Question

When the membrane potential of a cell is equal to the equilibrium potential of sodium (+60mV), the intracellular concentration of sodium is smaller than the extracellular concentration.

I'm probably just missing a tiny link. But I can't seem to understand why this is WRONG.

Will the sodium concentration be equal on both sides?

thx in advance!

Answer 1

Answer:

Equilibrium potential is basically membrane potential (potential inside of cell compared to outside) which would result in no met movement of a particular charge. In the above case, we know the fact that equilibrium potential of sodium is around +60mV. So basically what does this mean?

Actually, concentration of sodium is more extracellularly and less intracellularly. This concentration gradient is also maintained by Na+/K+ ATPase which actively pumps sodium outside against the concentration gradient. As the sodium is more extracellularly, it has the tendency to have influx into the cell. However, if somehow, we make membrane potential more positive, specifically around +60mV, then despite of having higher concentration extracellularly, there will be no net movement of sodium inside the cell. This is because the positive evaluation outside the cell now somewhat matches the inside of cell.

Therefore, equilibrium potential doesn't necessarily mean equal concentration of sodium on both the sides. It's actually no net movement of the charge due to charge distribution on both the sides in such a way which inhibits their natural tendency to move either into (sodium) or out (potassium) of the cell.