Donnan Equilibrium

Some ions inside a living cell, called fixed ions, become effectively trapped and never diffuse back out across the membrane once they are inside - yet their electrical charge still has to be balanced somehow by ions carrying the opposite charge. Where a pool of such fixed anions builds up inside the membrane, the cell has to draw in extra cations, over and above whatever amount ordinary ion exchange with the outside solution alone would require, purely to keep the overall charge balanced. The practical consequence is that the concentration of cations measured inside the cell ends up genuinely higher than the concentration measured in the external soil solution - a state of both electrical and diffusional balance across the membrane that is called the Donnan equilibrium, and it is one further reason why c …