactions. Reducing NADP⁺ to NADPH is driven by electrons from the ETC, not the proton gradient.
5. Conclude: The primary purpose of the electrochemical gradient generated in the light reactions is to drive the synthesis of ATP via ATP synthase, which provides energy for subsequent processes like the Calvin cycle.