All cells have an electrical potential difference across their membranes. That is, if you place an electrode in the cytosol on the inside of a cell and connect it to an electrode in the extracellular fluid, you will find a difference in electrical energy between these two spaces that will produce a flow of electrons in the connecting wire. This potential difference, called the resting membrane potential, can be measured.

The resting membrane potential is present in all cells, regardless of type, and can constitute a definition of life for cells - if the cell lacks a membrane potential it is dead! A lot of the chemical machinery of cells is involved with the maintenance of this membrane potential.

A block diagram of the ions in a model cell is shown at left. As can be seen, there is quite a difference between the chemical composition of the inside (cytosol) and the outside (extracellular fluid or ECF) of a cell. The relative concentration of each ion is shown by the height of its portion of the block. It is important to note that the height of the positive and negative ions' columns are the same. This shows that the positive and negative ions are always perfectly balanced. It is also important to understand that these differences are actively maintained and regulated by the cell. The relative imbalance in concentration of sodium (Na+) and potassium (K+) between the cytosol and ECF is what produces the electrical potential difference across the cell membrane - The Resting Membrane Potential.

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