Membranes electrical potentials across

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 ... 

II. SPONTANEOUS OSCILLATION OF ELECTRICAL POTENTIAL ACROSS A LIQUID MEMBRANE... 

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. 

The distribution of electric potential across the membrane and the dependence of the membrane potential on the concentration of fixed ions in the membrane and of the electrolyte in the solutions in contact with the membrane is described in the model of an ion-exchanger membrane worked out by T. Teorell, and K. H. Meyer and J. F. Sievers. 

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. 

It has no net charge, so that transport is not limited by any change in electrical potential across a membrane. 

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 electrical potential across a LB film of dioleoyl-lecithin deposited onto a fine-pore membrane, imposed between equimolar aqueous solutions of NaCl and KC1, was reported to exhibit rhythmic and sustained pulsing or oscillations of electrical potential between the two solutions. These oscillations were attributed to the change of permeability of Na+ and K+ ions across the membrane, which originated from the phase transition of lecithin. 

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. 

The basic mechanism underlying the toxicity of salicylate is the uncoupling of oxidative phosphorylation. For oxidative phosphorylation to take place, there is a requirement of a charge difference between the intermembrane space and the matrix of the mitochondria (Fig. 7.60). This is achieved when electrons move down the chain of multienzyme complexes and electron carriers (the electron transport chain), causing protons to move from the mitochondrial matrix to the intermembrane space. Consequently, a pH difference builds up, which is converted into an electrical potential across the membrane of approximately 200 mV over 8 nm. 

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... 

Neurons maintain electrical potential across a membrane by pumping sodium ions from their inner hollow channels. 

Unlike transport across the membranes of the ER, transport across plasma membranes of bacteria often requires both hydrolysis of ATP and energy provided by the membrane electrical potential.33 38 44-48 Secretion into the periplasmic space has been well characterized but less is known about transport of proteins into the external membranes of E. coli48 A16 kDa periplasmic chaperone may be required.483... 

The difference of the electrical potential across the membrane can be determined from either Equation (12.121) or Equation (12.122). We choose... 

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. 

We now consider a capsule which consists of liquid surrounded by a closed semi-permeable membrane (figure 2) details are provided in [3,4], Water and salt can pass through the membrane from side 1 (inside the capsule) to side 2 (outside), and vice versa, but large polymer molecules cannot. Trapped inside the capsule are n p polyelectrolyte molecules of valence zp and partial molar volume Tip. The resulting Donnan equilibrium is reviewed in [5, 6], Inside the capsule, electroneutrality requires zpn p + z+n + + Z-ri - = 0. We now assume the salt to be monovalent. At equilibrium there is a jump in electrical potential across the membrane inside the capsule x +xi- r X2+X2- x2 with x = (Q =F zpX p) where... 

There is difference in electric potential across the membrane. So there is an electric field in the membrane, but there are no electric fields in the two bulk phases. The electrical work required to move charge dQ across the center of the membrane is (cj>B — charge transport because the potential difference is constant. Since dnCx = — dncp, equation 8.3-3 can be used to show that at phase equilibrium,... 

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). 

The developed H+ concentration gradient plus an electric potential across the membrane supply the driving force for ATP synthesis from ADP and Pi, a thermodynamically unfavorable reaction catalyzed by ATP synthase (Karrasch and Walker, 1999). The latter is a mitochondrial enzyme located on, and spanning, the inner mitochondrial membrane. At least when in submitochondrial particles, ATP synthase saturation kinetics involve ADP positive site-site interactions in catalysis. One group has proposed that ADP saturation in vivo also shows site-site interactions ( , the interaction or Hill coefficient increasing from 1, meaning no interaction, to 2) however, others have not found this, so this issue at this time must be considered to remain unresolved. 

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