Sodium electrochemical potential

One basic oscillatory system uses the kinetic behavior of protein channels for sodium and potassium ion in a nerve membrane. Before elecflical excitation, the sodium channels are closed to sodium ion flow and the electrochemical gradient of 110 mV possible with the sodium ion gradient does not develop. The more permeable potassium channels tap the potassium ion gradient to produce an internal electrical potential of —60 mV relative to the external solution as ground. On excitation, the transient channels open and then close. As the sodium channels open, the sodium electrochemical potential of 110 mV appears to produce a peak internal potential of -60-1-110 = + 40 mV, The one-shot oscillation is completed as the sodium channel closes, the sodium-ion induced potential is lost, and the internal potential returns to the potassium channel dominated potential of -60 mV. The entire oscillation is controlled only by the sodium and potassium channels and the sodium and potassium ion concentration gradients across the membrane. 

The rate of the active transport of sodium ion across frog skin depends both on the electrochemical potential difference between the two sides of this complex membrane (or, more exactly, membrane system) and also on the affinity of the chemical reaction occurring in the membrane. This combination of material flux, a vector, and chemical flux (see Eq. 2.3.26), which is scalar in nature, is possible according to the Curie principle only when the medium in which the chemical reaction occurs is not homogeneous but anisotropic (i.e. has an oriented structure in the direction perpendicular to the surface of the membrane or, as is sometimes stated, has a vectorial character). 

It is assumed that the chloride ion is transported passively across the membrane. Using an approach similar to the formulation of Eqs (2.1.2), (2.3.26) and (2.5.23), relationships can be written for the material fluxes of sodium and chloride ions, /Na+ and Jcr (the driving force is considered to be the electrochemical potential difference), and for the flux of the chemical reaction, Jch ... 

The difference in the hydrogen ion electrochemical potential, formed in bacteria similarly as in mitochondria, can be used not only for synthesis of ATP but also for the electrogenic (connected with net charge transfer) symport of sugars and amino acids, for the electroneutral symport of some anions and for the sodium ion/hydrogen ion antiport, which, for example, maintains a low Na+ activity in the cells of the bacterium Escherichia coli. 

A number of suggestions have been made that calcium may be transported because it is coupled to the movement of other ions or because it moves passively down an electrochemical gradient established by the movement of some other ions600. Thus, a sodium-induced potential has been found which was sufficient to account for the passive movement of calcium into the shell gland of the domestic fowl during egg shell formation. In the mollusc, the shell side of the mantle is normally positive relative to blood and a potential of this type would, of course, tend to move calcium away from the extrapallial fluid. A potential of this orientation could be produced by the movement of an anion into the animal (mollusc) and the low chloride concentration of the extrapallial fluid could be accounted for on this basis. 

For the electrochemical measurements reported herein, all cyclic voltammetry measurements are performed in CH2C12 with 0.1 M tetra-n-butylammonium tetrafluoroborate (Bu4NBF4) as supporting electrolyte, while measurements in CH3CN use 0.1 M tetra-ethylammonium perchlorate. Cyclic voltammetry measurements are performed in a three-electrode, one-compartment cell equipped with a Pt working electrode, a Pt auxiliary electrode, and a saturated sodium chloride calomel (SSCE) reference electrode. E1 2 = (Ep.a + Ep.c)/2 AEP = Ep,e - Ep,a-Ei/2 and AEP values are measured at 100 mV/sec. Ferrocene is used as a reference in the measurement of the electrochemical potentials. 

The coupling between chemical reactions and transport in biological membranes, such as the sodium and potassium pumps, is known as active transport, in which the metabolic reactions cause the transport of substances against the direction imposed by their thermodynamic force of mainly electrochemical potential gradients. 

Equations (10.152) and (10.153) can be used to analyze sodium flow in frog skin. The flow of sodium chloride across the skin comprises the flow of sodium ions. /Na, which is coupled to the metabolic process Jrtot, while the flow of the chloride ions JC may be assumed to be passive transport. The driving forces for the ionic flows are the electrochemical potential differences, and are given for a component i in a simple system as follows ... 

Potassium leaves the cell, while the net flow of sodium is inward. A nonequilibrium stationary state for the cell at rest is maintained by the sodium and potassium pumps, which pump out the entering sodium ions and pump the leaking potassium ions back into the cell interior, using a certain metabolic output. The sodium transfer is coupled with the chemical reaction. The electrochemical potential difference for sodium ions is expressed as... 

The above model is useful however, biological membranes, which transport various substances, are complex systems. Such membranes are close to composite membranes with series and parallel elements. A value of q < 1 shows an incomplete coupling, where a metabolic energy must be expended to maintain an electrochemical potential difference of sodium even in the absence of active transport, that is (./, )y =0 A0. 

To construct the potential pH diagrams of the different elements, all their possible redox processes with water, oxygen, and hydrogen have to be taken into account, and the electrochemical potentials have to be calculated. In addition, the dissolution/precipitation equilibria (e.g., hydrolysis) have to be taken into consideration, as well. The main dissolved ions in groundwater (calcium, magnesium, sodium, and potassium cations hydrocarbonate/carbonate, chloride,... 

Sodium is selected as the solid state transported reactant in PEVD. This is because not only is Na" a component in the PEVD product phase Na COj, but also the mobile ionic species in the solid electrolyte (Na "-[3"-alumina) and in the auxiliary phase of the sensor. Thus, PEVD can take advantage of the solid electrochemical cell (substrate) of the sensor to transport one reactant (sodium) across the substrate under an electrochemical potential gradient. This gradient... 

When the process is limited by the diffusion of Na"" inside the product, the first step will increase the electrochemical potential of the sodium ion at the surface of the PEVD product (location (111)). Thus, the thermodynamic driving force will decrease causing further transport of Na"" in the product to pursue the... 

Pumps are proteins that can transport ions against electrochemical potential gradients using adenosine-5-triphosphate (ATP) as an energy source. Sodium-potassium pumps maintain intracellular sodium and potassium concentrations in animal cells and also control salt and water absorption by the epithelial cells in the intestine and kidney. The sodium-potassium pump transports three sodium ions out of the cell and two potassium ions into the cell at the cost of one molecule of ATP. The 3 2 coupling ratio results in net loss of sodium ions into the cell down an electrochemical gradient and maintains cell volume. Currently, considerable research is attempting to elucidate the structures of the various isoforms and subunits of sodium potassium pumps. 

Fig. 23 Combined EQCM/PBD responses of a polypyrrole film to redox switching in sodium salicylate solution. Electrode Au (area = 0.23 cm ) on 10-MHz AT-cut quartz crystal. Solution aqueous 0.5 mol dm sodium salicylate. Potential step program as shown in panel (a). Dotted line in panel (d) represents prediction, based on current trace of panel (b), of salicylate but no sodium transfer to satisfy electroneutrality. (Reproduced from Ref. [147] with permission from The Electrochemical Society.)...

It is well known that the resting and dynamic electrical activity of the brain is a consequence of electrochemical potentials across membranes. Many other aspects of electrochemistry are also familiar in the neurosciences. Hence it may seem paradoxical to have suggested that the electro-analytical techniques are far afield of the mainstream of neurobiology. However, neuronal membrane potentials depend on ionic charge distributions and fluxes insofar as is known, electron current plays no role. Just the opposite is true for electroanalytical techniques—ionic conductance is of minimal importance but electron flow (current) is the essence of the measurement. The electrodes employed do not sense membrane potentials or respond to sodium or potassium fluxes rather, they pass small but finite currents because molecules close to their surface undergo oxidation or reduction. Such electrochemical measurements are called faradaic (because the amount of material converted at the electrode surface can be calculated from Faraday s law). 

Let us now consider an ion-exchange membrane with a fixed negative charge (R ) with Na as the counterion placed in contact with a dilute sodium chloride (NaCl) solution, as shown in figure V - 32. If it is assumed that the solution behaves ideal, the activities can be put equal to the concentrations (a = q ). The Na" and Q ions and the water molecules can freely diffuse from the solution to the membrane phase, although the Na" " ions can only diffuse in combination with a Or ion. At equilibrium, the electrochemical potentials are equal in both phases. 

Figure 3.2. Coupling of methylmalonyl-CoA decarboxylation and generation of the electrochemical potential of sodium ions across the cytoplasmic membrane. The sodium gradient may drive ATP synthesis according to two possible mechanisms (A and B) discussed in the text. Reproduced from Dimroth (1988), with permission.

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