Facilitated transport systems

In the liver, the bilirubin is removed from albumin and taken up at the sinusoidal surface of the hepato-cytes by a carrier-mediated saturable system. This facilitated transport system has a very large capacity, so that even under pathologic conditions the system does not appear to be rate-limiting in the metabolism of bilirubin. 

Since this facilitated transport system allows the equilibrium of bilirubin across the sinusoidal membrane of the hepatocyte, the net uptake of bilirubin will be dependent upon the removal of bilirubin via subsequent metabolic pathways. 

Carrier-mediated passage of a molecular entity across a membrane (or other barrier). Facilitated transport follows saturation kinetics ie, the rate of transport at elevated concentrations of the transportable substrate reaches a maximum that reflects the concentration of carriers/transporters. In this respect, the kinetics resemble the Michaelis-Menten behavior of enzyme-catalyzed reactions. Facilitated diffusion systems are often stereo-specific, and they are subject to competitive inhibition. Facilitated transport systems are also distinguished from active transport systems which work against a concentration barrier and require a source of free energy. Simple diffusion often occurs in parallel to facilitated diffusion, and one must correct facilitated transport for the basal rate. This is usually evident when a plot of transport rate versus substrate concentration reaches a limiting nonzero rate at saturating substrate While the term passive transport has been used synonymously with facilitated transport, others have suggested that this term may be confused with or mistaken for simple diffusion. See Membrane Transport Kinetics... 

Although lipophilic and able to diffuse across the cell membrane TH can enter cells more rapidly by facilitated transport systems that tend to be specific for T4 or T3. Based on studies on isolated hepatocytes and RBCs, TH uptake in fish can be blocked by inhibitors of protein binding (phloretin, bromosulphothalein, 5,5 -di phenylhydan-toin and 8-anilino-1-naphthalene sulphonic acid) and by certain sulphydryl blockers (p-hydroxymercuribenzoate and /V-cthylmaleimide)59,63,64. 

The porosity of the support refers to the percentage of the total volume which is void space. The porosity determines the total volume of the liquid membrane which can be immobilized in the pore volume. The volume of liquid membrane solvent and the carrier solubility determine the maximum amount of carrier which can be immobilized in the membrane. Increasing the amount of carrier in the membrane will increase the solute fluxes. The strength of the functional dependence of the solute flux on carrier concentration will depend on whether the facilitated transport system is reaction or diffusion limited. Consequently, a high porosity support is desirable for liquid membrane applications. 

Besides transport through complexation with the carrier, it also occurs nonspecific gas transport through the solvent phase in which the carrier is dissolved. Therefore, the characteristic of facilitated transport is the occurrence of a reversible complexation process in combination with the diffusion process. Two different cases have to be taken in account (i) diffusion is rate limiting (fast reaction), (ii) reaction is rate Hmiting (slow reaction and relatively fast diffusion) [2]. The second case does not occur frequendy and only the first one wiU be considered. The selective transport of gases in facilitated transport systems can be described in analogy to a dual-mode transport mechanism [5, 6, 8]. 

The number of gases for which suitable carriers are currently available is smaU and most effort has been devoted to the cleanup of acid gases. Many studies on facilitated transport of gases such as O2, CO2, H2S, SO2, NO, and CO through liquid membranes are reported in literature. The first studies on facilitated transport systems for different gaseous permeants are reported in Table 7.4. 

Table 7,4 First studies on facilitated transport systems for different gaseous permeants...

There are two primary transport systems in neurons facilitated and active. Facilitated transport systems do not require metabolic energy from the cell. These systems are believed to be carrier mediated, since the presence of carriers can account for the kinetic findings observed in facilitated transport systems, such as saturability, competition by other substrates, and specificity toward certain families of molecules. Facilitated transport systems can... 

Because the driving force for diffusion is a concentration gradient, active transport pumps, such as the sodium-potassium pump, create gradients of these two ions that are continually (though slowly) degraded by diffusion. Note in Table 10.6 that sodium and potassium ions do not have facilitated transport systems, so their permeability constants are very low. 

Facilitated transport (or facilitated diffusion) - Includes pore-facilitated transport and carrier-facilitated transport systems. One notable feature of facilitated transport systems is that even though the driving force is also the process of diffusion and the end result is the same as diffusion, facilitated transport systems speed up diffusion by a factor of up to 10,000,000-fold. 

Pore-facilitated transport - Band 3 protein of the erythrocyte is an example of a pore-facilitated transport system (Figure 10.20a). It contains a highly specific channel to transport bicarbonate ions out of cells as it transports chloride ions in. Note that the net charge difference in the transport is zero, so there is no electrical polarization of the membrane. 

Carrier-facilitated transport - Valinomycin, an antibiotic, is an example of a carrier-facilitated transport system. It contains a hydrophobic exterior for interacting with the hydrophobic portion of the membrane s lipid bilayer and an interior designed specifically to accommodate a potassium ion. It transports by the mechanism depicted in Figure 10.20b. 

Figure 5.11 Facilitated transport proteins in cell membranes. Unlike simple diffusion through the membrane bilayer, facilitated transport systems become saturated as the solute concentration difference increases. In this hypothetical example, the permeability of the membrane in simple diffusion is 0.4 (units of flux/concentration).

Both facilitated and simple diffusion depend on concentration gradients net solute transport always occurs from high to low concentration. Unlike diffusion, facilitated transport systems are specific, since they depend on binding of the solute to a site on the transport protein. For example, the D-glucose... 

Figure 5.12 Transport proteins in cell membranes, (a) Energy-dependent, ATP-powered ion pumps such as the Na /K exchange ATPase (b) channels, gated or non-gated, which permit diffusion through an aqueous pathway, such as voltage-gated Na channels (c) passive, facilitated transport systems, which can act in uniport, symport, or antiport modes, such as the glucose transporter. The filled arrow indicates gradient of molecules indicated by the filled symbol.

Both facilitated transport systems and active transport mechanisms can be saturated, and the rates of transport are similar. In addition, both carrier-mediated transport systems have less transport capacity than channel-mediated transport or simple diffusion (Table 5.6). 

Regulating Permeability by Active or Facilitated Transport Systems... 

This overview chapter has the objective of introducing the SyiQ>oslum Series volume and the subject of liquid membrane technology. If membranes are viewed as semi-permeable phase separators, then the traditional concept of membranes as polymer films can be extended to Include liquids and liquid-swollen polymers. The addition of a mobile complexatlon agent to the membrane Is known as facilitated liquid membrane separation. Often, In liquid phase facilitated transport systems, the solute flux Is coupled to the opposite flux of another species. This process, common in metal ion recovery schemes, is known as coupled transport. 

These predictions were made for 200 um membranes. Industrial application of this technology will require the use of membranes which are two orders of magnitude thinner. In order to use the model to predict facilitation factors for thinner membranes, it is necessary to determine whether the reaction equilibrium assumption still applies. The parameter (tanh )/ has a value of 0 if the system is diffusion limited and 1 if the facilitated transport system is reaction rate limited. At a thickness of Ipm, the value of (tanh X)/X is of the order 10 , which implies that the system is diffusion limited and that the simplified analytical model can be used to predict facilitation factors. If the solubility of HjS, the pressure and temperature dependence of the equilibrium constant and the diffusion coefficients are known, then F could be estimated at industrial conditions. 

Kimura and Walmet (11) and other investigators (14) have also reported that in facilitated transport systems, Qqq values were found to Increase as APm.. was lowered. This is probably due to the fact that... 

The Approximate Solution. Recently, Teramoto (75) developed an approximate solution for the facilitation factor in facilitated transport membranes where a reaction A(permeant) + B(carrier) = C(co plex) occurs. Very recently, this approximation method was extended to the facilitated transport system where reaction (a) occurs in the membrane (77). It was confirmed that this approximation method provides... 

We will now identify some other facilitated transport systems of considerable interest Separation of CO2 from inert gases like O2, N2, CH4 has been achieved by using alkaline-concentrated HCOj/COj solutions ... 

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