I've noticed several contributors on here implicating that ATP synthase could operate outside on the mitochondria. Under Peter Mitchels chemosomotic theory it is possible that the membrane potential created by calcium ion concentrations in endoplasmic reticulums could drive hydrogen ions down ATP synthase if present in the ERs membrane. I've spent sometime thinking how this could be demonstrated, unfortunately I have fallen short as the majority of assays i'm aware of use false substrates for the citric acid cycle to demonstrate ATP synthesis..