<P> Maintenance of the resting potential can be metabolically costly for a cell because of its requirement for active pumping of ions to counteract losses due to leakage channels . The cost is highest when the cell function requires an especially depolarized value of membrane voltage . For example, the resting potential in daylight - adapted blowfly (Calliphora vicina) photoreceptors can be as high as - 30 mV . This elevated membrane potential allows the cells to respond very rapidly to visual inputs; the cost is that maintenance of the resting potential may consume more than 20% of overall cellular ATP . </P> <P> On the other hand, the high resting potential in undifferentiated cells can be a metabolic advantage . This apparent paradox is resolved by examination of the origin of that resting potential . Little - differentiated cells are characterized by extremely high input resistance, which implies that few leakage channels are present at this stage of cell life . As an apparent result, potassium permeability becomes similar to that for sodium ions, which places resting potential in - between the reversal potentials for sodium and potassium as discussed above . The reduced leakage currents also mean there is little need for active pumping in order to compensate, therefore low metabolic cost . </P> <P> As explained above, the potential at any point in a cell's membrane is determined by the ion concentration differences between the intracellular and extracellular areas, and by the permeability of the membrane to each type of ion . The ion concentrations do not normally change very quickly (with the exception of Ca, where the baseline intracellular concentration is so low that even a small influx may increase it by orders of magnitude), but the permeabilities of the ions can change in a fraction of a millisecond, as a result of activation of ligand - gated ion channels . The change in membrane potential can be either large or small, depending on how many ion channels are activated and what type they are, and can be either long or short, depending on the lengths of time that the channels remain open . Changes of this type are referred to as graded potentials, in contrast to action potentials, which have a fixed amplitude and time course . </P> <P> As can be derived from the Goldman equation shown above, the effect of increasing the permeability of a membrane to a particular type of ion shifts the membrane potential toward the reversal potential for that ion . Thus, opening Na channels pulls the membrane potential toward the Na reversal potential, which is usually around + 100 mV . Likewise, opening K channels pulls the membrane potential toward about--90 mV, and opening Cl channels pulls it toward about--70 mV (resting potential of most membranes). Because--90 to + 100 mV is the full operating range of membrane potential, the effect is that Na channels always pull the membrane potential up, K channels pull it down, and Cl channels pull it toward the resting potential . </P>

Concentration of cl inside and outside the cell