The electron transport chain, which is embedded in the mitochondrial membrane, exists primarily to generate new molecules of ATP for use by the cell. This is accomplished by a positive gradient of H+ ions that are formed outside the membrane which then pass back through a specialized channel known as ATP synthase. The energy created from this phosphorylates an ATP to an ATP, known as oxidative phosphorylation. The mechanism by which H+ returns to the inside of the mitochondria is known as what?