Nutrient absorption membrane transport processe

Several types of absorptive mechanisms exist for nutrients including active transport, passive diffusion, facilitated diffusion, and endocytosis. End-ocytosis occurs when the outer plasma membrane surrounds soluble or particulate nutrients in the GI tract and engulfs the contents. This process is similar to phagocytosis. 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

In addition to the passive diffusional processes over lipid membranes or between cells, substances can be transferred through the lipid phase of biological membranes through specialized systems, i.e., active transport and facilitated diffusion. Until recently, the active transport component has been discussed only for nutrients or endogenous substances (e.g., amino acids, sugars, bile acids, small peptides), and seemed not to play any major role in the absorption of pharmaceuticals. However, sufficient evidence has now been gathered to recognize the involvement of transporters in the disposition of pharmaceuticals in the body [50, 127]. 

Facilitated diffusion is very similar to passive diffusion with the difference that transfer across membranes is assisted by the participation of carrier proteins embedded in the membrane bilayer. Again, the direction of passage will be from the side of the membrane with high concentration of a chemical to the side with low concentration this also occurs without energy expenditure by the cell. Such a process is somewhat specific in the sense that it applies to molecules that are able to bind to a carrier protein. Absorption of nutrients such as glucose and amino acids across the epithelial membrane of the gastrointestinal tract occurs by facilitated diffusion. Since a finite number of carriers are available for transport, the process is saturable at high concentrations of the transported molecules and competition for transport may occur between molecules of similar structure. 

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