Final answer:
H+ ions power ATP synthase as they move down their electrochemical gradient through chemiosmosis, enabling the synthesis of ATP.
Step-by-step explanation:
As H+ flow back across the inner mitochondrial membrane into the matrix, the central rotor unit rotates, powering ATP synthase. This complex protein acts much like a tiny turbine, utilizing the electrochemical gradient of H+ ions, which flow down their gradient from the intermembrane space to the matrix. During this process, known as chemiosmosis, the rotation of the ATP synthase enables the conversion of ADP and inorganic phosphate into ATP, a crucial energy-carrying molecule in cells.