Electric Potentials in the Cell

As explained at the beginning of Sec. 3.8, the electric potential at a point in space is defined as the change in the electrical potential energy of an infinitesimal test charge when it is brought to this point from a position infinitely far from other charges, divided by the charge. 

We are concerned with the electric potential within a phase— the inner electric potential, or Galvani potential. We can measure the difference between the values of this electric potential in the two terminals of a galvanic cell, provided the terminals have the same chemical composition. If the terminals were of different metals, at least one of them would have an unknown metal-metal contact potential in its connection to the external measuring circuit. 

Since we will be applying the concept of electric potential to macroscopic phases, the value of the Galvani potential at a point in a phase should be interpreted as the average value in a small volume element at this point that is large enough to contain many molecules. 

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Next we consider the second case, where the liquid crystal cell is connected to the voltage source such that the voltage applied V across the cell is fixed. The electric potential in the cell is (z). On top of the cell (z = h), the potential (z = h) = 2 is low. At the bottom of the cell (z = 0), the potential i — 4>2= V- The electric field is in the + z direction. The free surface charge density on the top surface of the liquid crystal cell... 

CHAPTER 14 GALVANIC CELLS 14.2 Electric Potentials in the Cell... 

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 ... 

This can be accomplished by applying an electrical potential in the external circuit in such a manner that an emf occurs in opposition to that of the galvanic cell. The opposing emf is varied by means of a potentiometer until the current flow from the cell is essentially zero. Under these conditions, the cell may very well approach reversibility. This is readily tested by changing the direction of the current and allowing an infinitesimally small current flow in the opposite direction. If the cell is reversible, the cell reaction will proceed in the reverse direction with the same efficiency as in the forward direction. For a reversible reaction... 

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. 

What makes a neuron special is the presence of protein molecules that are sensitive to the voltage across its membrane. Membranes do not allow ions to pass—one of the main jobs of a membrane is to regulate the flow of substances into and out of the cell—but certain proteins embedded in the membrane contain channels to allow ions to pass through. The flow of ions constitutes an electrical current. In neurons, these proteins, known as ion channels, can open and close quickly, producing currents that briefly change the electrical potential of the cell. 

Under open circuit conditions, the electric current /= J) Zj-F-Jj vanishes. As long as tQ2- = 1 this means thaty o2- = To2- 7o2- - 0. or equally V /02- = Zj-F-Vtp = 0. This is true since oxygen ions are the mobile majority species with a constant chemical potential independent of any variation in the oxygen potential. It follows that the electrical potential in the oxide electrolyte of a galvanic cell is constant under open circuit conditions, despite the different oxygen potentials at the two electrodes. 

Chemical aspects of taste receptor functions can he studied by recording the patterns of electrical potentials in receptor cells while the cells are being stimulated with pure chemicals of known structures and properties. Since the mid-1950s, when this method was first successfully applied to single taste receptor cells, using receptors on Ihe mouth parts of a fly. many earlier theories of taste stimulation have been revised. 

The difference between the electrical potentials in the two copper wires is determined by the difference [/l"(Cu) — e(Cu)] under equilibrium conditions with certain restrictions. (The single prime refers here to all parts of the cell to the left of the boundary between the two solutions, and the double prime to all parts to the right of the boundary.) The restrictions are that the boundaries between the various parts of the cell are permeable only to certain species. Without such restrictions the electrical potential difference of the electrons in the copper wires would be zero at equilibrium. The boundary between the copper and platinum or between the copper and silver is permeable only to electrons that between the platinum with adsorbed hydrogen and the first solution is permeable to hydrogen ions but not electrons that between the second solution and the silver chloride is permeable to chloride ions but not electrons and that between the silver chloride and silver is permeable only to silver ions. We ignore the presence of the boundary between the two solutions, for the present. The conditions of equilibrium in terms of the chemical potentials are then ... 

The oxidation rate depends not only on the gas composition and the temperature parameter, but also on the electric potential difference between the electronically conductive part of the anode electrode and the ionically conductive electrolyte. Defining the electric potential of the solid part of the anode electrode as zero potential, the reaction rate depends on the electric potential in the electrolyte, other hand, the reduction reaction rate depends on the electric potential difference at the cathode electrode, which is the difference between the given cell voltage, Uceii, and the electrolyte potential, equilibrium constants are determined by the... 

Although electrochemistry has the stigma of being difficult to use, and therefore is often overlooked as an analysis option, potentiometric measurements are probably the most common technique encountered. Many analytical chemists make potentiometric measurements daily, whenever they use a pH meter. Potentiometry is based on the measurement of the potential between two electrodes immersed in a test solution. As the electrical potential of the cell is measured with no current flow between the electrodes, potentiometry is an equilibrium technique. The first electrode, the indicator electrode, is chosen to respond to the activity of a specific species in the test solution. The second electrode is a reference of known and fixed potential. The design of the indicator electrode is fundamental to potentiometric measurements, and should interact selectively with the analyte of interest so that other sample constituents do not interfere with the measurement. Many different strategies have been developed to make indicator electrodes that respond selectively to a number of species including organic ions. 

To assess the consistency of this KMC model, a variety of materials-independent, materials-dependent, and geometrical parameters was investigated, and the ionic current calculated from the model was used as the primary metric. The materials-independent parameters included the oxygen partial pressure, system temperature, and the external applied potential. Of these parameters, the oxygen pressure had a weak influence on the current (Figure 4), unless its value falls below a threshold of approximately 0.05 atm. As the temperature increased (from 200 to 800°C), the current showed an exponential increase, owing to the thermally activated ion transport in YSZ. As the applied electric potential of the cell increased, a similar increase was found in the calculated ionic current. The materials-dependent parameters included the dopant level (i.e., Y2O3... 

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). 

We shall now consider what happens in the axon of the nerve (Fig. A2.4). The cell membrane of the axon also has sodium and potassium ion channels but they are different in character from those in the cell body. The axon ion channels are not controlled by neurotransmitters, but by the electric potential of the cell membrane. 

These second messengers interact with yet another type of protein, the ion channels. They open the gates on the ion channels and allow positively charged ions to flow into the cell, therefore creating an electrical potential between the cell and its environment. This is relieved when the synapse at the other end of the cell, in the olfactory bulb, fires across to the next nerve cell in line and the transduction process is set in motion. The second messengers also set in train a process which deactivates the receptor by phosphorylation, and a dephosphorylation is necessary to make it active again. 

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. 

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