Membrane, biological cell potential

The Nernst equation is widely used to estimate the emf of cells under nonstandard conditions. In biology it is used, among other things, to estimate the potential difference across biological cell membranes, such as those of neurons. 

Ferrier, J., Crygorczyk, C., Grygorczyk, R., et al. (1991) Ba2-induced action potentials in osteoblastic cells. Journal of Membrane Biology 123 255-259... 

More recent penetration experiments were carried out in biological systems, that is, large intact nuclei [35], giant liposomes [36], and mammalian cells [37]. Such experiments can provide information about the distribution of electroactive species inside the cell, potentials, and ion transfers across biological membranes (see Section V.F). 

Whole cell biosensors may be divided into two groups depending on whether or not it is necessary for the cellular membranes to be intact. If the cells are being used simply as a source of one or more bound enzymes then the integrity of the cellular membranes may be of little import whereas if the enzymes are not bound within the cells then the contiguity of the cellular membranes is of vital importance. The status of the cell membranes is of particular importance when it is necessary that the cells be alive (i.e., potentially or actually capable of cell division, etc.). This requirement imposes additional difficulties on the fabrication of devices. However, biosensors that contain intact whole cells have the potential to act as convenient surrogates for traditional applications of biological cells and systems. At the forefront are those applications... 

Several in vitro studies proved that treatment of intact erythrocytes with nitrites causes the oxidation of hemoglobin to methemoglobin by radical generation along with a decrease in reduced glutathione (GSH) level associated with erythrocyte membrane dysfunctions and namely altered cell ionic flux, lipid peroxidation, and perturbation of membrane transport (Batina et al 1990 May et al 2000). Nitrate/ nitrite-induced oxidation of biological molecules potentiates reactions, which interfere in the oxidative chain and which can affect some antioxidant systems. 

A biological cell can be compared to a concentration cell for the purpose of calculating its membrane potential. Membrane potential is the electrical potential that exists across the membrane of various kinds of cells, including muscle cells and nerve cells. It is responsible for the propagation of nerve impulses and heart beat. A membrane potential is established whenever there are unequal concentrations of the same type of ion in the interior and exterior of a cell. For example, the concentrations of ions in the interior and exterior of a nerve cell are 400 mM and 15 mM, respectively. Treating the situation as a concentration cell and applying the Nemst equation, we can write... 

For many years, it has been known that many individual biological cells maintain different distributions in ionic concentrations and an electrical potential difference between their intracellular and extracellular phases at the resting state (Table 19). In some cells, upon application of an appropriate stimulus (electrical depolarization or chemical stimulus), the cells exhibit a time-dependent response via a potential difference across the cell membranes which does not necessarily follow Ohm s law. The former potential is called a resting membrane potential and the latter an excitation potential. We would like to review the origins of these electrical potential differences across the cell membrane. 

Formerly, the membrane potential in biological cells was thought to be due to this Donnan equilibrium potential. Bernstein suggested that the resting membrane potential was determined by the ratio of potassium ion concentration inside and outside the cell. The relative impermeability of Na" across the cell membrane was observed by Boyle and Conway. The validity of the formula of the membrane potential for biological cells... 

The membrane potential of many biological cells has been analyzed quantitatively by using the above equation [Eq. (126)] with known bulk ion concentrations and ion permeabilities across the membrane (Figure 27). The latter (permeabilities) were determined from isotope flux measurements across membranes together with the use of the flux equation [Eq. (124)]. However, it should be remembered that the flux equation [Eq. (124)] used to obtain the permeability was derived under the conditions of the constant field assumption. Therefore, if this membrane potential theory is not a correct expression for biological membranes, the permeability values obtained using Eqs. (124) and (126) will lose their validity. 

This equation is often used to describe the liquid junction potential for multi-univalent ions. However, this equation is not often used to describe the membrane potential of biological cells because of difficulties in obtaining the mobilities of the ionic species within the membrane. 

In these situations (biological membranes and polymer-resin membranes), the fixed charge membrane theory with various aqueous pore sizes may be a better theoretical model to explain the mechanism of the observed transmembrane potential. Ohki attempted to analyze the transmembrane potential for a membrane having an aqueous pore the surface of which possesses fixed charges. He used a number of assumptions to simplify the mathematical treatment. Further development of such a membrane potential theory therefore seems to be necessary in order to describe the observed membrane potential for biological cell systems. 

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