In biological cells that are electrically "at rest," the cytosol possesses a uniform electric potential or voltage of about - 0.01 V or -100 mV compared to the extracellular solution. This voltage is the resting cell potential, also sometimes called the transmembrane potential of the resting cell. Cells whose voltage is more negative than typical are said to be hyperpolarized, and those more positive are said to be depolarized. Healthy cells do not naturally hyperpolarize or depolarize except for brief intervals, for example during an action potential. Among other roles, the cell potential acts as a reservoir for metabolic energy, which cells use to drive the transport of solute molecules across the membrane, to communicate with other cells and to trigger intracellular events.
Between the inside and outside of the cell (which like the cytosol is typically uniform electrically) the voltage rises very steeply just at the boundary created by the membrane. This gives rise to the transmembrane electric field, which exerts a force on ions and controls voltage-gated ion channels. Integral membrane proteins such as channels, pumps, and exchangers establish the membrane potential by transporting specific ions in or out. In essence, resting cells are negative because positively charged potassium ions, which are more concentrated inside than outside, are allowed to leak out. The resulting negative voltage difference between inside and out is therefore approximately equal to the reversal potential for potassium. Sodium-potassium exchangers maintain intracellular potassium at a high concentration while pumping sodium into the extracellular solution, where the concentration of sodium typically is high.
A reservoir for metabolic energy
While cells expend energy to transport ions and establish a transmembrane potential, they use this potential in turn to transport other ions and metabolites such as sugar. The transmembrane potenial of the mitochondria drives the prodution of ATP, which is the common currency of biological energy.
Cell potential changes
Cells may draw on the energy they store in the resting potential to drive action potentials or other forms of excitation. These changes in the membrane potential itself can enable communication with other cells--as in the case of the impulses that travel nerves--or initiate changes inside the cell that undergoes them--such as the changes in an egg when it is fertilized by a sperm.
Excitation involves a rush of sodium ions into the cell through sodium channels, resulting in depolarization, while recovery involves an outward rush of potassium through potassium channels. Both these fluxes occur by passive diffusion and tend to neutralize the concentration differences painstakingly established by the sodium-potassium exchanger and other pumps. As a result, cells cannot become excited many times in quick succession, and they require both time and metabolic energy to recover.