The outside of a cell membrane is usually electrically charged or polarized with respect to the inside. This is due to an unequal distribution of ions on either side of the membrane and is of particular importance to the functions of nerve and muscle cells.
When nerve cells are at rest (that is, not conducting impulses), there is a relatively greater concentration of sodium ions (Na) on the outside of their membranes and a relatively greater concentration of potassium ions (K) on the inside.
In the cytoplasm of these cells, there are large numbers of negatively charged ions, including those of phosphate, sulfate, and protein, that cannot diffuse through the cell membranes.
On the other hand, the membranes are very permeable to potassium ions and only slightly permeable to sodium ions.
Consequently, in a resting cell, potassium ions tend to diffuse freely through the membrane to the outside, and sodium ions diffuse inward more slowly.
At the same time, however, the cell membrane expends energy to actively transport these ions in the opposite directions, preventing them from reaching equilibrium by diffusion.
Sodium ions are, therefore, actively transported outward through the membrane, and potassium ions are transported inward.
Since potassium ions can readily diffuse out again and sodium can enter only with difficulty, the net effect is for more positively charged ions to leave the cell than to enter it.
As a result, the outside of the membrane becomes positively charged with respect to the inside, which is negative.
The difference in electrical charge between the inside and the outside of the membrane is about -85 milli-volts and is called the resting potential. As long as the nerve cell membrane is undisturbed it remains in this polarized state.
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