C. Coupled reactions establish an electrochemical gradient across a membrane.
Explanation:
During cellular respiration in mitochondria, and during photosynthesis in chloroplasts, the electron transport chain requires a proton gradient to pump protons across the membrane by active transport.
Protons flow back across the membrane by facilitated diffusion through ATP synthase, which utilizes them to phosphorylate ADP to ATP.
This process of ATP synthesis by harnessing the elctrochemicaal gradient geenrated by the diffusion of protons across the biological membrane through ATP synthase is called chemiosmosis.
