Pharmacology Notes > BIOL10832 Excitable Cells Notes

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e-learning III - Nernst Equation

The Nernst Equation predicts when an electrical gradient (E) and a chemical gradient are in balance. It was originally formulated to describe the behaviour of electrochemical cells

(batteries).

In this equation, z is the charge on the ion, R is the ideal gas constant (8.314 J.K-1.mol-1),

F is the Faraday constant (9.65x104 C.mol-1), T is the temperature in Kelvin (273 K = 0 ˚C)

and E is the equilibrium potential for the ion (in V).

Use this information to determine the equilibrium potential at 21 ˚C, for Ca 2+in a cell where

[Ca2+ outside] = 5 mM and [Ca2+ inside] = 0.1 mM.

E=

8.314 ×(21+273)

5 ×10−3

[

] = 0.0495V (49.5 mV)

ln 2 ×9.65 ×10 4

0.1 ×10−3

At 37˚C, the equilibrium potential for Cl- in a cell is -61.5 mV. If the intracellular concentration of Cl- is 15 mM, what is the concentration of chloride outside the cell?

-61.5x10-3 V =

x 8.314 ×(37+273)

]

ln[

4 15 ×10−3

−1 × 9.65× 10

-61.5x10-3 V = -0.0267 ln[

2.303 = ln[

x ]

15 ×10−3

x ]

15 ×10−3

e2.303 (10.0041) =

x 15× 10−3

10.0041 x 15x10-3 = 0.15M (150mM)

At 30˚C the equilibrium potential for ion X is 30.1 mV. The concentration of X outside the cell is 10 mM and inside the cell it is 1 mM. What is the valence of ion X?

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