H+ ions flowing down their gradient enter a half channel in a stator, which is anchored in the membrane. H+ ions enter binding sites within a rotor, changing the shape of each subunit so that the rotor spins within the membrane. Each H+ ion makes one complete turn before leaving the rotor and passing through a second half channel in the stator into the mitochondrial matrix. Spinning of the rotor causes an internal rod to spin as well. This rod extends like a stalk into the knob below it, which is held stationary by part of the stator. Turning of the rod activates catalytic sites in the knob that can produce ATP from ADP and P_i.
