The neurotransmitter acetylcholine is made in the cytosol and then transported into synaptic vesicles, where its concentration is more than 100-fold higher than in the cytosol. When synaptic vesicles are isolated from neurons, they can take up additional acetylcholine added to the solution in which they are suspended, but only when ATP is present. Na+ ions are not required for the uptake, but, curiously, raising the pH of the solution in which the synaptic vesicles are suspended increasing the rate of uptake. 

Furthermore, transport is inhibited when drugs are added that make the membrane permeable to H⁺ ions. Suggest a mechanism that is consistent with all of these observations.