Diffusion, facilitated

The transport mechanisms that operate in distribution and elimination processes of drugs, drug-carrier conjugates and pro-drugs include convective transport (for example, by blood flow), passive diffusion, facilitated diffusion and active transport by carrier proteins, and, in the case of macromolecules, endocytosis. The kinetics of the particular transport processes depend on the mechanism involved. For example, convective transport is governed by fluid flow and passive diffusion is governed by the concentration gradient, whereas facilitated diffusion, active transport and endocytosis obey saturable MichaeUs-Menten kinetics. 

Figure 11.1 Schematic examples of passive diffusion, facilitated transport and coupled transport. The facilitated transport example shows permeation of oxygen across a membrane using hemoglobin as the carrier agent. The coupled transport example shows permeation of copper and hydrogen ions across a membrane using a reactive oxime as the carrier agent...

Question The movement of an ionized solute across certain types of membranes does not always give rise to equal concentrations of the ion on both sides of the membrane, even when only simple diffusion, facilitated diffusion, or both are available. How can this be explained ... 

Glucose ATP out—A DP in H+ Na+ out—K+ in Glucose Glucose Facilitated diffusion Facilitated diffusion Primary active (uses ATP) Primary active (uses ATP) Secondary active (Na+ cotransport) Group translocation Most animal cells Mitochondria Stomach epithelia Animal cells Some animal cells Many bacteria... 

Weissenborn, D. L., Wittekindt, N., and Larson, T. J. (1992). Structure and regulation of the glpFK operon encoding glycerol diffusion facilitator and glycerol kinase of Escherichia coliK-12.J Biol. Chem. 267, 6122-6131. 

Characteristic Diffusion Facilitated diffusion Active transport... 

Facilitated diffusion has certain general characteristics. As already mentioned, the net flux is toward a lower chemical potential. (According to the usual definition, active transport is in the energetically uphill direction active transport may use the same carriers as those used for facilitated diffusion.) Facilitated diffusion causes fluxes to be larger than those expected for ordinary diffusion. Furthermore, the transporters can exhibit selectivity (Fig. 3-17) that is, they can be specific for certain molecules solute and not bind closely related ones, similar to the properties of enzymes. In addition, carriers in facilitated diffusion become saturated when the external concentration of the solute transported is raised sufficiently, a behavior consistent with Equation 3.28. Finally, because carriers can exhibit competition, the flux density of a solute entering a cell by facilitated diffusion can be reduced when structurally similar molecules are added to the external solution. Such molecules compete for the same sites on the carriers and thereby reduce the binding and the subsequent transfer of the original solute into the cell. 

Several types of absorptive mechanisms exist for nutrients including active transport, passive diffusion, facilitated diffusion, and endocytosis. End-ocytosis occurs when the outer plasma membrane surrounds soluble or particulate nutrients in the GI tract and engulfs the contents. This process is similar to phagocytosis. 

The placenta is both a transport and a metabolizing organ. Transport is accomplished by simple diffusion, facilitated diffusion, active transport across membranes, and by special processes such as pinocytosis, phagocytosis, specific transport molecules, and channels in the barrier . The placenta also contains a full complement of mixed function oxidases located in the microsomal and mitochondrial subcellular fractions capable of induction and metabolism of endogenous and exogenous chemicals. 

Optimum Composition Theory predicts that the optimum current-voltage performance is obtained with 33 vol% of ionomer, as validated by experiments [122, 123]. The optimum catalyst utilization is, however, found with 50 vol% of ionomer, as can be seen in Fig. 15. This discrepancy is due to nonoptimal gas diffusion. Facilitating oxygen diffusion would bring the two optima together. As mentioned before, a result of the statistical theory is that the maximum catalyst utilization does not exceed 40%. 

The three ways by which substances, including drugs, can cross cell membranes are simple diffusion, facilitated diffusion and active transport. 

Figure 7.5 (A) Time course of Internal drug concentration In a cell when exposed to a constant external concentration In which there Is simple diffusion, facilitated diffusion, or active transport. (B) Drug flux as a function of drug concentration for diffusion, active transport, or a combination of both diffusion and active transport Into a cell.

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. 

The different types of diffusion (facilitated versus simple diffusion) can be distinguished because facilitated systems have only a fixed number of sites through which transport occurs, but simple diffusion occurs across the entire surface of the cell. Thus, facilitated transport has a rate which is limited by the number of sites. Researchers attempting to increase the rate by increasing the concentration of transported molecule find that facilitated systems reach a maximum rate than cannot... 

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