The Gibbs—Donnan effect also known as the Donnan's effect , Donnan law , Donnan equilibrium , or Gibbs—Donnan equilibrium is a name for the behaviour of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly across the two sides of the membrane. Because small cations are attracted, but are not bound to the proteins, small anions will cross capillary walls away from the anionic proteins more readily than small cations. Thus, some ionic species can pass through the barrier while others cannot. The solutions may be gels or colloids as well as solutions of electrolytes , and as such the phase boundary between gels, or a gel and a liquid, can also act as a selective barrier. The electric potential arising between two such solutions is called the Donnan potential.
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The Gibbs-Donnan effect is of course not really a mechanism of transport across cell membranes; rather, transport across cell membranes is the mechanism of Gibbs-Donnan effect; but such objections are pointlessly academic. Apparently, a large number of the exam candidates confused it with the electrochemical gradients which produce and maintain the resting membrane potential, which the examiners viewed as a minor disaster. If one is temperamentally unsuited to piracy, one can pay for these textbooks and find these references inside them.
Instead, the paper is an excellent, well written long-form explanation of the effect, probably better than anything subsequently published in glossy colour textbooks. Is it the Donnan effect, or is it the Gibbs-Donnan effect? Donnan never called his effect "the Donnan effect", but from onward it became known as such, and at this stage there was zero Gibbs in the public mentions of this concept.
W Gibbs was predominantly a physicist and mathematician who contributed massively to chemistry some decades before Donnan came along. S Adair , who found a Gibbsian equation from which was essentially identical to Donnan's equation.
Subsequent publications by Donnan eg. Donnan, are well-furnished with appropriate attributions, i. Donnan even went on to publish what appears to be a two-volume hagiography of Gibbs' scientific works.
So, whose effect is it? Because of some inherent lassitude on the part of the author, what follows is essentially a recapitulation of the original description Donnan gave for his own effect in , but with potassium substituted for sodium. Behold, these two compartments.
For the purposes of maintaining some attachment to college syllabus documents, let us label them "intracellular" and "extracellular". In these compartments, some ions are dissolved. The concentration of electrolytes in each compartment is equal, and electroneutrality of each compartment is maintained.
If one were that way inclined, one might be able to represent this equilibrium as an equation, where "int" means intracellular and "ext" means extracellular. Now, intracellular and extracellular concentrations of potassium remain the same and so the potassium is not inclined to diffuse anywhere , but now there is a concentration gradient for the chloride ions.
So, because the membrane is permeable to chloride ions and now there's a concentration gradient, some of the chloride ions diffuse into the intracellular compartment.
By necessity, they are accompanied by some potassium ions, so that electroneutrality is preserved. The chloride ions are also repelled by the negatively charged protein in the intracellular compartment, and so the bulk of the chloride remains on the extracellular side of the membrane. So; electroneutrality is preserved. Now, of course, because there is an electrical gradient as well as a chemical diffusion gradient acting on the ions, there will be a slightly unequal distribution of charge across the membrane, leading to a potential difference.
This is a familiar concept discussed at great lengths in the chapter on the resting membrane potential. In short, the Gibbs-Donnan effect sets up a transmembrane potential difference because the distribution of charged ions across the membrane is uneven. This potential difference is apparently quite small.
Sperelakis gives a value of mV, though it is not clear where that number comes from. Obviously, that does not happen in vivo. The terrible sodium permeability of the cell membrane means that the sodium generally keeps to the extracellular compartment, maintaining the osmolality there.
As a result, a second Donnan effect this time with the non-diffusible ions being extracellular sodium is established across the membrane, which maintains an osmotic counter-gradient for water movement. With ion pumps disabled, the cells increased in size markedly.
Apart from influencing the confusing ATP-pump-infested environment of the cell, the Gibbs-Donnan effect also influences other macroscopic environments, and through a detailed discussion of these matters falls outside the remit of this chapter, it would be amiss to completely ignore these applications of the concept. In short, wherever a membrane separates compartments and isolates a non-diffusible substance within one of them, we can find some application of the Gibbs-Donnan effect.
To maintain some vestiges of exam focus, these have been omitted from the discussion below. In short, again we are presented with two compartments, this time interstitial and intravascular. Let us fill these with physiologically plausible concentrations of electrolytes. All the ions are staying put. There are no forces shifting them around. Now lets add some anionic protein, as before. Now, there is an electrostatic force repelling chloride out of the intravascular compartment.
Consequently, more chloride collects in the interstitial fluid. The same force is attracting sodium back into the intravascular compartment. This competes with the concentration gradient.
One can almost imagine little ions sliding down them. The attractive force of anionic protein for sodium competes with the concentration gradient sucking it back into the interstitial compartment. At a certain concentration, some sort of equilibrium is reached. Of course, in reality this is not a true equilibrium. There is still unequal particle concentration on both sides of the membrane. Water is osmotically attracted into the vascular compartment.
The movement of water would then dilute the concentration of the ions, and there would be a change in their concentration gradients. So there is no stable steady state. There is movement of some ions out of the intravascular space, but at Gibbs-Donnan equilibrium there are still more particles in the vascular compartment, exerting an oncotic pressure.
The oncotic force sucking water into the capillaries is opposed by the capillary hydrostatic pressure, which is applied by the pumping action of the heart.
If this pressure becomes too great eg. Oedema ensues. The distribution of ions in the interstitial and intravascular compartments can be expressed in terms of a coefficient factor which describes the distribution of the ion in the interstitial fluid as a proportion of its concentration in the plasma.
This is generally referred to as the Gibbs-Donnan Factor. The value of this factor for monovalent cations is 0. For monovalent anions, its 1. Divalent cations like calcium are partially protein bound, and the Gibbs — Donnan effect only applies to the ionized forms.
For them, the factor is 0. Donnan, Frederick George. A contribution to physical-chemical physiology. Adair, G. Nguyen, Minhtri K. Masuda, Takashi, Geoffrey P. Dobson, and Richard L. Tosteson, D. Wilson, T. Russo, M. Van Rossum, and T.
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