The Gibbs-Donnan equilibrium is a phenomenon of solutions that contributes to the formation
of an electrical potential across a cell membrane.
The Gibbs-Donnan equilibrium can be illustrated using a beaker separated into two chambers
by a semi-permeable membrane.
A solution containing potassium chloride is added to one side I of the beaker and distilled
water is added to side II.
The membrane is completely permeable to both the K+ and Cl- ions.
Initially, the K+ and Cl- ions diffuse passively through the membrane, from their high concentrations
on side I, to their lower concentrations on side II.
The ions continue to diffuse back and forth between sides I and II until an equilibrium
is attained and equal concentrations of the K+ and the Cl- ions occur on both sides of
the membrane.
At equilibrium, equal numbers of both ions move back and forth from one side to the other
to maintain both equal concentrations of ions and electrical neutrality on each side of the membrane.
However, the system becomes more complicated if an ion is present on one side of the membrane that is not permeable to the membrane
In this illustration, Pr represents anionic protein – an impermeable anion common in cells
K+ and Cl– easily penetrate the membrane separating the two chambers.
The impermeable anion is retained on side I.
As we shall see in the next scene,
the presence of an impermeable ion on one side of a semi-permeable membrane
has profound effects on the final distribution of permeable ions
as they diffuse to an equilibrium in the solution.