Diffusion, facilitated, definition

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. 

Facilitated transport has been briefly described in Chapter 1. In facilitated transport, the selective transport medium is a liquid or molten salt contained or immobilized in a porous support. The liquid membrane is held tightly in the support pores by capillary forces. The liquid or molten salt selectively reacts with a gas or vapor species and the reacting species diffuses across the liquid or salt and desorbed on the other side of the facilitated transport membrane. The major advantage of the facilitated transpoa is that diffusion is generally several orders of magnitude faster than diffusion through solid membranes. The support is, therefore, not a membrane by definition. Comprehensive... 

The original definition of facilitated diffusion given by Danielli related to solute translocation through non-aqueous membranes, but there is no difference in principle between facilitated diffusion in aqueous and non-aqueous media. Eqns. 4-6 are therefore of general interest as a summary of... 

To solve this inconsistency, one should definitely take into account the discrete ionic structure of the double layer. In Levich s theory, use is made of a one-dimensional potential distribution model so that the anion diffusing toward the electrode everywhere meets the same repulsion field. If the discrete nature of charges in the double layer is taken into consideration, certain parts of the repulsion field turn out to be stronger than the average, while other parts of this field, near the cations, are weaker. As a result, directions appear in which anion diffusion is facilitated. No strict quantitative theory of diffusion in a discrete double layer has been elaborated so far, but semiquantitative evaluations of these effects, made by Krylov, indicate that diffusion of anions in the model of a discrete double layer should, in fact, be much faster than in a one-dimensional model, and the above discrepancy is removed at least for doubly charged anions. 

Since fn(t) goes to 0 for all n as t increases, the total fraction of solute in the slab goes to 1. Comparison of solute sorption with different diffusivities and for slabs of different thickness is facilitated by definition of a dimensionless time, T ... 

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