|MadSci Network: Medicine|
There are transporter "pumps" to transport specific compounds across the plasma membrane in almost every cells in the body. They can either make use of the energy derived from ATP hydrolysis or depend on the so- called "co-transportation" mechanisms to "pump" ions or proteins against their electrochemical gradients. In case of the renal tubular epithelial cell, the co-transporters use energy derived from ONE ion running DOWN the electrochemical gradient to power another UPHILL transport so that the ion concerned can be concentrated on one side of the membrane. If the transport is in the same direction it is called symporter, otherwise it is termed antiporter. Notice that in co-transportation, the electrochemical gradient of the species running downhill (usually hydrogen ions or sodium ions) has to be eventually maintained by another counter-acting active tranport mechanism, usually by means of the sodium-potassium ATPase, so often it is also called secondary active transport. Just to give an example, in the luminal membrane of the epithelial cells in the renal proximal convoluted tubule, there is a symporter for glucose and sodium co- transportation, i.e. glucose and sodium are being transported into the epithelial cell in the same direction. Quite obviously, glucose is transported actively against the concentration gradient using the energy derived from sodium running down its gradient. The electrochemical gradient of sodium, on the other hand, is maintained by the sodium- potassium ATPase in the baso-lateral membrane. For more details, you can consult any of the standard physiology textbook, e.g. Guyton & Hall, 'Textbook of Medical Physiology', Saunders, gives a very detailed account. Joshua Chai Medical Student University of Cambridge
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