Membrane potential question

[deleted678432]

Hi.
I was doing homework and came across this question in my physiology class, hopefully somebody can help explain.

The question reads: True or False: If a positive ion is in higher concentration inside the cell than outside the cell, it will make the resting membrane potential more positive, even if the cell is not permeable to that ion.

The answer is false, yet I do not understand why. In my brain I see that: there are two compartments and one compartment has more positive charge than the other compartment. This means that the inside will be positive in relation to the outside, regardless of if the ions are diffusible or not, seeing as though the charges are already in the cell. Where does my terminology go wrong? Is permeability required, even if there is already an ion concentration that is seemingly leading to a membrane potential?

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[luisfigo]

Maybe you're forgetting the other ions. Just because potassium, for example, is higher inside the cell it doesn't offset the sodium outside, which along with the permeability of potassium it makes the resting membrane potential negative. Otherwise in an isolated condition with only one ion, I don't understand it.

 

[deleted678432]

Maybe you're forgetting the other ions. Just because potassium, for example, is higher inside the cell it doesn't offset the sodium outside, which along with the permeability of potassium it makes the resting membrane potential negative. Otherwise in an isolated condition with only one ion, I don't understand it.

So let me ask in other words. If there is say 50 positive ions inside but only 25 outside, yet the membrane is not permeable, why would there not be a membrane potential? Do you interpret this as what the question is asking?? Or is it asking about if this was in an actual real life cell?

 

[schmoob]

So let me ask in other words. If there is say 50 positive ions inside but only 25 outside, yet the membrane is not permeable, why would there not be a membrane potential? Do you interpret this as what the question is asking?? Or is it asking about if this was in an actual real life cell?

If there are 50 positive K ions inside the cell and 25 outside, but let's say there are also hypothetically 100 Na ions outside but only 50 inside. You still maintain a negative potential because you have 25 more extracellular cations.

Permeability does play a role as well.

EDIT: I wrote out a whole huge response that was better, but I accidentally hit the back button like a bozo. So let me try again...

Membrane potential (E) ⇌ permeability
If we are talking about cations here, we are discussing Na and K. Permeability is selective, so K, although bigger is more permeable (about 20X) because H2O tends to cluster around the smaller Na. So here is where membrane potential comes into play:

When 1 cation is permeable, we use the Nernst equation:

• If it is Potassium, then the potential shifts until equilibrium is reached when E(K+) = -90mV
• If it is Sodium, then the potential shifts until equilibrium is reached when E(Na+) = +60mV

Keep in mind the concentration gradients. If only K is permeable, it will go down its concentration gradient OUT of the cell, and make it more negative. The opposite goes for sodium; since the concentration is higher extracellularly, sodium will move to the area of lower concentration (inside) and create a more positive membrane potential.

With 2 ions permeable, we use the Goldman-Hodgkin-Katz equation. This is where we find out where the membrane potential shifts BASED ON THE PERMEABILITY.

• If Na+ is more permeable, it will shift in the positive direction towards +60mV. The opposite goes for K+.

So although the concentration of ions in your problem is different on either side of the membrane, there is no permeability so there is no change in membrane potential.

Hopefully this should answer your question. Not sure how in-depth your course is, but if you understand this concept, hopefully it will be solidified once you start looking at polarization. Feel free to ask any more questions.

P.S. If I totally botched this explanation, I would appreciate if someone can come in and correct my error; I would not want to provide bad information.