Final answer:
ATP is formed when hydrogen ions pass back across the membrane through ATP synthase channels.
Step-by-step explanation:
ATP is formed when hydrogen ions pass back across the membrane through ATP synthase channels. The process by which this occurs is known as chemiosmosis, where the ions move from an area of high concentration to one of low concentration across a semi-permeable membrane.
This gradient, or proton motive force, drives hydrogen ions through the ATP synthase, which functions as both a channel and an enzyme, to catalyze the conversion of ADP and inorganic phosphate into ATP, a process called oxidative phosphorylation.