The Donnan phenomenon is not only limited to thermodynamic concepts in physical chemistry. It can be used to help us understand how living cells function. This post presents the significance of this phenomenon in every day molecular processes. The chemical aspect of the topic is captured in Chapter 7 of the Physical Chemistry book.
The Donnan equilibrium
The Gibbs-Donnan Equilibrium or Donnan Equilibrium is the basis for electrical charges that are found across the membranes of many cells (e.g. nerve and muscle cells).
It refers to the uneven distribution of charged particles on one side of a semipermeable membrane. These particles are not able to evenly distribute themselves by diffusion across both sides of the membrane.
How does the Donnan Equilibrium work?
When two solutions of differing concentrations are separated by a semipermeable membrane their concentrations will equalize as a result of diffusion.Diffusion occurs when substances move from areas of high concentration to low concentration down a concentration gradient.
However if there is an impermeable solute in one of the solutions, the concentration of the solution does not equalize. The concentration of the solution with impermeable solutes remains high even at equilibrium. This effect is called the Donnan equilibrium (refer to Fig. 7.25 above).
The Donnan Equilibrium in living cells
The Donnan effect can be correlated to living cells. Cell membranes are selectively permeable, which means that they allow some molecules to pass through while keeping others out.
The flow of molecules and ions between a cell and its environment is regulated by the Donnan effect. Living cells contain impermeable anionic colloids, which are mostly made up of proteins and organic phosphates; and these colloidal anions cannot cross the cell membrane.
As a result of this, there is a high concentration of non-diffusible anions across the cell membrane, thus creating the Donnan Equilibrium. This means that there are more ions inside the cell than outside. For ease of explanation let’s call this Donnan Equilibrium 1.
What does this do to cells? Water will continuously move into the cell by the process of osmosis. If this process continues, the cells will inevitably rupture.
What is the mechanism that prevents cells from swelling and rupturing?
The answer is the sodium pump (Na⁺- K⁺ ATPase) in the cell membrane. It is the most ubiquitous system in animal cells. The presence of the ATP-driven Na⁺ and K⁺ pump is nature’s way of preventing cells from rupturing by continuously pushing out excess ions.
The pump together with the membrane’s low permeability to sodium effectively prevents sodium from entering the cell. The sodium pump renders the membrane impermeable to sodium, setting up a second Donnan Equilibrium (let’s call this Donnan Equilibrium 2).
The earlier Donnan effect (Donnan Equilibrium 1) with respect to impermeable anionic colloids balances the latter Donnan effect (Donnan Equilibrium 2) of impermeable extracellular sodium. The balancing act between these two effects is by way of allowing cells to maintain and regulate normal cell volume in living functions.