General Question Regarding RMP Permeability and the Nernst Eq'n

[Messier83]

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For the scope of the MCAT and using the Nernst equation, under resting conditions do we consider the membrane to be permeable only to Potassium?

I realize that there is a slight permeability of the membrane to sodium which gives a slightly less negative RMP, but for our purposes do we ignore that in our RMP calculations using the Nernst equation?

FYI this is in reference to Passage 45, Q2 in the TPR Science Workbook (Biology).

 

[inasensegone]

For the scope of the MCAT and using the Nernst equation, under resting conditions do we consider the membrane to be permeable only to Potassium?

I realize that there is a slight permeability of the membrane to sodium which gives a slightly less negative RMP, but for our purposes do we ignore that in our RMP calculations using the Nernst equation?

FYI this is in reference to Passage 45, Q2 in the TPR Science Workbook (Biology).

First of all... it is super unlikely that you will have to use the nernst equation for much else other than a qualitative type question: e.g., if you change the concentration of X, how is the EMF affected? Increase, decrease, no change, etc.

Now in regards to that passage, it is true that we have a slightly less negative RMP than the nernst equation would lead us to calculate... this is in part due to the slight permeability of sodium ions. In fact, the nernst equation would give a value of approximately -90 mV while the actual value is closer to -70 mV. So what does this mean for the MCAT? Well if they're going to ask you about this, you can decide on the applicability of sodium permeability in the nernst equation based off of what they give you in the passage. The TPRH S/W passage was kind of misleading and not really helpful in this matter. In fact, I'm beginning to think that most of their "Advanced Passages" are just convoluted and not necessarily more difficult. Anyway, I digress... the nernst equation is used to calculate the EMF of just one ion to which the membrane is permeable... not slightly permeable. Because it is just used for one ion, you won't have to worry about sodium if they ask you about potassium. The Goldman-Hodgkin-Katz equation is used to calculate the RMP that results from the concentrations of all the influential ions, but that almost definitely won't be on the MCAT.

TL;DR fuhgeddaboutit

 

[Messier83]

First of all... it is super unlikely that you will have to use the nernst equation for much else other than a qualitative type question: e.g., if you change the concentration of X, how is the EMF affected? Increase, decrease, no change, etc.

Now in regards to that passage, it is true that we have a slightly less negative RMP than the nernst equation would lead us to calculate... this is in part due to the slight permeability of sodium ions. In fact, the nernst equation would give a value of approximately -90 mV while the actual value is closer to -70 mV. So what does this mean for the MCAT? Well if they're going to ask you about this, you can decide on the applicability of sodium permeability in the nernst equation based off of what they give you in the passage. The TPRH S/W passage was kind of misleading and not really helpful in this matter. In fact, I'm beginning to think that most of their "Advanced Passages" are just convoluted and not necessarily more difficult. Anyway, I digress... the nernst equation is used to calculate the EMF of just one ion to which the membrane is permeable... not slightly permeable. Because it is just used for one ion, you won't have to worry about sodium if they ask you about potassium. The Goldman-Hodgkin-Katz equation is used to calculate the RMP that results from the concentrations of all the influential ions, but that almost definitely won't be on the MCAT.

TL;DR fuhgeddaboutit

cogent. thanks BenFranklinWithASweetStache.

So moral of the story: slightly permeable is NOT considered permeable unless stated in the passage. It must be fully permeable to use Nersnt (if they ever force you to).

 

[inasensegone]

cogent. thanks BenFranklinWithASweetStache.

So moral of the story: slightly permeable is NOT considered permeable unless stated in the passage. It must be fully permeable to use Nersnt (if they ever force you to).

Haha.

Pretty much... I took issue with the TPRH explanation as well when I did this passage so I read a medical physiology textbook. It basically said that the membrane isn't solely permeable to Potassium ions... so while the nernst equation, as utilized solely with Potassium would yield -90 mV, this doesn't factor in the other ions. (i.e. sodium and chloride also exert an effect)

I hate the half-as*ed explanations in TPRH. They're almost as bad as AAMC's explanations. After using TBR, they feel wildly insufficient. TBR sometimes might go a little TOO detailed in their explanations, but at least they're there if you need them.

Anyway, the RMP is largely affected by Potassium, Sodium, and Chloride. The membrane is significantly more permeable to potassium than sodium, but TPRH is almost dismissive in regards to sodium.

This is in regards to the RMP as affected by Na, K, Cl... hopefully this helps to clarify what my long-winded explanation tried to convey:

Also, notice how this paragraph seemingly invalidates TPRH's answer to that question.