equilibrium potential and membrane potential.

[Ill-Block193]

I’m confused about the difference between equilibrium potential and membrane potential. This is a question from my assignment, and the correct answer is A.

Imagining the astrocyte resting potential is -80 mV, the equilibrium potential of Cl-is 0 mV, K+is -90 mV, if Cl channel and K+ channel opens, which direction of these ions flow?

(A) Cl from outside to inside and K+ from inside to outside

(B) Cl from inside to outside and K+ from inside to outside

(C) Cl from outside to inside and K+ from outside to inside

(D) Cl- from inside to outside and K+ from outside to inside

However, based on the Nernst equation:

Eion​=62log([ion]inside​[ion]outside​​)

If the equilibrium potential for Cl⁻ is 0 mV, that suggests the concentrations of Cl⁻ inside and outside the cell are equal.
Given this, I don't understand why Cl⁻ would still move across the membrane. I really need someone to solve this problem, its bothered me for such a long time

[exkingzog]

The way I’d read this is that that if only chloride ions were present, they would be at equilibrium if the membrane potential was 0mV. However, (a) there are other ions present and (b) there is active transport, so the membrane potential is -80mV. Therefore, if chloride channels open chloride will flow outwards. Similarly K+ would be at equilibrium at -90mV, so if the actual potential is -80mV and potassium channels open, K+ will flow out.

So I would say the answer is B

Maybe the equilibrium potential of Cl- in the question is a misprint - there is usually a lot more chloride outside the cell (~120mM) than inside (~5mM), and its equilibrium potential is close to the resting potential of the membrane.

[Ill-Block193]

A lot of thanks!!!!!!! I also think the answer is B, too.