You may find the following online simulation useful when calculating the GHK equ

Question

You may find the following online simulation useful when calculating the GHK equation: http://www.nemstgoldman.physiology.arizona.edu/. The simulator also shows ion concentrations and this can be a useful tool in helping you to visualize what happens when a variable is altered. However, make sure that you also know how to solve the equation with your calculator. Given the following concentrations inside and outside the giant squid axon: The relative are: P_k: P_Na: P_Cl = 1: 0.04: 0.45 a. Use the GHK equation to determine how much CI contributes to V_r? b. Explain your answer to given that P_Clis relatively large and E_cl = -60 mV.

Explanation / Answer

1) Using GHK equation,

Em,Cl-=RT/zF ln{ pcl[cl-]out/pcl-[cl-]in}=RT/F ln{ [cl-]out/[cl-]in}

Where Em=membrane potential in volts

pCl-=permeability of cl- ion

[Cl-]out=concentration of Cl- outside the cell

[Cl-]in =concentration of Cl- inside the cell

Z=charge transfer=-1 in case of Cl-

R=ideal gas constant=8.314 J/k.mol

T=37 deg C (body temperature)=37+273=310K

F=faraday’s constant=9.6*10^4C/mol=9.6*10^4 J/mol.V

RT/F=61.5 mV/z

Em,Cl-=-61.5 mV log { [cl-]out/[cl-]in}

     =-61.5 mV log 560/52=-61.5 mV (1.032)=-63.48 mV

2) According to the GHK equation the contribution to Vt (voltage ) by any ion depends on the difference of its concentration inside and outside the cell and  pCl- or permeability ,it is large so Cl- can easily move in and out of the cell .As both pCl- and concentration gradient is large so its contribution to Vt or membrane potential is maximum.

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