Electrical potentials across cell

Inhalation of certain hydrocarbons, including some anesthetics, can make the mammalian heart abnormally sensitive to epinephrine, resulting in ventricular arrhythmias, which in some cases can lead to sudden death (Reinhardt et al. 1971). The mechanism of action of cardiac sensitization is not completely understood but appears to involve a disturbance in the normal conduction of the electrical impulse through the heart, probably by producing a local disturbance in the electrical potential across cell membranes. The hydrocarbons themselves do not produce arrhythmia the arrhythmia is the result of the potentiation of endogenous epinephrine (adrenalin) by the hydrocarbon. 

Rubidium typically exists in the human body at the level of only 1/1,000 of 1 percent, and cesium content is even lower. Rubidium and cesium are both absorbed from soil by plants and are, therefore, present in small quantities in vegetables and up the food chain to meat products and humans. Rubidium is known to stimulate mammalian metabolism, probably because of its physical and chemical similarity to potassium, which plays a crucial role in electrical pulse transmission along nerve fibers protein synthesis acid-base balance and formation of collagen, elastin, and muscle. Its likeness to potassium may be the reason rubidium enhances growth in some plants. For particular insects, however, the introduction in the laboratory of rubidium to the bloodstream has been shown to drastically reduce fluid secretion and to change the electric potential across cell membranes. Excess rubidium is almost never encountered, however, in nature. 

Many ions have been shown to alter the release of insulin (K+, Mg++, Ca-H-, Na+, Ba-H- and Li+). Their effects are believed to be related to electrical potentials across cell raembranes j S. Changes in potentials across isolated islets have been measured and found to correlate with the release of insulin. 

The existence of electrical potentials across cell membranes suggests that electrodes could be used to control in vivo processes as well as to monitor their activity. Indeed, in order to measure dopamine release, it was shown earlier in this report that electrodes can be used to stimulate the activity of neurons. This has been recognized by biologists for many years, and the use of local electrical stimulation has been used to probe the anatomy and function of neuronal circuits. 

The following factors affect net diffusion of a substance (1) Its concentration gradient across the membrane. Solutes move from high to low concentration. (2) The electrical potential across the membrane. Solutes move toward the solution that has the opposite charge. The inside of the cell usually has a negative charge. (3) The permeability coefficient of the substance for the membrane. (4) The hydrostatic pressure gradient across the membrane. Increased pressure will increase the rate and force of the collision between the molecules and the membrane. (5) Temperature. Increased temperature will increase particle motion and thus increase the frequency of collisions between external particles and the membrane. In addition, a multitude of channels exist in membranes that route the entry of ions into cells. 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

Space-clamped (HH) equations relate the difference in electrical potential across the cell membrane (V) and gating variables (0 < m, n, h < 1), for ion channels to the stimulus intensity (7J and temperature (T), as follows ... 

Alteration of Electrical Potential (PD). Study of the Influence of allelochemicals on the electrical potentials across plant cell membranes has been restricted to phenolic acids. Glass and Dunlop (42) reported that at pH 7.2, 500 yM salicylic acid depolarized the electrical potential in epidermal cells of barley roots. The electrical potential changed from -150 mV to -10 mV within 12 min. Recovery of the PD was very slow over about 100 min when the salicylic acid was removed. As the concentration of the allelochemical was increased, the extent of depolarization increased, but the time required for depolarization and recovery were constant. 

Benzoic acid derivatives also altered the electrical potential across the cell membrane in neurons of the marine mollusk Navanax lnermls (46). Salicylic acid (1-30 mM) caused a depolarization very rapidly (1-2 min) and decreased the ionic resistance across the membrane. As pH was decreased, more salicylic acid was required to reverse the effect of pH on the membrane potential (47). This result is contradictory to the influence of pH on the amount of salicylic acid required to affect mineral absorption in roots (32). The ability of a series of salicylic and benzoic acid derivatives to increase PD correlated with their octanol/water partition coefficients and pKa values (48). The authors proposed that the organic acid anions bound directly to membranes to produce the observed results. 

Up to now in this chapter, we have concentrated on the measurement via electric field sensitive dyes of the transmembrane electrical potential, which by itself should produce a linear drop in the electrical potential across a membrane. However, at least through the lipid matrix of a cell membrane, the electrical potential, /, at any point does not change linearly across the membrane. Instead, it follows a complex profile (see Fig. 6). This is due to contributions other than the transmembrane electrical potential to /. The other contributions come from the surface potential and the dipole potential. Both of these can also be quantified via electric field sensitive dyes. 

Pulsed electric field is another alternative to conventional methods of extraction. PEF enhances mass transfer rates using an external electrical field, which results in an electric potential across the membranes of matrix cells that minimizes thermal degradation and changes textural properties. PEF has been considered as a nonthermal pretreatment stage used to increase the extraction efficiency, increasing also permeability throughout the cell membranes. 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primary cells. The discharging reaction is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondary cells), however, the reaction is reversible that is, application of an electrical potential across the electrodes in the opposite direction will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, electroplating, and electrowinning industries are based. 

Most cells possess an electrical potential across then-plasma membrane, which is positive on the external surface. This is known as the resting potential. The neurone is no exception it has a potential of between 50 and 75 millivolts (mV). The resting potential arises from the following ... 

The electric potential throughout the body of a homogeneous phase is constant, but the possibility of obtaining an electromtjtivo force from the terminals of a cell consisting entirely of homogeneous phases and their interfaces proves that in general there exist differences of electric potential across the latter. 

Each of their receptors transmits its signal across the plasma membrane by increasing transmembrane conductance of the relevant ion and thereby altering the electrical potential across the membrane. For example, acetylcholine causes the opening of the ion channel in the nicotinic acetylcholine receptor (AChR), which allows Na+ to flow down its concentration gradient into cells, producing a localized excitatory postsynaptic potential—a depolarization. 

Neurons oxidize glucose by glycolysis and the citric acid cycle, and the flow of electrons from these oxidations through the respiratory chain provides almost all the ATP used by these cells. Energy is required to create and maintain an electrical potential across the neuronal plasma membrane. The membrane contains an electrogenic ATP-driven antiporter, the Na+K+ ATPase, which simultaneously pumps 2 K+ ions into and 3 Na+ ions out of the neuron (see Fig. 11-37). The resulting... 

In this discussion we have emphasized the difference of electrical potential across a membrane without reference to a galvanic cell. Cells can be devised by which such differences can be experimentally determined. 

Here binding of the ligand again causes a conformational change in the protein but this time such that a specific ion channel is opened (Fig. 3). This allows a certain ion to flow through that subsequently alters the electric potential across the membrane. For example, at the nerve-muscle junction the neurotransmitter acetylcholine binds to specific receptors that allow Na+ ions to flow into and K+ ions out of the target cell (see Topic N3). 

The voltage (or electrical potential) across the circuit is described as the cell voltage and defined by the symbol cell. When the half-cells contain 1 M solutions at 25°C and 1 atm pressure, the cell voltage is called the standard cell voltage or standard emf and is labeled as E°cell. The cell voltage is determined by the difference between the two electrode potentials. The cell voltage is determined according to Equation 18.1 below ... 

The condition of equilibrium does not require that the various forces acting on a substance are zero rather, it requires that they balance each other. In the example for C. corallina, the factors that tend to cause K+ to move are the differences in both its activity (or concentration) and the electrical potential across the membranes. The activity of K+ is much higher in the central vacuole than in the external solution (a K a see Fig. 3-3). The activity term in the chemical potential therefore represents a driving force on K+ that is directed from inside the cell to the bathing solution. 

According to Equation 6.7, the amount of Gibbs free energy stored or released is directly proportional to the difference in electrical potential across which the electrons move. Moreover, this equation indicates that the flow of electrons toward more positive electrical potentials (AE > 0) corresponds to a decrease in the free energy (AG < 0), and so the transfer proceeds spontaneously. We emphasize that two half-cells are necessary to... 

Membranepotential is the electric potential across a membrane. It results fiom the difference in charges at either side of the membrane. The resting potential varies in animal cells, but is usually about -70mV. A Na" " /K. -ATPase exchanges intracellular Na" for extracellular < and is responsible for the maintenance of low intracellular Na concentrations. 

The cell can be charged again by applying an electrical potential Across the terminals, and causing the above electrode reactions to take place in the opposite directions. The charged cell produces an electromotive force of slightly over 2 volts. 

Since it is impossible to measure the individual electric potential differences at the phase boundaries, we shall hereinafter speak only in terms of the difference in electric potential across the two terminals connected to the electrodes of the battery. When in a battery the current is not flowing or tends to zero, the measurable potential difference across the two terminals is called the open-circuit voltage (OCV), fJc, and it represents the battery s equilibrium potential (or voltage). Since it is related to the free energy of the cell reaction, the OCV is a measure of the tendency of the cell reaction to take place. Indeed, while the conversion of chemical into electric energy is regulated by thermodynamics, the behavior of a battery under current flow (the current is a measure of the electrochemical reaction rate) comes under electrochemical kinetics. 

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