Perform the calculations and answer the questions on your ow

Perform the calculations and answer the questions on your own. If you have questions, ask your instructor (Kevin). Calculate the equilibrium potentials for each of the following ions. Assume your measurements are taken at 15 degree C. Show your calculations: [K^+]_out = 15 mM; [K^+]_in = 400 mM [Na^+]_out = 350 mM; [Na^+]_in = 30 mM [Ca^2+]_out = 20 mM; [ca^2+]_in = 0.08 mM [Na^+]_out = 20 mM; [Na^+]_in = 350 mM Use the information in the table below to determine the resting V_m for a squid axon that is at 18 degree C. Calculate V_m if the axon is 40 times more permeable to K^+ than to any other ion type. Determine the change in membrane potential that would be produced by increasing the extracellular K^+ concentration by a factor of five (5X). Assume the permeability of ions has not changed from (a). How has the V_m changed compared to your answer in (a)? Using the concentrations of ions in the table, what would V_m be if Na^+ permeability was increased by 1000 times? Assume the K^+ permeability is still 40 times greater than the permeability of Cl. In terms of electrical and chemical potential, comment on how the movement of K^+ and Na^+ ions across the plasma membrane were affected in the axon when the conditions changed from (b) to (c).

Solution

1. E_k = RT/nF ln ( X out /X in )

R (Rhydberg constant ) = 8.314 J/K ; F Faraday constant )= 9.6 X 10⁴ C/mol.

T = 273+15 = 288 K

RT / F = 25 mV

a) Kout = 15 mM ; Kin = 400 mM. n=1

Ek = 25 x ln (15 /400) =25 x-3.28 = - 82 mV.

b )Na out =350 mM ; Na in = 30.

Ek = 25xln (350/30) = 25 X 2.45 =62 mV .

c) Ca out = 20mM ; Ca in = 0.08 mM, n=2

Ek = (25/2) X ln (20/0.08) = 25 X 5.5 =137/2 =68.5 mV

d) Na out = 20 mM Na in =350 mM, n=1

Ek = 25 ln (20/350)= 25x -2.86 = -71.5 mV.