The cell membrane, made up of roughly 70% lipids and 30% protein, serves as an effective barrier to maintain the differences in composition between the intracellular and the extracellular space. The majority of the energy required to sustain cellular function is expended in maintaining large differences in electrolyte ion concentrations across the cell membrane. The lipid bilayer of the membrane serves this barrier role remarkably well by establishing a nonpolar region through which an ion must pass to cross the membrane. The energy required to move a hydrated ion from the aqueous phase into the nonpolar lipid phase is extremely high, thus providing a strong impediment to passive ion diffusion across the lipid bilayer. On the other hand, many of the membrane proteins facilitate and regulate membrane ion transport. The presence of these proteins thus enhances the ability of the mammalian cell membrane to actively transport certain ions.