Final answer:
The electrochemical gradient for chemiosmosis in the light reactions of photosynthesis is established across the thylakoid membrane, and is essential for ATP production used in the Calvin cycle.
Step-by-step explanation:
The electrochemical gradient that drives chemiosmosis in the light reactions of photosynthesis is formed across the thylakoid membrane. The light-dependent reactions of photosynthesis utilize the energy from sunlight to transfer electrons from water to NADP+, producing NADPH. In the process, protons are moved across the thylakoid membrane into the thylakoid lumen, creating a high concentration of hydrogen ions there. This gradient is then used by ATP synthase to produce ATP, which is utilized in the Calvin cycle to convert CO₂ into organic compounds.