ABSTRACT
Na+ and Cl-ions are present at higher concentrations in the extracellular fluid, and K+ ions and large negatively charged ions (anions) like proteins are present at higher concentration inside the cell. These differential concentrations of Na+
and K+ ions in the intracellular and extracellular compartments are set up by the action of specific ion pump and ion channel proteins embedded in the plasmalemma. The most important of these is the Na+/K+ ATPase or Na+/K+ pump. This pump is at work in most cells almost all of the time, and serves to pump Na+ out of the cell and K+ into the cell, a process that is dependent on a supply of ATP from cellular respiration. Three Na+ ions are exchanged for two K+ ions in each cycle of the pump, and so it is slightly electrogenic, making the extracellular environment slightly more electropositive than the intracellular space. In most cells, this pump is fed by K+ ions that can leak slowly out of the cell through a K+-specific ion channel embedded in the membrane, but membrane permeability to other ions is low because other ion channels (those for Na+, for example) are usually closed at rest. At any rate, the permeability of the plasmalemma to K+ is about 100-fold higher than for any other ion at rest.
