 This process involves storing energy as a proton gradient, also known as the proton motive force (PMF), across the mitochondrial inner membrane.This energy is used to synthesize ATP when the protons flow across the membrane (down their concentration gradient) through the ATP synthase enzyme; this is known as chemiosmosis.In endotherms, body heat is maintained by signaling the mitochondria to allow protons to run back along the gradient without producing ATP (proton leak).