Gibbs free energy (G) is the thermodynamic potential indicating the amount of useful work produced in a thermodynamic system at constant conditions. The rate of change of the free energy (ΔG)of the system to the difference in the number of moles of individual particles (∆ν) is called chemical potential (µ):
µ = ΔG/∆ν.
Biological membranes represent systems that are characterized by the movement of ions and molecules through the membrane. This transport changes the concentrations of these substances on both sides of the membrane. In addition, this transport is associated with work and free energy changes. Finally, ions that move through the membrane possesses charges and generate the electrostatic field. In general, the characteristic of membranes that reflect the change of free energy during the transport of ions is called electrochemical membrane potential. As a result, the change of free energy (ΔG) during the transport of ions through the membrane that is associated with the change of the number and concentration (∆ν) of these ions reflects the membrane potential.