The Membrane Potential

It is universally found that electrical potentials are associated with surfaces in biology. In pre-electrodic days, this called for some special explanation. Many measurements of the so-called membrane potentials showed values in the region of 50-100 mV but occasionally measurements were much lower, or stretched to values of 250 mV (Table 1). The large edifice of electrophysiology is intimately associated with these potentials. 

The membrane potential in biology came to prominence in the days in which electrode phenomena were treated exclusively in terms of equilibrium thermodynamics. Between 1892 (Nernst ) and 1911 (Donnan ), three treatments were given of membrane potentials. They form such a durable part of electrochemistry, not because of their importance per se, or even of their direct relevance to biological phenomena, but because one of them was the origin of the best-known of bioelectrochemical theories, the Hodgkin-Huxley-Katz mechanism for the passage of electricity through nerves. 

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Interaction of the analyte with the membrane results in a membrane potential if there is a difference in the analyte s concentration on opposite sides of the membrane. One side of the membrane is in contact with an internal solution containing a fixed concentration of analyte, while the other side of the membrane is in contact with the sample. Current is carried through the membrane by the movement of either the analyte or an ion already present in the membrane s matrix. The membrane potential is given by a Nernst-like equation... 

An electrode in which the membrane potential is a function of the concentration of a particular ion in solution. 

The membrane potential when opposite sides of the membrane are in contact with identical solutions yet a nonzero potential is observed. 

The membrane potential for a Ag2S pellet develops as the result of a difference in the equilibrium position of the solubility reaction... 

If a mixture of an insoluble silver salt and Ag2S is used to make the membrane, then the membrane potential also responds to the concentration of the anion of the added silver salt. Thus, pellets made from a mixture of Ag2S and AgCl can serve as a Ck ion-selective electrode, with a cell potential of... 

The membrane potential for a E ion-selective electrode results from a difference in the solubility of LaE3 on opposite sides of the membrane, with the potential given by... 

Below a pH of 4 the predominate form of fluoride in solution is HF, which, unlike F , does not contribute to the membrane potential. For this reason, an analysis for total fluoride must be carried out at a pH greater than 4. 

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

Electroporation. When bacteria are exposed to an electric field a number of physical and biochemical changes occur. The bacterial membrane becomes polarized at low electric field. When the membrane potential reaches a critical value of 200—300 mV, areas of reversible local disorganization and transient breakdown occur resulting in a permeable membrane. This results in both molecular influx and efflux. The nature of the membrane disturbance is not clearly understood but bacteria, yeast, and fungi are capable of DNA uptake (see Yeasts). This method, called electroporation, has been used to transform a variety of bacterial and yeast strains that are recalcitrant to other methods (2). Apparatus for electroporation is commercially available, and constant improvements in the design are being made. 

The electrostatic free energy of a macromolecule embedded in a membrane in the presence of a membrane potential V can be expressed as the sum of three separate terms involving the capacitance C of the system, the reaction field Orffr), and the membrane potential field p(r) [73],... 

Simple considerations show that the membrane potential cannot be treated with computer simulations, and continuum electrostatic methods may constimte the only practical approach to address such questions. The capacitance of a typical lipid membrane is on the order of 1 j.F/cm-, which corresponds to a thickness of approximately 25 A and a dielectric constant of 2 for the hydrophobic core of a bilayer. In the presence of a membrane potential the bulk solution remains electrically neutral and a small charge imbalance is distributed in the neighborhood of the interfaces. The membrane potential arises from... 

Maintenance of electrical potential between the cell membrane exterior and interior is a necessity for the proper functioning of excitable neuronal and muscle cells. Chemical compounds can disturb ion fluxes that are essential for the maintenance of the membrane potentials. Fluxes of ions into the cells or out of the cells can be blocked by ion channel blockers (for example, some marine tox-... 

Depletion of ATP in the cells prevents maintenance of the membrane potential, inhibits the functioning of ion pumps, and attenuates cellular signal transduction (e.g., formation of second messengers such as inositol phos phates or cyclic AMP). A marked ATP depletion ultimately impairs the activ-itv of the cell and leads to ceil death. 

Reported values for A and ApH vary, but the membrane potential is always found to be positive outside and negative inside, and the pH is always more acidic outside and more basic inside. Taking typical values of A F = 0.18 V and ApH = 1 unit, the free energy change associated with the movement of one mole of protons from inside to outside is... 

Assuming that 3 H are transported per ATP synthesized in the mitochondrial matrix, the membrane potential difference is 0.18 V (negative inside), and the pH difference is 1 unit (acid outside, basic inside), calculate the largest ratio of [ATP]/[ADP] [P,] under which synthesis of ATP can occur. 

In chloroplasts, the value of AT is typically —50 to —100 mV, and the pH gradient is equivalent to about 3 pH units, so that — (2.3 i T/S ) ApH = —200 mV. This situation contrasts with the mitochondrial proton-motive force, where the membrane potential contributes relatively more to bsp than does the pH gradient. 

The transmembrane potential derived from a concentration gradient is calculable by means of the Nemst equation. If K+ were the only permeable ion then the membrane potential would be given by Eq. 1. With an ion activity (concentration) gradient for K+ of 10 1 from one side to the other of the membrane at 20 °C, the membrane potential that develops on addition of Valinomycin approaches a limiting value of 58 mV87). This is what is calculated from Eq. 1 and indicates that cation over anion selectivity is essentially total. As the conformation of Valinomycin in nonpolar solvents in the absence of cation is similar to that of the cation complex 105), it is quite understandable that anions have no location for interaction. One could with the Valinomycin structure construct a conformation in which a polar core were formed with six peptide N—H moieties directed inward in place of the C—O moieties but... 

An Action Potential is a stereotyped (within a given cell) change of the membrane potential from a resting... 

The current-voltage relationship is a plot of the current through a channel versus the voltage of the membrane potential. 

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

Table 1). Further determinants of blocking potency are the membrane potential and the state in which the sodium channel is in (resting, activated, inactivated). The tertiary amine group can be protonated giving most local... 

The voltage sensor is the part of a channel protein responsible for detection of the membrane potential. A voltage sensor of the voltage-dependent Na+ channel was predicted by Hodgkin and Huxley in 1952. Positively charged amino acid residues in S4 of each repeat play an essential role as the voltage sensor. 

Boguslavsky, L. I. Electron Transfer Effects and the Mechanism of the Membrane Potential 18... 

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