Transport of ions and water in epithelia: molecular insights Transport of ions and water in epithelia: molecular insights
Three epithelia are examined as a means of exploring molecular mechanisms of fluid and ion transport. The Malpighian tubules of insects are the site of urine production and can transport a variety of ions and organic compounds. The energy for these processes is generated by a hydrogen ATPase at the apical membrane while a Na+/K+ ATPase energizes the basal surface. The chloride cells of freshwater animals are the sites of active uptake of sodium and chloride ions from dilute media. These cells are also powered by hydrogen and Na+/K+ ATPases. Unlike the Malpighian tubules that transport isoosmotic fluid in an apical direction, chloride cells transport ions in a basal direction with little water following. The collecting tubules of mammalian kidneys can vary their osmotic permeability through the insertion and removal of aquaporin molecules. These epithelia incorporate many of the physiological specializations employed by animals to move fluid, transport ions, and modify osmotic permeability.
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