Free energy and transport. The free-energy change in transporting (A) an uncharged solute from a compartment at concentration c₁ to one at c₂ and (B) a singly charged species across a membrane to the side having the same charge as that of the transported ion. Notice that the free-energy change imposed by a membrane potential of 59 mV is equivalent to that imposed by a concentration ratio of 10 for a singly charged ion at 25°C.