13.12:
Transcellular Transport of Solutes
Transcellular transport is a cellular process that allows the movement of substances across polarized cells, such as endothelial and epithelial cells, with biochemically distinct membrane compositions on two opposite sides.
It is observed during nutrient absorption in the small intestine and selective transport of solutes across the blood-brain barrier.
For instance, the epithelial lining has a compact arrangement of cells. The tight junctions between these cells block solute movement between the cells.
So, solutes like monosaccharides, amino acids, and ions are transported across the cell with the help of membrane transport proteins present on the opposite sides of the cell.
Solutes can enter or leave the cell via three important ways, depending on the type of cell and the solute being transported.
Transcellular diffusion, where solutes such as glucose and fructose can diffuse from higher to lower concentrations.
Active transport, where solutes like calcium and copper ions are transported against their concentration gradient using energy from ATP hydrolysis.
Transcytosis, where solutes such as albumin, and lipoproteins are transported via vesicles across the cell.
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.
Suggested Reading
- Chen, Lihong, Biguang Tuo, and Hui Dong. "Regulation of intestinal glucose absorption by ion channels and transporters." Nutrients 8, no. 1 (2016): 43.
- Fung, Karen YY, Gregory D. Fairn, and Warren L. Lee. "Transcellular vesicular transport in epithelial and endothelial cells: Challenges and opportunities." Traffic 19, no. 1 (2018): 5-18.
- Fronius, Martin, Wolfgang G. Clauss, and Mike Althaus. "Why do we have to move fluid to be able to breathe?." Frontiers in physiology 3 (2012): 146.