This animation explains the Gibbs-Donnan equilibrium. The Gibbs-Donnan equilibrium is a property of solutions and is the electrochemical basis for the unequal distribution of ions and electrical charges across a semi-permeable membrane. The Gibbs-Donnan equilibrium is a major contributor to the electrical charge across cellular membranes. The video describes solution equilibria and membrane differential permeability, and how the presence of an impermeable ion causes the unequal redistribution of permeable ions. The summary scene presents the Nernst equation for calculating the charge across a membrane based on ion distributions, and solves a sample calculation. The final scene explains the impact of the equilibrium at the membrane level of an intact cell.