Note that Sides 1 and 2 are no longer in osmotic equilibrium (i.e. the total osmolytes on each side are not the same)
In vivo, ion balance does equilibriate at the proportions that would be predicted by the Gibbs–Donnan model, because the cell cannot tolerate the attendant large influx of water. This is balanced by instating a functionally impermeant cation, Na, extracellularly to counter the anionic protein. Na does cross the membrane via leak channels (the permeability is approximately 1/10 that of K, the most permeant ion) but, as per the pump-leak model, it is extruded by the Na/K-ATPase.