13.12: Transcellular Transport of Solutes
Transcellular transport of solutes is the movement of substances like monosaccharides and amino acids through polarized cells. This transport mechanism is primarily seen in epithelial and endothelial cells aided by membrane transport proteins such as channels and transporters. The tight junctions between these cells confine the membrane proteins to the two sides of the cell. The epithelial cells have distinct apical and basolateral domains. In contrast, the endothelial cells show the luminal and abluminal domains, defining the opposite ends for the entry and exit of solutes through these cells. The membrane transport proteins confined on the apical region of the epithelial cells or the luminal side of the endothelial cells help incorporate the solute into the cell. The transport proteins at the basolateral region of the epithelial cells (or the abluminal side of the endothelial cells) allow the solute to leave from the opposite end.
Examples of the different cellular processes in the body that use this transport mechanism include processes for nutrient movement, water balance, and selective transport of substances from the blood to the brain.
The transepithelial transport of glucose from the intestinal lumen to the bloodstream takes place through the intestinal epithelial cells. The key players in the transport process are the sodium-glucose transporters and glucose uniporters present on the intestinal epithelial cells. This transport becomes crucial when severe dehydration threatens homeostasis during diarrhea or vigorous exercise. Drinking only water does not hydrate the body fast enough or thoroughly enough because it is excreted from the gastrointestinal tract almost as soon as it enters. Therefore, a sugar-salt rehydration formula is administered that allows the sodium and sugar transepithelial uptake. This leads to an osmotic gradient that causes water to be drawn from the intestinal lumen through the epithelial layer into the blood rehydrating the body.
In the lungs, maintenance of the air-blood barrier is critical for breathing. The fluid (water) volume covering the alveolar epithelium of the lungs must be maintained to facilitate gaseous exchange (carbon dioxide and oxygen) between the blood and inhaled air. This alveolar fluid is maintained by the transepithelial transport of sodium, chloride, and potassium ions that help regulate the water influx. Malfunctions of the ion transport processes that impair the lungs' fluid balance are associated with severe diseases, such as pulmonary edema and cystic fibrosis.
The blood-brain barrier formed by the tightly packed endothelial cells containing transporters helps move molecules into the brain for therapeutic purposes or the normal functioning of the neurons.