出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/08/16 14:15:25」(JST)
The Gibbs–Donnan effect (also known as the Donnan's effect, Donnan law, Donnan equilibrium, or Gibbs–Donnan equilibrium) is a name for the behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly across the two sides of the membrane.[1] The usual cause is the presence of a different charged substance that is unable to pass through the membrane and thus creates an uneven electrical charge.[2] For example, the large anionic proteins in blood plasma are not permeable to capillary walls. 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.
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.
The effect is named after the American physicist Josiah Willard Gibbs and the British chemist Frederick G. Donnan.[3]
The Donnan equilibrium is prominent in the triphasic model for articular cartilage proposed by Mow and Lai, as well as in electrochemical fuel cells and dialysis.
The Donnan effect is extra osmotic pressure attributable to cations (Na+ and K+) attached to dissolved plasma proteins.
The presence of a charged impermeant ion (for example, a protein) on one side of a membrane will result in an asymmetric distribution of permeant charged ions. The Gibbs–Donnan equation at equilibrium states (assuming permeant ions are Na+ and Cl−):
[NaSide 1] × [ClSide 1] = [NaSide 2] × [ClSide 2]
Example:
| Start | Equilibrium | Osmolarity |
|---|---|---|
| Side 1: 9 Na, 9 Cl Side 2: 9 Na, 9 Protein |
Side 1: 6 Na, 6 Cl Side 2: 12 Na, 3 Cl, 9 Protein |
Side 1: 12
Side 2: 24 |
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 not 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 extracellularly to counter the anionic protein, Na+. 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.
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| リンク元 | 「ドナン膜平衡」「ギブズ-ドナン膜平衡」 |
| 関連記事 | 「equilibrium」「Donnan equilibrium」「Gibbs」 |