The distribution of Na⁺ ions across a typical biological membrane is 10. mmol · L^–1 inside the cell and 140 mmol · L^–1 outside the cell. At equilibrium the concentrations would be equal, but in a living cell the ions are not at equilibrium. What is the difference in Gibbs free energy of Na⁺ ions across the membrane at 37°C (normal body temperature)? The concentration differential must be maintained by coupling to reactions that have at least that difference of Gibbs free energy.