Nutrients transport mechanisms

U Kompella, KJ Kim, VHL Lee. (1992). Active ion and nutrient transport mechanisms of the pigmented rabbit conjunctiva. Pharm Res 9(Suppl) 3. 

A simplified diagram representing the various reservoirs and transport mechanisms and pathways involved in the cycles of nutrient elements at and above the surface of the Earth is given in Eigure 1. The processes are those considered to be the most important in the context of this article, but others of lesser significance can be postulated. Eor some of the elements, notably carbon, sulfur, chlorine, and nitrogen, considerable research has been done to evaluate (quantitatively) the amount of the various elements in the reservoirs and the rates of transfer. 

Eig. 1. Generalized cycle of the various reservoirs and transport mechanisms and pathways involved in the circulation of nutrient elements. The numbered arrows represent processes by which elements transfer among the reservoirs. Processes shown are those considered to have the most important influence... 

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... 

Most hydrophilic, or water-soluble, substances are repelled by this hydrophobic interior and cannot simply diffuse through the membrane. Instead, these substances must cross the membrane using specialized transport mechanisms. Examples of lipid-insoluble substances that require such mechanisms include nutrient molecules, such as glucose and amino acids, and all species of ions (Na+, Ca++, H+, Cl, and HC03). Therefore, the plasma membrane plays a very important role in determining the composition of the intracellular fluid by selectively permitting substances to move in and out of the cell. 

However, the receptor-mediated endocytosis of iron-transferrin studies [63] does not explain the initial uptake of iron from nutrients in the intestinal (villus) cells, since apotransferrin is generally not available in the lumen, except in a limited amount from biliary excretion. Work on other iron transport mechanisms has mainly been reported in the last five years. 

Primarily to elucidate transporter localization and function, vesicles enriched in trophoblast apical or basolateral membranes have frequently been utilized. To give a few instances, they have been used to investigate P-gp-mediated transport, mechanisms of transport of cationic compounds, drug interactions with nutrient transport, and differences in amino acid transport in pathological conditions of the placenta [36, 40-42], Briefly, for preparation of microvillus membrane vesicles the cord, amniochorion and decidua are removed from placenta, and the tissue cut on the maternal side. The mince is stirred to loosen... 

As mentioned in Section II, arsenate is present in seawater at a fairly uniform concentration (about 0.5-2 ng As/liter), and in nutrient-deficient waters its concentrations may exceed that of the essential phosphate (146). In oxygenated seawater, the species exist predominantly as H2PO4 and H2As04, and algae may absorb arsenate because it is similar to the phosphate anion (147). In terrestrial plants and other organisms, arsenic is taken up by the phosphate transport mechanism... 

It is assumed that transport processes for sugars arose early in the evolution of living systems since synthesis of simple sugars by the formol condensation mechanism would occur at an early stage on the primtive earth. At the level of the unicellular organism, development of transport systems for nutrients present in the environment would give competitive advantages when the supply became limited. It is presumed that development of specific and efficient transport mechanisms has been under... 

Stimulation of active H+ extrusion from roots (Cesco, 1995 Pinton et al., 1997 Table 9.1) and transmembrane potential hyperpolarization (Slesak and Jurek, 1988) indicated the involvement of the PM H+-ATPase in the increased nutrient uptake generally observed in the presence of humic substances. Direct proof of an interaction between humic molecules and the PM H+-ATPase has been obtained by Vara-nini et al. (1993), who demonstrated that low-molecular-weight (<5kDa) humic molecules at concentrations compatible with those present in the rhizosphere can stimulate the phospho-hydrolytic activity of this enzyme in isolated PM vesicles (Table 9.1). Further proof of the action of humic molecules on PM FT-ATPase activity and on nutrient uptake mechanisms was obtained when studying the effect of these molecules on NO3 uptake. Transport of this nutrient is a substrate-inducible process and involves FT co-transport. At higher uptake rates, the levels and activity of root PM FT-ATPase increased (Santi et al., 1995). The short-term (4h) contact... 

Such methods are used in more fundamental studies to elucidate the soil chemistry, to determine the structure and composition of soil components and to improve understanding of the processes in the soil that control the mobilisation and retention of nutrient and toxicant elements in soil as well as to illuminate their transport mechanisms. They are, therefore, more important for the soil physical chemist than the functionally defined procedures that are the main concern of the agronomist. Both methods are of major interest to the environmental scientist particularly in the study of the fate of environmental pollutants. Many of the extractants intended to target particular species are also used in a functional speciation role. 

The small intestine contains a wide variety of transporters (amino acid transporters, oligopeptide transporters, glucose transporters, lactic acid transporters etc.) on the apical membrane of the epithelial cells, which serve as carriers to facilitate nutrient absorption by the intestine. On the basolateral membrane, the presence of amino acid and oligopeptide transporters has been demonstrated. Active transport mechanisms for di- and tri-peptides have also been demonstrated in the nasal and buccal epithelia. 

Various transport mechanisms exist in the brain endothelium for the uptake of nutrients into the CNS. These transport mechanisms may also potentially take up drugs. The types of transport mechanisms available include ... 

Y He, Z., and Stoffells, P.J., Molecular mechanisms of heavy metal hyperaccumulation and phytoremediation, J. Trace Elem. Med. Biol. 18, 339-353, 2005 Mackenzie, S.A., Plant organellar protein targeting a traffic plan still under construction. Trends Cell Biol. 15, 548-554, 2005 Thompson, M.V., Phloem the long and the short of it. Trends Plant Sci. 11, 26-32, 2006 Takahashi, H., Yoshimoto, N., and Saito, K., Anionic nutrient transport in plants the molecular basis of the sulfate transporter gene family. Genet. Eng. 27, 67-80, 2006. 

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