Question

If a cell with a resting membrane potential of -70 mV is treated so that the resting membrane potential is at the equilibrium potential of K+, and then the permeability of the membrane for all ions is returned to normal: why WONT K+ move into the cell across the cell membrane until the resting membrane potential is restored but Na+ will?

Answer 1

At rest, there is small conductance (leakage) of K+ down its concetration gradient (i.e towards the outside of the cell) This creates a negative voltage (resting potential) inside the cell. At rest the membrane potential is about -70mV

As potassiums flow out due to the concentration gradient, the inside of the cell becomes more negative, which attracts the potassium (K+) back inside the cell because opposite charges attract and the potassium ion channels are always open. When the electrical gradient is pulling something in is as strong as the concentration gradient pushing it out, then it reaches equilibrium. The electrical polarity on the inside is about 70-90mV and it’s strong enough to keep the potassiums in.

This is occurring because only potassium ion channels are open. If sodiums were able to move in, then this wouldn’t happen, but remember potassiums are the only ones that can move out of the cell. This is what creates the electrical polarity.

1) The concentration gradient causes K+ to diffuse out of the cell.

2) The electrical gradient that develops, causes K+ to diffuse back into the cell.