Gibbs-Donnan potential

Q.23.1 What is the Gibbs-Donnan potential What would happen if the charge on the proteins were switched off Based on this, suggest a possible mechanism for a cell to modulate the Gibbs-Donnan potential. 

The Gibbs-Donnan potential occurs when a nonpermeant ion (for biological systems, usually a polyion such as a protein) is unequally distributed between the two electrolyte solutions separated by a permselective membrane, which allows certain electrolyte ions to move freely between the two solutions. The second law of thermodynamics and the principle of electroneutrality restrict this movement. The first restriction requires that each permeant ion species moves only down its electrochemical potential gradient, the latter requires the sum of all positive charges (cations) to be equal to that of all negative charges (anions) in each solution. When the system reaches its equilibrium, there is no flux of any permeant species i, = 0. 

The movement of solute across a semipermeable membrane depends upon the chemical concentration gradient and the electrical gradient. Movement occurs down the concentration gradient until a significant opposing electrical potential has developed. This prevents further movement of ions and the Gibbs-Donnan equilibrium is reached. This is electrochemical equilibrium and the potential difference across the cell is the equilibrium potential. It can be calculated using the Nemst equation. 

A related phenomenon occurs when the membrane in the above-mentioned experiment is permeable to the solvent and small ions but not to a macroion such as a polyelectrolyte or charged colloidal particles that may be present in a solution. The polyelectrolyte, prevented from moving to the other side, perturbs the concentration distributions of the small ions and gives rise to an ionic equilibrium (with attendant potential differences) that is different from what we would expect in the absence of the polyelectrolyte. The resulting equilibrium is known as the Donnan equilibrium (or, the Gibbs-Donnan equilibrium) and plays an important role in... 

Be able to compute the equilibrium state, osmotic pressure, and membrane potentials of proteins and other charged species (Gibbs-Donnan equilibrium) (Sec. 

The encapsulation of the impermeable solute and its counterions creates an ion gradient across the membrane, causing ions to flow out of the hypertonic volume if both cations and anions are permeable. This effect reduces the osmotic pressure exerted by the impermeable solute. The result is a transmembrane potential and an altered distribution of ions, known as the Gibbs-Donnan equilibrium [116,117]. For a permeable cation, the ratio of its interior to exterior concentration (r) is equal to... 

In the definitions of T, two variables in addition to the ion chemical potential must also be specified as constant. In an equilibrium dialysis experiment, these are temperature and the chemical potential of water. This partial derivative is known as the Donnan coefficient. (Note that the hydrostatic pressure is higher in the RNA-containing solution.) In making connections between T and the Gibbs free energy, it is more convenient if temperature... 

Membrane phenomena cover an extremely broad field. Membranes are organized structures especially designed to perform several specific functions. They act as a barrier in living organisms to separate two regions, and they must be able to control the transport of matter. Moreover, alteration in transmembrane potentials can have a profound effect on key physiological processes such as muscle contraction and neuronal activity. In 1875, Gibbs stated the thermodynamic relations that form the basis of membrane equilibria. The theory of ionic membrane equilibrium was developed later by Donnan (1911). From theoretical considerations, Donnan obtained an expression for the electric potential difference, commonly known as the membrane potential between two phases. 

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