Carrier-mediated transport facilitated diffusion

Carrier-Mediated Transport Facilitated Diffusion and Active Transport... 

Gibbs-Donnan equilibrium determines the concentration difference across simple membranes made of polymers, porous ceramic media, and other ultrafiltration devices. However, the difference of ion concentrations across the membranes of living cells and nerves is more complicated because of the existence of ion pumps as a result of carrier-mediated or facilitated diffusion, so that the concentrations of some ions are not in thermodynamic equilibrium. For example, there is a much higher sodium con- centration outside cells than there is inside, while the reverse is true for potassium ions. This occurs because there is a carrier (probably a lipoprotein) that binds with a sodium ion inside the cell, transports the ion across membrane, and then releases it into the fluid outside the cell. The carrier is then transformed and binds with a potassium ion, which is then transported into the cell. This mechanism is discussed in courses... 

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. 

Carrier-mediated transport (or facilitated diffusion) consists of the transfer of a substrate across a membrane, facilitated by a carrier molecule located in the membrane. It is a cyclic process comprising four steps (1) formation of the carrier-substrate complex at one interface (2) diffusion of the complex through the membrane phase (3) release of the substrate at the other interface (4) back diffusion of the free carrier. 

Out of this concept grew the cardinal idea of carrier mediated transport. Necessary for this was the development of a more coherent theoretical analysis built upon the general notion of facilitated diffusion. The major insight here came from Widdas who proposed in 1952 that carrier mediated transport would explain earlier data such as the transport of glucose across the sheep placenta, as well as his own observations on glucose entry into the erythrocyte. There were three assumptions made in developing this quantitative hypothesis ... 

In SLM extraction, the transport mechanism is influenced primarily by the chemical characteristics of the analytes to be extracted and the organic liquid in the membrane into which the analytes will interact and diffuse. Analyte solubility in the membrane and its partition coefficient will have the main impact on separation and enrichment. Analyte transport in SLM extraction can be substantially categorized into two major types one is diffusive transport (or simple permeation) and the other covers facilitated transport (or carrier-mediated transport).73... 

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. 

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). 

In Section 3.2 we introduced the basic processes of advection, diffusion, and drift, by which material is transported in biophysical systems. In this chapter we focus on a specialized class of transport transport across biological membranes. Transport of a substance across a membrane may be driven by passive permeation, as described by Equation (3.60), or it may be facilitated by a carrier protein or transporter that is embedded in the membrane. Thus transport of substances across membranes mediated by transporters is termed carrier-mediated transport. The most basic way to think about carrier proteins or transporters is as enzymes that catalyze reactions that involve transport. 

The distinction between facilitated diffusion through channels and carrier-mediated transport is somewhat artificial/ but may be justified on the basis of specificity. For example/ 3-lactams in general can pass through nonselective bacterial outer membrane porin (e.g./ OmpF) channels via passive diffusion/ whereas imipenem (and related zwitterionic carbapenems) can also utilize OprD channels/ which preferentially recognize basic amino acids and dipeptides. The identification of mutants that selectively confer imipenem resistance suggests that more intimate protein-drug associations are involved in carrier-mediated transport than in facilitated diffusion/ which may be limited only by pore diameter. 

The characteristic of a facilitated or carrier-mediated transport is the occurrence of a reversible chemical reaction or complexation process in combination with a diffusion process. This implies that either the diffusion or the reaction is rate limiting The total flux of a permeant A will thus be the sum of both the Fickian diffusion and the carrier-mediated diffusion as illustrated in Equation 4.19 [46] ... 

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. 

Facilitated or carrier-mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to fonn a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes the carrier can be chemically or physically bound to the solid matrix (Jixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier [2, 8], see Fig. 7.1. The state of the art of supported liquid membranes for gas separations will be discussed in detail in this chapter. 

Facilitated or carrier mediated transport is a transport process that combines a chemical reaction with a diffusion process. The solute has first to react with the carrier to form a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. 

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. 

Carrier mediated transport can be further divided into two subclasses based on the means by which the carriers move the substrate molecule across the membrane. The first type of carrier mediated transport is called facilitated diffusion. In this case substrate molecules bind to the transporter but the driving force for their transport across the membrane is still a favorable chemical gradient. In the second type of carrier... 

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). 

In molecular terms, the movement of an ion across a membrane via a channel can best be described as a series of "hops" along the stationary channel molecule. The ion-transporter interactions that facilitate ion translocation replace ion-water interactions on either side of the membrane. The energy barrier for direct ion diffusion across the membrane is directly due to the instability of the ion in the nonpolar membrane interior. lon-chatmel interactions significantly lower the barrier and thus facilitate diffusion.A similar argument applies to carrier-mediated transport, with the result that chaimels and carriers are kinetically indistinguishable. However, activation barriers for channel-mediated diffusion are expected to be much lower than the activation barrier for carriers. " with the result that the maximum transport rates of channels are typically several orders of magnitude higher than those of carriers. 

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