What exactly is proton motive force?

Question

in regards to ATP synthesis

Answer 1

A proton-motive force is "energy that is generated by the transfer of protons or electrons across an energy-transducing membrane and can be used for chemical, osmotic or mechanical work."  It can be "generated by a variety of phenomena uncluding the operation of an electron transport chain, illumination of a purple membrane, and the hydrolysis of ATP by a proton ATPase.  "

The proton motive force .."can be described as the storing of energy as a combination of a proton and voltage gradient across a membrane. The chemical potential energy refers to the difference in concentration of the protons and the electrical potential energy as a consequence of the charge separation (when the protons move without a counter-ion).