Transporters and Pumps

Abstract

Ionic channels are not the only mechanism that cells use to transport impermeant species across membranes. Cells have developed a great variety of transport proteins for moving both ions and molecules from one cellular compartment to another. For example, to maintain the concentration imbalance of Na⁺, K⁺, and Ca²⁺ across the plasma membrane it is necessary to pump ions against significant concentration gradients. In the case of Ca²⁺ ions the ratio of concentrations outside to inside is greater than four orders of magnitude (ca. 2 mM outside and 0.1 μM inside). In addition to pumps, there are numerous specific cotransporters and exchangers that allow ions and small molecules to be transported selectively into internal compartments or out of the cell. Unlike ionic channels, for which the driving force is a passive combination of electrical potential and ionic concentration differences, most transporters and pumps expend considerable energy. In many animal cells, for example, it has been estimated that nearly 25% of the ATP that is utilized is devoted to maintaining low cytoplasmic Na⁺ and high cytoplasmic K⁺ concentrations via Na⁺/K⁺ pumps (Cooper 1997).