Concentration gradient carrier-mediated transport

Ability to transport substances against a concentration gradient. Carrier-mediated active transport may be able to carry substances uphill —that is, from areas of low concentration to areas of high concentration. 

Depending upon the mechanism that is employed by the organism to accumulate the solute, internalisation fluxes can vary both in direction and order of magnitude. The kinetics of passive transport will be examined in Section 6.1.1. Trace element internalisation via ion channels or carrier-mediated transport, subsequent to the specific binding of a solute to a transport site, will be addressed in Section 6.1.2. Finally, since several substances (e.g. Na+, Ca2+, Zn2+, some sugars and amino acids) can be concentrated in the cell against their electrochemical gradient (active transport systems), the kinetic implications of an active transport mechanism will be examined in Section 6.1.3. Further explanations of the mechanisms themselves can be obtained in Chapters 6 and 7 of this volume [24,245]. 

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. 

Facilitated transport is essentially the same as carrier-mediated transport, except that besides a carrier molecule, another transport facilitator is essential. For example, vitamin B12 attaches to the intrinsic factor, and the vitamin B12-intrinsic factor complex then attaches to the carrier molecule and is transported. This transport process does not require energy and does not proceed against a concentration gradient (Figure 1.3). 

Facilitated diffusion involves carrier-mediated transport down a concentration gradient. The existence of the carrier molecules means that diffusion down the concentration gradient is much greater than would be expected on the basis of the physicochemical properties of the drag. A much larger number of substances are believed to be transported by facilitated diffusion than active transport, including vitamins such as thiamine, nicotinic acid, riboflavin and vitamin B6, various sugars and amino acids. 

For carrier-mediated transport [Fig. 6-11(a)] there must be a finite number of carrier molecules in the membrane. At low [S]c, only some of these molecules will be bound to the solute, but at high [S]0, most of the carrier molecules will be occupied, and there is therefore a maximal value for v (Fmax). In simple diffusion [Fig. 6-11(6)1, there is no carrier to saturate, and v is higher at high [SJ0 because the concentration gradient of solute across the membrane, which determines the rate of diffusion, is greater than it is at low [S]0. 

As the preceding question shows, there are two types of carrier-mediated transport (1) facilitated diffusion (which allows the concentration of solute on both sides of a membrane to be equalized) and (2) active transport (which allows the solute to move up, or against, a concentration gradient). 

Fab portion (Fab fragment) That part of the antibody molecule containing the antigen binding site, facilitated diffusion Carrier-mediated transport of molecules along a concentration gradient across the cell membrane with no expenditure of energy, facilitation Increase in responsiveness of a post-synaptic membrane to successive stimuli. 

FIGURE 9.5 Graph showing initial velocity of transport processes across lipid membranes. Passive diffusion (compound dissolves directly into lipid membrane) is driven by a concentration gradient and is not saturable. In contrast, carrier-mediated transport is saturable, reaching a maximal rate when the carrier molecules are saturated with substrate. Transport proteins mediate these processes. 

Another form of facilitated diffusion involves membrane proteins called carriers (sometimes referred to as passive transporters). In carrier-mediated transport, a specific solute binds to the carrier on one side of a membrane and causes a conformational change in the carrier. The solute is then translocated across the membrane and released. The red blood cell glucose transporter is the best-characterized example of passive transporters. It allows D-glucose to diffuse across the red blood cell membrane for use in glycolysis and the pentose phosphate pathway. Facilitated diffusion increases the rate at which certain solutes move down their concentration gradients. This process cannot cause a net increase in solute concentration on one side of the membrane. 

Analytes that are derived from the blood plasma can be conveniently divided into those that are actively transported and typically maintained at homeostatic levels, as opposed to those that diffuse down concentration gradients. In between is carrier-mediated transport, which is rate limited typically by a saturable enzymatic transporter system. The passive diffusion of macromolecules such as proteins is influenced by the molecular sieving effect of various barriers that are constituted by basement membranes with varying degrees of effective pore size . 

Carrier-mediated transport involves cotransport of the absorbable species with a proton. The required proton gradient is hypothesized to be maintained by a Na+-H+ exchanger. The lumen of the intestine is acidic relative to the epithelial cell cytosol. The low cytosolic sodium concentration, required to produce the transporter driving force, is maintained by the Na K ATPase in the basolateral membrane. The sodium/proton exchanger working in concert with the sodium/potassium ATPase, therefore, results in a transport mechanism for the uptake of di- and tripeptides into the intestinal wall (Ganapthy and Leibach, 1985). 

The second nme of carrier-mediated transport is cotransport. Here a. solute A is transported through the membrane together with a solute B. Both solutes are located on the same side of the membrane and the driving force is the concentration gradient of one of the solutes, for example of B. This means that solute A can be transported even against its own concenuation gradient. 

In so far as transport into the brain is concerned, studies using rat-brain synaptosomes have demonstrated that there is a rapid influx of L-tryptophan which is temperature-dependent and partially inhibited by ouabain and cyanide. An intracellular gradient concentration of four is achieved and competitively inhibited by L-phenylalanine and para-chlorophenylalanine (pCPA). These facts strongly suggest a carrier-mediated transport. Such a transport process is easily modified by an extracellular amino-acid imbalance, as is seen in many types of congenital hyper-aminoacidemia, which in turn can affect intracellular concentrations of tryptophan and thereby alter the rate of 5-HT biosynthesis in the central nervous system (Fig. 6). 

Although the absorption of most drugs can be explained by passive diffusion, some compounds have specific transport mechanisms. An example is the absorption in the intestine of some penicillin derivatives, e.g. cyclacillin (1 aminocyclohexylpenicillin). This process is saturable, proceeds against an unfavourable concentration gradient and shows temperature dependence. Transport of amoxicillin is also carrier mediated but it is not an active process. Since these materials are xenobiotics, the transport mechanism is probably one which serves some other function in the body. The two penicillins probably share the same carrier since they are mutually competitive. Digitalis and other cardioselective glycosides also demonstrate behaviour not compatible with simple partition theory which suggests carrier-mediated transport. 

Several other conditions can provoke this reverse pump type of release. One is when the transmembrane ionic gradient is reversed. Experimentally this is achieved by reducing extracellular Na+. Because the neuronal uptake of monoamines from the synapse by the transporter requires co-transport of Na+ and Cl , reversing the ionic gradient (so that the Na+ concentration is lower outside, than inside, the terminals) will drive the transporter in the wrong direction. Such carrier-mediated release could explain the massive Ca +-independent release of noradrenaline during ischaemia which increases intracellular Na+ concentration and reduces intracellular K+. 

As described above, because MAO is bound to mitochondrial outer membranes, MAOIs first increase the concentration of monoamines in the neuronal cytosol, followed by a secondary increase in the vesicle-bound transmitter. The enlarged vesicular pool will increase exocytotic release of transmitter, while an increase in cytoplasmic monoamines will both reduce carrier-mediated removal of transmitter from the synapse (because the favourable concentration gradient is reduced) and could even lead to net export of transmitter by the membrane transporter. That MAOIs increase the concentration of extracellular monoamines has been confirmed using intracranial microdialysis (Ferrer and Artigas 1994). 

Historically, the absorption of lipid-soluble nutrients has been considered to be carrier-independent, with solutes diffusing into enterocytes down concentration gradients. This is true for some lipid-soluble components of plants (e.g. the hydroxytyrosol in olive oil Manna et al., 2000). However, transporters have been reported for several lipid-soluble nutrients. For example, absorption of cholesterol is partly dependent on a carrier-mediated process that is inhibited by tea polyphenols (Dawson and Rudel, 1999) and other phytochemicals (Park et al., 2002). A portion of the decreased absorption caused by tea polyphenols may be due to precipitation of the cholesterol associated with micelles (Ikeda et al., 1992). Alternatively, plant stanols and other phytochemicals may compete with cholesterol for transporter sites (Plat and Mensink, 2002). It is likely that transporters for other lipid-soluble nutrients are also affected by phytochemicals, although this has not been adequately investigated. 

Facilitated transport combines some properties of both mechanisms discussed above. This type of transport is carrier mediated so that there is substrate specificity, a transport maximum, and competitive inhibition. However, facilitated transport is not energy-dependent and is unable to transport a substrate against a concentration gradient. 

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