Electrical potentials across cell membranes

Figure 8.43 Electric double layer at a cell membrane. The potential across the Helmholtz layer drops linearly while the further drop in the Guy—Chapman layer is exponential.

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 potential x as the difference of electrical potential across the interface between the phase and gas, is not measurable. But its relative changes caused by the change of solution composition can be determined using the proper voltaic cells (see Section IV). The name surface potential is unfortunately also often used for the description the ionic double layer potential (i.e., the ionic part of the Galvani potential) at the interfaces of membranes, microemulsion droplets and micelles, measured usually by the acid-base indicator technique (Section V). 

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. 

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. 

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 ... 

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... 

Action potentials are waves of depolarization and repolarization of the plasma membrane. In a resting nerve cell, the electric potential gradient (At//) across the plasma membrane is about —70 mV, inside negative. This potential difference is generated mainly by the unequal rates of diffusion of K+ and Na+ ions down concentration gradients maintained by the Na+-K+ ATPase. 

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. 

In this case, the energy required for the transport of the molecule across the membrane is derived from the coupled hydrolysis of ATP, for example the movement of Na+ and K+ ions by the Na+/K+-ATPase. All cells maintain a high internal concentration of K+ and a low internal concentration of Na+. The resulting Na+/K+ gradient across the plasma membrane is important for the active transport of certain molecules, and the maintenance of the membrane electrical potential (see Topic N3). The movement across the membrane of Na+, K+, Ca2+ and H+, as well as a number of other molecules, is directly coupled to the hydrolysis of ATP. 

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). 

Electric potential differences can exist in living systems. Changes in electrical potential energy are produced by ions in solutions whose electric potentials change across cell membranes. The potential energy per mole species at the potential t(/ is obtained from... 

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. 

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. 

Nearly all cells in the body exhibit a difference in electrical voltage between their interior and exterior, the membrane potential. Some cells, including the conducting and contracting cells of the heart, are excitable an appropriate stimulus alters the properties of the cell membrane, ions flow across it and elicit an action potential. This spreads to adjacent cells, i.e. it is conducted as an electrical impulse and, when it reaches a muscle cell, causes it to contract this is excitation-contraction coupling. 

Fig. A2.2 Generation of electric potential across a cell membrane.

To conclude, the movement of potassium across the cell membrane sets up an electric potential across the cell membrane which opposes this same flow. Charged protein structures are unable to move across the membrane, while sodium ions cross very slowly, and so an equilibrium is established. The cell is polarized and the electric potential at equilibrium is known as the resting potential. 

As mentioned above, potassium ions are able to flow out of potassium ion channels. However, not all of these channels are open in the resting state. What would happen if more were to open The answer is that more potassium ions would flow out of the cell and the electric potential across the cell membrane would become more negative to counter this increased flow. This is known as hyperpolarization and the effect is to destimulate the nerve (Fig. A2.3). 

Electrodialysis involves the use of a selectively permeable membrane, but the driving force is an electrical potential across the membrane. Electrodialysis is useful for separating inorganic electrolytes from a solution, and can therefore be used to produce freshwater from brackish water or seawater. Electrodialysis typically consists of many cells arranged side by side, in a stack. Figure 9.12 illustrates a two-cell stack. 

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. 

The use of potential-sensitive fluorescent probes to monitor the electrical potential across a cell membrane permits an accurate, noninvasive measurement of membrane potential changes in a wide variety of cells, vesicles, and organelles without the external electrical or mechanical manipulation required by micro-... 

The ionic gradients and electric potential across the plasma membrane play a role in many biological processes. As noted previously, a rise in the cytosolic Ca concentration is an Important regulatory signal. Initiating contraction in muscle cells and triggering secretion of digestive enzymes... 

Here we discuss the origin of the membrane electric potential In resting cells, how Ion channels mediate the selective movement of Ions across a membrane, and useful experimental techniques for characterizing the functional properties of channel proteins. 

A EXPERIMENTAL FIGURE 7-14 The electric potential across the plasma membrane of living cells can be measured. 

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