Cotransport process

Several types of cells are equipped with carrier proteins to transport essential nutrients such as glucose and amino acids that cannot cross the plasma membrane freely because of their hydrophilicity. Intestinal and renal epithelia have long been known to possess specialized Na+ cotransport processes for glucose [205], amino acids [206], and di- and tripeptides [207],... 

In many epithelia Cl is transported transcellularly. Cl is taken up by secondary or tertiary active processes such as Na 2Cl K -cotransport, Na Cl -cotransport, HCOJ-Cl -exchange and other systems across one cell membrane and leaves the epithelial cell across the other membrane via Cl -channels. The driving force for Cl -exit is provided by the Cl -uptake mechanism. The Cl -activity, unlike that in excitable cells, is clearly above the Nernst potential [15,16], and the driving force for Cl -exit amounts to some 2(f-40mV. 

Ion-dependent solute transport processes such as Na+-glucose and Na+-amino acid cotransporters can be identified in epithelial tissues by observing an elevation in /sc following solute addition in Na+-containing but not Na+-free... 

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Additionally, amino acids may be reclaimed as dipeptides. The transport mechanisms for dipeptides are less specific than those for individual amino acids but require the dipeptide to carry a net positive charge so there is cotransport of protons, rather than of Na+ as for free amino acids. A potential advantage of dipeptide transport process is the favourable cell-lumen concentration gradient, which exists for peptides compared with free amino acids. 

The resorption process is facilitated by the large inner surface of the intestine, with its brush-border cells. Lipophilic molecules penetrate the plasma membrane of the mucosal cells by simple diffusion, whereas polar molecules require transporters (facilitated diffusion see p. 218). In many cases, carrier-mediated cotransport with Na"" ions can be observed. In this case, the difference in the concentration of the sodium ions (high in the intestinal lumen and low in the mucosal cells) drives the import of nutrients against a concentration gradient (secondary active transport see p. 220). Failure of carrier systems in the gastrointestinal tract can result in diseases. 

Alternatively, both the first and the second solutes may pass through the membrane bound to the same carrier (cotransport or symport). Another form of active transport is group translocation, a process in which the substance to be transported undergoes covalent modification, e.g., by phosphorylation. Tire modified product enters the cell and within the cell may be converted back to the unmodified substance. Transport processes, whether facilitated or active, often require the participation of more than one membrane protein. Sometimes the name permease is used to describe the protein complexes utilized. 

Copper-sulfide cluster 884s Coproporphyrin III 843,845s Comified cell envelope 439 Corrin in transmethylation 592 Corrin ring 867, 868 Corrinoid-dependent synthesis of acetyl-CoA 876, 877 Cosmarium 22 COSY-NOESY diagram 143 Cotransport (symport) process 411,416,417 Coulomb 283... 

It is clear that numerous facilitated transport processes may still be set up, especially for anions, salts or neutral molecules, and that the active research in receptor chemistry will make available a variety of novel carrier molecules. Of special interest are those transport effectors, derived from coreceptors, that allow coupled transport (cotransport) to be performed. 

Fig. 4. Ion-driven cotransport mechanisms, (a) Symport process involving a symporter (e.g. Na+/glucose transporter) (b) antiport process involving an antiporter (e.g. erythrocyte band 3 anion transporter).

Like the 5-HT1A receptor (see Section 2.1), the 5-HT2A receptor can regulate several transport processes. The 5-HT2A receptor activates the type 1 sodium-proton exchanger (NHE-1) in renal mesangial cells (187,227) and vascular smooth muscle cells (222), the Na+K+-AIPase (sodium pump) in airway smooth muscle cells (228), and the Na+/K+/2Cr cotransporter when 5-HT2A receptor transfected... 

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