The equilibrium potential for an ion is the
membrane potential at which there is no net movement of the ion. The flow of any inorganic ion, such as
Na+ or
K+, through an
ion channel (since membranes are normally impermeable to ions) is driven by the
electrochemical gradient for that ion. This gradient consists of two parts, the difference in the concentration of that ion across the membrane, and the voltage gradient. When these two influences balance each other, the electrochemical gradient for the ion is zero and there is no net flow of the ion through the channel; this also translates to no current across the membrane so long as only one ionic species is involved. The voltage gradient at which this equilibrium is reached is the equilibrium potential for the ion and it can be calculated from the
Nernst equation.
Mathematical models and the driving force We can consider as an example a positively charged ion, such as
K+, and a negatively charged membrane, as it is commonly the case in most organisms. The membrane voltage opposes the flow of the potassium ions out of the cell and the ions can leave the interior of the cell only if they have sufficient thermal energy to overcome the energy barrier produced by the negative membrane voltage. However, this biasing effect can be overcome by an opposing concentration gradient if the interior concentration is high enough which favours the potassium ions leaving the cell. An important concept related to the equilibrium potential is the
driving force. Driving force is simply defined as the difference between the actual membrane potential and an ion's equilibrium potential V_\mathrm{m}-E_\mathrm{i}\ where E_\mathrm{i}\ refers to the equilibrium potential for a specific ion. Relatedly, the membrane current per unit area due to the type i ion channel is given by the following equation: :i_\mathrm{i} = g_\mathrm{i} \left(V_\mathrm{m}-E_\mathrm{i}\right) where V_\mathrm{m}-E_\mathrm{i}\ is the driving force and g_\mathrm{i} is the
specific conductance, or conductance per unit area. Note that the ionic current will be zero if the membrane is impermeable to that ion in question or if the membrane voltage is exactly equal to the equilibrium potential of that ion. ==Use in research==