Question #2: Membrane Potential

(permeable to a single ion species)
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In a hypothetical cell the intra and extracellular environments are as follows:

  inside outside
Sodium 50 440
Chloride 100 560
Calcium .4 10
Potassium 400 20

1. What would the membrane potential be if the membrane were only permeable to potassium? Be able to show your work (the Nernst potential alone does not give you Vm!). (-79.24mV)

2. What would happen to the membrane potential if the concentration of potassium in the extracellular media were doubled? (-60.90mV; depolarizes) Explain.

3. What would the membrane potential be if the membrane were only permeable to calcium? (+85.14mV)

 

Questions, Comments:

(4/3/98, 1:32:54 PM) (sbs8650@oberlin.edu) wrote:

You mention in your answer that the nerst potential alone does not give you Vm. But in this case, when the membrane is only permeable to K, since the permeabilities cancel out, isn't Vm equal to the Nernst potential?
(4/3/98, 1:34:17 PM) () wrote:

Why, in the GHK equation, are the outside concentrations for K, Na, and Ca in the numerator, while the inside concentration for Cl is in the denominator?
(4/5/98, 12:06:32 PM) () wrote:

Whoops, got that mixed up. why are the outside concentrations for K, Na, and Ca in the numerator, while the _outside_ concentration for Cl is in the denominator?
(4/5/98, 5:32:17 PM) AMB in response to #1 sbs8650 (fborroni@oberlin.edu) wrote:

When the membrane is only permeable to one ion Vm (the membrane potential) is infact equal to the Nerst Potential for that ion. However, it is due to the fact that the permeabilities cancel out in the GHK equation not because the Nernst potential is necessarily the same as Vm. If you begin the discussion of membrane potential believing that the Nernst potential is the membrane potential it is very difficult to understand how to calculate the membrane potential under conditions where the membrane is permeable to more than one ion. This is why I made the point of saying that the Nernst potential is not Vm but is 'used' to calculate Vm. Permeability to one ion is a special case where the GHK equation used to calculate membrane potential reduces to the Nernst potential for the permeable ion.
(4/5/98, 5:34:30 PM) AMB in response to question 2&3 (fborroni@oberlin.edu) wrote:

The reason for the reversal in the inside/outside ratio for chloride is because chloride has a negative charge and therefore behaves differently than the other ions. This odd behavior is taken into account in the equation by reversing the calculation of the inside/outside ratio.
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