Very simply Action potential past paper question

Yes, you’re right, I tried to simplify it for your level but since you’re asking I will go into it. The sodium-potassium pump is important in MAINTAINING the resting membrane potential. Yes, the misconception is that the resting membrane potential (usually around -70mV) is due to the fact that the sodium-potassium pump pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell, thereby making the inside of the cell more negative relative to the outside - this is FALSE. This mechanism only accounts for a few mV. The main reason that the resting membrane potential is around -70mV is because the membrane, at rest, is far more permeable (around 40 times more) to potassium ions than sodium ions. At rest, voltage-gated ion channels are closed however, so-called ‘leak channels’ are open, and allow sodium and potassium ions to move across the membrane. However, there are far more leak potassium channels than leak sodium channels, and this is why the membrane is more permeable to potassium ions.

Now consider the factors that determine the rate at which ions will move across the membrane. One of these factors is the electrochemical gradient of a particular ion (the difference in charge and concentration across the membrane) and the other is the permeability of the membrane to the ion.

Imagine a scenario where the membrane is permeable ONLY to potassium ions. Potassium ions would move from inside the cell to the outside via these leak channels via their concentration gradient. As a result, the outside of the cell would become more positive and the inside of the cell would become more negative (as positive ions are leaving the cell) - making the resting membrane potential more negative. Now, the build up of positive charge on the outside of the cell would begin to reduce the rate at which potassium ions are leaving the cell across the membrane as the positive charge outside the cell would repel the positively charged potassium ions. The membrane potential at which there would be no net flow of potassium ions (the equilibrium potential) would be around -84mV.

Now imagine if the membrane were only permeable to sodium ions. Sodium ions would move from the outside to the inside of the cell (down its concentration gradient). This would make the inside of the cell more positive relative to the outside, however, the build up of sodium ions inside the cell would make the membrane potential positive. This would then begin to slow down the movement of sodium ions from the outside to the inside of the cell. The membrane potential at which the chemical and electrical forces (due to the concentration and electrical gradient) would be balanced, and therefore there would be no net flow of sodium ions, would be around +62mV.

However, since the membrane is more permeable to potassium, the resting membrane potential is much closer to the equilibrium potential of potassium (-80mV) rather than sodium (+62mV). It’s quite complicated and I’m not sure if I have explained it very well, but hopefully that gives you an idea of the reason why the resting membrane potential is the value that it is.