Gibbs-Donnan equilibria

The bicarbonate-carbonic acid buffer system plays a major role in regulating the pH of fluids in tissue spaces outside blood vessels. This fluid, commonly referred to as interstitial fluid and separated from plasma by the membrane barrier known as the capillary endothelium, primarily 

Magnetic resonance image (T1 weighted) of brain from a patient with multiple sclerosis. The image obtained is a horizontal section at the level of the head of the caudate nucleus showing characteristic marked increase of signal as indicated by arrows. (Courtesy of Robert M. DiMauro and John M. Hardman.) 

Plasma proteins are polyionic at pH 7.4 and cannot diffuse across membranes. The normal difference in concentrations of diffusible ions between the plasma and interstitial compartments is due to the presence of nondiffusible protein in plasma, shown in Table 1-5. 

The difference is explained by Gibbs theory of equilibria and was studied experimentally by Donnan the overall process is known as the Gibbs-Donnan equilibrium. 

Gibbs-Donnan equilibria can best be understood in a two-compartment system. Compartment 1 contains the sodium salt of an anionic protein (Na+P ) at an initial concentration Ci, with n representing the number of charges compartment 2 contains NaCl at an initial 

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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. 

Donnan equilibrium phys chem The particular eq ul 11 bri u m set up when two coexisting phases are subject to the restriction that one or more of the ionic components cannot pass from one phase into the other commonly, this restriction is caused by a membrane which is permeable to the solvent and small ions but impermeable to colloidal ions or charged particles of colloidal size. Also known as Gibbs-Donnan equilibrium. dO-non e-kwo lib-re-om ... 

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... 

This is called the Gibbs-Donnan equilibrium and provides an insight into the reasons for the higher concentration of sodium ions in the intracellular fluid. 

The composition of ICF can differ markedly from that of the ECF because of the separation of these compartments by the ceU membrane. The composition differences are a consequence of both the Gibbs-Donnan equilibrium and active and passive transport of ions. 

Gibbs-Donnan equilibrium exists in systems consisting of two fluid compartments separated by a semipermeable membrane, which permits diffusion of some ions (e.g., Na+, Cl ) but is impermeable to protein anions. Note that the concentration of diffusible cation in compartment 1 is greater than in compartment 2 and that osmotic differences exist between compartments 1 and 2. 

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

Note that the sodium ion concentrations in the two compartments are quite different in this last case, as are the hydroxyl ion concentrations. This is the Gibbs-Donnan equilibrium effect. The fact that each ion partitions unevenly between the two compartments results in a contribution to the osmotic pressure from the ions in addition to that which results... 

Gibbs-Donnan equilibrium determines the concentration difference across simple membranes made of polymers, porous ceramic media, and other ultrafiltration devices. However, the difference of ion concentrations across the membranes of living cells and nerves is more complicated because of the existence of ion pumps as a result of carrier-mediated or facilitated diffusion, so that the concentrations of some ions are not in thermodynamic equilibrium. For example, there is a much higher sodium con- centration outside cells than there is inside, while the reverse is true for potassium ions. This occurs because there is a carrier (probably a lipoprotein) that binds with a sodium ion inside the cell, transports the ion across membrane, and then releases it into the fluid outside the cell. The carrier is then transformed and binds with a potassium ion, which is then transported into the cell. This mechanism is discussed in courses... 

Derive e analogue of Eqs.. 15.7-6 for Gibbs- Donnan equilibrium involving a negatively charged protein. 

In the blood, uric acid is distributed between red cells and serum. The proportion found in each fraction of the blood is determined by the Gibbs-Donnan equilibrium. Two types of methods are available for determining uric acid in blood—colorimetric and enzymic analyses. The colorimetric methods require preliminary precipitation of blood proteins, which entails coprecipitation of urates. In addition, the color tests are never rigidly specific for urates. For these reasons, accurate uric acid determinations are obtained only by treating the serum with uricase. The disappearance of uric acid from the sample can be determined by a spectrophotometric reading at 292 mp before and after enzymic treatment. 

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... 

Most of the extracellular fluid is interstitial fluid (ISF) in the tissue spaces, providing the transport medium between capillaries and cells. The sodium concentration in plasma is slightly above that in ISF because plasma contains more proteins, notably albumin, which do not readily escape into ISF across the capillary membranes, and the effect of their negative charges is to hold more positively charged ions, notably sodium, in circulation (Gibbs-Donnan equilibrium). 

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