[File:Basis of Membrane Potential2.png|thumb|right|350px|Differences in the concentrations of ions on opposite sides of a cellular membrane lead to a voltage called the membrane potential.

Typical values of membrane potential are in the range +40 mV to –70 mV. Many ions have a concentration gradient across the membrane, including  potassium (K⁺), which is at a high concentration inside and a low concentration outside the membrane.  Sodium (Na⁺) and chloride (Cl^-) ions are at high concentrations in the extracellular region, and low concentrations in the intracellular regions. These concentration gradients provide the potential energy to drive the formation of the membrane potential. This voltage is established when the membrane has permeability to one or more ions.

In the simplest case, illustrated here, if the membrane is selectively permeable to potassium, these positively charged ions can diffuse down the concentration gradient to the outside of the cell, leaving behind uncompensated negative charges. This separation of charges is what causes the membrane potential. Note that the system as a whole is electro-neutral. The uncompensated positive charges outside the cell, and the uncompensated negative charges inside the cell, physically line up on the membrane surface and attract each other across the lipid bilayer. Thus, the membrane potential is physically located only in the immediate vicinity of the membrane. It is the separation of these charges across the membrane that is the basis of the membrane voltage.

Note also that this diagram is only an approximation of the ionic contributions to the membrane potential. Other ions including sodium, chloride, calcium, and others play a more minor role, even though they have strong concentration gradients, because they have more limited permeability than potassium.

Key: pentagons – sodium ions; squares – potassium ions; circles – chloride ions; rectangles – membrane-impermeable anions (these arise from a variety of sources including proteins). The large structure with an arrow represents a transmembrane potassium channel and the direction of net potassium movement.]]