Transporters and Pumps
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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).
KeywordsTransport Rate Passive Transport Glut Transporter Directional Diagram Transport Cycle
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Suggestions for Further Reading
- •.Free Energy Transduction and Biochemical Cycle Kinetics, Terrell L. Hill. There are many books that introduce the basic concepts of transport rates. However, this book stands out in explaining both the basic concepts and the diagrammatic method in a concise and complete fashion. Hill also has numerous more advanced books on the subject (Hill 1977).Google Scholar
- •.The Fluctuating Enzyme, edited by G. Rickey Welch. For a somewhat higher level discussion of transporter kinetics, see the chapter “Not Just Catalysts: Molecular Machines in Bioenergetics” by G. Rickey Welch and Douglas B Kell (Welch and Kell 1986).Google Scholar
- •.Electrogenic Ion Pumps, Peter Läuger. This chapter gives a development of the basic physical chemical basis behind many of the electrogenic ion pumps covered here (Läuger 1991).Google Scholar