Cell Facilitated diffusion

Simple diffusion probably accounts for the uptake of undissociated organic acids by the yeast cell. Facilitated diffusion may be involved in the transport of sugars [17], although the fact that some are taken up against a concentration gradient could indicate that active transport occurs [18]. Active transport processes are used to transport amino acids and the ions of potassium, magnesium, phosphorus and sulphate. 

Calcium is absorbed from the intestine by facilitated diffusion and active transport. In the former, Ca " moves from the mucosal to the serosal compartments along a concentration gradient. The active transport system requires a cation pump. In both processes, a calcium-binding protein (CaBP) is thought to be required for the transport. Synthesis of CaBP is activated by 1,25-DHCC. In the active transport, release of Ca " from the mucosal cell into... 

Mutations in bacteria and mammalian cells (including some that result in human disease) have supported these conclusions. Facilitated diffusion and active transport resemble a substrate-enzyme reaction except that no covalent interaction occurs. These points of resemblance are as follows (1) There is a specific binding site for the solute. (2) The carrier is saturable, so it has a maximum rate of transport (V Figure 41-11). (3) There is a binding constant (Al) ) for the solute, and... 

Certain solutes, eg, glucose, enter cells by facilitated diffusion, along a downhill gradient from high to low concentration. Specific carrier molecules, or transporters, are involved in such processes. 

Unphosphorylated functioning according to Fig. 5 catalyzes facilitated diffusion of mannitol across the membrane. The same process has been reported for purified II reconstituted in proteoliposomes [70]. The relevance of this activity in terms of transport of mannitol into the bacterial cell is probably low, but it may have important implications for the mechanism by which E-IIs catalyze vectorial phosphorylation. It would indicate that the transmembrane C domain of Il is a mannitol translocating unit which is somehow coupled to the kinase activity of the cytoplasmic domains. We propose that the inwardly oriented binding site which is in contact with the internal water phase (Ecyt Mtl, see Fig. 5) is the site from where mannitol is phosphorylated when transport is coupled to phosphorylation. Meehan-... 

By far the most complete study of the kinetics of mammalian passive glucose transporters has been done on the GLUT-1 isoform in the human erythrocyte. The transport of glucose in this cell type is a classic example of facilitated diffusion, the... 

All-trans lycopene is rapidly isomerized to an equilibrium mixture with its cis isomers both in cell culture medium (Liu et al. 2006) and in vivo in prostate tissue (Clinton et al. 1996, van Breemen et al. 2002). The cis isomers of lycopene are absorbed better than the all -trans isomers when fed to humans (Unlu et al. 2007). The 5 -cis isomer predominates in plasma (Gustin et al. 2004). Since lycopene absorption by prostate cells might be due to facilitated diffusion (Liu et al. 2006), it is likely that the cis isomers of lycopene form a significant proportion of intracellular lycopene in the experiments that are reviewed later. 

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. 

Dipeptides and tripeptides are also presented to the brush border of the absorptive cells. As the nutrient molecules are absorbed, aminopeptidases split them into their constituent amino acids. The activity of aminopeptidases accounts for approximately 60% of protein digestion. The amino acid molecules then exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Formation of Na+ channels in the luminal membrane of the tubular epithelial cells (facilitates passive diffusion of Na+ ions into the cell)... 

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 of glucose across the blood-brain barrier and into brain cells is catalyzed by GLUT1,- 2 and -3, products of the SLC2 gene superfamily 90... 

Cell membranes or synthetic lipid vesicles with normal low permeability to water will, if reconstituted with AQP1, absorb water, swell and burst upon exposure to hypo-osmotic solutions. The water permeability of membranes containing AQP 1 can be about 100 times greater than that of membranes without aquaporins. The water permeability conferred by AQP1 (about 3 billion water molecules per subunit per second) is reversibly inhibited by Hg2+, exhibits low activation energy and is not accompanied by ionic currents or translocation of any other solutes, ions or protons. Thus, the movement of water through aquaporins is an example of facilitated diffusion, in this case driven by osmotic gradients. 

Conditional stability constants have been determined for cadmium binding to humic acid in freshwater, log Kk 6.5 [27], which may be comparable to binding to humic acid coated particles. The experiments demonstrated the importance of cadmium uptake from particles rather than from the dissolved phase. The authors recognised that the overall conclusion was similar to previous studies [28], but there remain inconsistencies in the uptake levels which may be related to the heterogeneity of the systems. Uptake from the intestine into the mucosal cells was not investigated. It was presumed that the material was digested extracellularly by hydrolytic enzymes and the released metal was taken up by facilitated diffusion. 

Despite the advantages offered by CNTs and CNFs, there are still many obstacles (cost, synthesis methods) to overcome to allow large-scale production. Another type of catalyst support material is mesoporous carbon that provides high surface area and conductivity [100, 141]. It can be classified into ordered (OMC) and disordered (DOMC) mesoporous carbon [100], OMCs have been extensively used as catalyst support materials for fuel cells [140,142-146], The large surface area and 3D connected monodis-persed mesospheres facilitate diffusion of the reactants, making them very attractive materials as catalyst supports [100]. 

Facilitated diffusion within organisms takes place when carriers or proteins residing within membranes—ion channels, for instance—organize the movement of ions from one location to another. This diffusion type is a kinetic, not thermodynamic, effect in which a for the transfer is lowered and the rate of diffusion is accelerated. Facilitated diffusion channels organize ion movements in both directions, and the process can be inhibited both competitively and noncompetitively. It is known that most cells maintain open channels for K+ most of the time and closed channels for other ions. Potassium-ion-dependent enzymes include NaVK+ ATPases (to be discussed in Section 5.4.1), pyruvate kinases, and dioldehydratases (not to be discussed further). 

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. 

Figure 4-6. Mechanism of facilitated diffusion mediated by a glucose transporter. This is an example of uniport. The reversible interconversion between conformations of the transporter in which the glucosebinding site is alternately exposed to the exterior and interior of the cell is called a ping-pong mechanism.

Free T4 and T3 can enter cells by carrier-mediated facilitated diffusion or active transport. After gaining access to the cell interior, T4 may undergo 5 -monodeiodina-tion to yield T3. The T3 thus mixes with T3 entering the... 

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