Potential, membrane

The electrical potential difference at both sides of a membrane separating two solutions of the same electrolyte but different concentrations (Ci, C2) is called membrane potential (AOm). The Teorell-Meyer-Sievers (or TMS) theory [37, 38] assumes the membrane potential can be considered as the sum of three terms associated with two different contributions  

Two Donnan potentials, one for each membrane/solution interface, related to the exclusion of co-ions in the membrane  

A diffusion potential due to the different mobility or transport number (t ) of the ions in the membrane (for diluted solutions concentration was used instead of solution activity, Ci a )  

Combining these two expressions, the membrane potential can be expressed as [39]  

Ion transport number (t+ for cations, t for anions) represents the fraction of the total current carried by each ion (ti = I /Ix) and for single salts t +t = l. 

Because the change in free energy is equal to the difference in chemical potential, fih in the permeation of i having charge z, through the membrane with transport number,,  

This is the general formula of the Nemst equation for the membrane concentration potential. 

When an ion exchange membrane separates solution ax from solution a2 of the same electrolyte ax a2), the membrane potential is 

When the Donnan equilibrium of the counter-ion is attained at membrane-solution interfaces, the Donnan potential at the interface 1, E, is 

The diffusion potential, diff, is calculated from Eq. (2.40) on the assumption that ions are linearly distributed across the membrane (an assumption made by the Henderson diffusion potential), 

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When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Membrane Potentials Ion-selective electrodes, such as the glass pH electrode, function by using a membrane that reacts selectively with a single ion. figure 11.10 shows a generic diagram for a potentiometric electrochemical cell equipped with an ion-selective electrode. The shorthand notation for this cell is... 

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. 

Selectivity of Membranes Membrane potentials result from a chemical interaction between the analyte and active sites on the membrane s surface. Because the signal depends on a chemical process, most membranes are not selective toward... 

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... 

Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

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. 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

Automa-ticity. Special cardiac cells, such as SA and AV nodal cells, His-bundle cells, and Purkinje fibers, spontaneously generate an impulse. This is the property of automaticity. Ectopic sites can act as pacemakers if the rate of phase 4 depolarization or resting membrane potential is increased, or the threshold for excitation is reduced. 

The resting membrane potential of most excitable cells is around —60 to —80 mV. This gradient is maintained by the activity of various ion channels. When the potassium channels of the cell open, potassium efflux occurs and hyperpolari2ation results. This decreases calcium channel openings, which ia turn preveats the influx of calcium iato the cell lea ding to a decrease ia iatraceUular calcium ia the smooth muscles of the vasculature. The vascular smooth muscles thea relax and the systemic blood pressure faUs. 

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. 

The pain appears to arise from the formation of melittin pores in the membranes of nociceptors, free nerve endings that detect harmful ( noxious —thus the name) stimuli of violent mechanical stress, high temperatures, and irritant chemicals. The creation of pores by melittin depends on the nociceptor membrane potential. Melittin in water solution is tetrameric. However, melittin interacting with membranes in the absence of a membrane potential is monomeric and shows no evidence of oligomer... 

The electron on the bj heme facing the cytosolic side of the membrane is now passed to the bfj evcie on the matrix side of the membrane. This electron transfer occurs against a membrane potential of 0.15 V and is driven by the loss of redox potential as the electron moves from bj = — O.IOOV) to bn = +0.050V). The electron is then passed from bn to a molecule of UQ at a second quinone-binding site, Q , converting this UQ to UQ . The result-... 

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... 

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