15 - Gibbs-Donnan Equilibrium Potentials

Abstract

This chapter discusses Gibbs-Donnan (G-D) potential across the cell membrane. It is mentioned that intracellular cytoplasm contains many colloids, including large nondiffusible polyvalent electrolytes, a Donnan equilibrium can be established across the cell membrane with an accompanying transmembrane Gibbs-Donnan (G-D) potential. In a Donnan equilibrium, all diffusible ions are in equilibrium across the membrane.. The chapter discusses mechanism for development of the Gibbs-Donnan potential, Gibbs-Donnan equilibrium, quantitation of the Gibbs-Donnan potential, and osmotic considerations. The extra osmotic pressure inside a cell produced by the presence of the negatively charged proteins and other impermeant large charged molecules is known as the colloid osmotic pressure (COP). The G-D equilibrium occurs because of the large impermeant charged macromolecules, such as proteins, inside the cell. Gibbs-Donnan potential is passive; that is, energy is not necessary for its establishment. In contrast, the resting potential is actively generated (indirectly or directly) by the action of the Na + -K + pump. The resting potential of most cells in the body, including nerve and muscle cells, however, is not because of a Donnan equilibrium, and the normal resting potential is not a Gibbs-Donnan potential. The differences between both these are outlined in the chapter. In the G-D equilibrium, the osmolarity of the cell becomes higher than the interstitial fluid bathing the cell, and so the cell tends to gain water and swell. When equilibrium is established, the product of the concentrations of the permeant ions inside the cell is equal to that outside the cell. The concentrations of the anions and cations outside the cell must be equal (law of electroneutrality). The gains in cations and anions inside the cell also must be equal to each other. From these required conditions, the equation can be solved algebraically to give the final concentrations at equilibrium, and from this, the calculated potential difference across the membrane.