This Demonstration shows the effects of ion concentration gradients across a membrane. We also consider the membrane permittivity for an ion acted upon by the membrane potential in a generic cell, as described by the Goldman–Hodgkin–Katz equation. Variation of the concentration of an ion (inside or outside the cell) with a higher membrane permittivity constant will mostly affect the membrane potential. For a membrane that is permeable to one ion only, permittivity constants in the numerator and denominator of the Goldman equation will cancel and the potential will depend only on the ratio of ion concentrations on the two sides of the membrane. In such instances, the Goldman equation reduces to the Nernst equation.