Potassium Nernst equation

Thus under standard conditions chloride ions are not oxidised to chlorine by dichromate(Vr) ions. However, it is necessary to emphasise that changes in the concentration of the dichromate(VI) and chloride ions alters their redox potentials as indicated by the Nernst equation. Hence, when concentrated hydrochloric acid is added to solid potassium dichromate and the mixture warmed, chlorine is liberated. 

When paint films are immersed in water or solutions of electrolytes they acquire a charge. The existence of this charge is based on the following evidence. In a junction between two solutions of potassium chloride, 0 -1 N and 0 01 N, there will be no diffusion potential, because the transport numbers of both the and the Cl" ions are almost 0-5. If the solutions are separated by a membrane equally permeable to both ions, there will still be no diffusion potential, but if the membrane is more permeable to one ion than to the other a diffusion potential will arise it can be calculated from the Nernst equation that when the membrane is permeable to only one ion, the potential will have the value of 56 mV. 

Voltage-gated potassium (Kv) channels are membrane-inserted protein complexes, which form potassium-selective pores that are gated by changes in the potential across the membrane. The potassium current flow through the open channel follows by the electrochemical gradient as defined by the Nernst equation. In general, Kv channels are localized in the plasma membrane. 

Thus, a 10 1 transmembrane gradient of a single monovalent ion, say potassium, will generate a membrane potential of 58 mV. See Resting Potential Action Potential Depolarization Threshold Potential Nernst Equation Goldman Equation Patch-Clamp Technique... 

If the permeability is significant for both potassium and sodium, the Nernst equation is not a good predictor of membrane potential, but the Goldman-Hodgkin-Katz equation may be used. 

Diffusion potentials for the primary biological ions (potassium, sodium, and chloride) represent the primary source of the resting membrane potential. The diffusion potential for a given ionic species can be calculated from the modified Nernst equation developed by Hodgkin and Huxley. The equation is ordinarily written as... 

Figure 10 shows the results of a stepwise addition of increasing concentrations of KCl to a group of cells. A clear dose-dependent increase in fluorescence is apparent as the cells become more depolarized. The actual changes in membrane potential can be estimated by using the Nernst equation to calculate membrane potential at various external potassium concentrations (68). 

Quantitatively, the usual resting membrane potential of — 70 mV Is close to but lower In magnitude than that of the potassium equilibrium potential calculated from the Nernst equation because of the presence of a few open Na channels. These open Na channels allow the net Inward flow of Na" ions, making the cytosolic face of the plasma membrane more positive, that is, less negative, than predicted by the... 

It would appear that changes in the intracellular or extracellular concentration of potassium ions markedly alter the resting membrane potential. For this reason, neurophysiologists treated an excitable cell as if it were an electrochemical, or Nernst, cell. The resting potential for one permeant species could therefore be explained by the familiar Nernst equation ... 

The transmembrane potential at the peak of the action potential can be predicted from the Nernst equation by substituting appropriate sodium ion concentrations for those of potassium ions. 

Whether these discrepancies are due to a lower activity coefficient of potassium inside of the cell as compared with the activity coefficient outside of the cell, or whether they are due to leakage of potassium outside and penetration of sodium inside the cell is not certain. Better agreement between theoretical and actual values can sometimes be obtained by calculating the potential by the Goldman equation rather than the Nernst equation. The Goldman equation does not express the membrane potential in function of the intracellular movements of potassium, but it takes into consideration the participation of other cations, particularly sodium. Thus, in Goldman s equation the potential is expressed as a function of the relative permeabilities and the electrochemical gradients of each ion. 

The first section of Table II represents the transepithelial parameters. The fluid/plasma K activity ratio of 1.9 0.2 is significantly greater than unity (P < 0.01) but is not significantly different from an inulin ratio of 1.5 0.2 in the latter part of the proximal tubule of the Necturus kidney. The potassium equilibrium potential (Ej ) calculated from the Nernst equation using the K" " activity ratio yield a value of 16.4 1.0 mV. This is not significantly different from the mean transepithelial PD of 14.5 A 1.5 mV. Thus K ion is in electro-chemical equilibrium distribution across the proximal tubular epithelium of Necturus. 

Chloride activity measurements are done in the same manner as the potassium measurements, except that the Nernst equation was used instead of the Nicolski equation to describe the potential difference due to the difference of chloride activities between the inside and outside of the cell. We know that there are no interfering ions in the solution on the outside. We assume that there are no interfering anions inside the cell. This assumption is necessary because we simply do not have any information about what ions are present and what their activities are. 

---