Final answer:
ATP synthase uses a proton gradient to restore ADP to ATP in a process known as chemiosmosis, which takes place during oxidative phosphorylation.
Step-by-step explanation:
The process by which ATP synthase uses a proton gradient to restore ADP back to ATP is called chemiosmosis. This occurs during oxidative phosphorylation, a key part of cellular respiration where the energy stored in the electrochemical gradient, developed due to the movement of protons (H+), is utilized. As protons flow through ATP synthase, much like water flowing through a turbine, they provide the energy needed for the enzyme to catalyze the production of ATP from ADP and inorganic phosphate (Pi).