When two solutions of different strengths are separated by a permeable membrane, their concentration on both sides of the membrane equalizes due to diffusion of the solutes as well as the solvent.
However, if there is an impermeable solute in one of the solutions, the concentration of the solutions do not equalize (Fig. 1.1). The final concentration of the solutions in such a situation is governed by the Gibbs Donnan equilibrium. One of the consequences of the Gibbs-Donnan equilibrium is that the concentration of the solution with impermeable solutes remains higher even at equilibrium.
Cells contain impermeable protein anions. Hence, its equilibrium with the extracellular fluid (ECF) is of the Gibbs-Donnan type. Thus, the intracellular fluid (ICF) always has a higher concentration of solutes than the ECF. This results in continuous osmosis of fluids into the cell, which threatens to burst the cell. To survive, the Na+-K+ pump in the cell membrane continuously pumps out excess cations.