Diffusive channels

Nikaido, H. Porins and specific diffusion channels in bacterial outer membranes. J. Biol. Chem. 269 3905-3908, 1994. 

Tybsre m is the total mass of analyte collected, D the molecular diffusion coefficient, A the area of the diffusion channel, L the diffusion path length, C the analyte concentration in the air, and Tt the sampling time. In deriving equation (8.7) it was assumed t. that the sorbent is effective sink for the analyte and,... 

Figure 13. The magnetoresistor chip is covered with a flow cell for the introduction of sample after immobilization of an array of DNA probes. The photo shows a diffuser channel placed over the 65 magnetoresistor sensors on the chip54.

Facilitated diffusion (channel), molecule moves down its electrochemical gradient. Active transport (pump) molecule moves up its electrochemical gradient (requires energy input). Pumps use energy (usually ATP hydrolysis). Na+ high outside/K+ high inside. 

Luckey, M. and Nikaido, H. (1980). Specificity of diffusion channels produced by lambda phage receptor protein of Escherichia coli, Proc. Natl Acad. Sci. USA, 11, 167-171. 

Phagocytosis is an important mechanism for the organism to rid itself of bacteria and pathogenic material, as well as cell debris and remnants of apoptosis. However, it can also provide a route for the uptake of pollutant particulate material. It is seen to be especially important in the incorporation of airborne particulate material, which often has serious health consequences (see Section 6.4). In terrestrial invertebrates, food is obtained either from particulate matter in the soil or from molecules dissolved in interstitial water. Most of these organisms have extracellular digestion, with nutrients and foreign material being absorbed by one or more of the routes available for transport across membranes, such as diffusion, channels or pinocytosis. There have been few studies to establish which route is taken. 

Facilitated diffusion within organisms takes place when carriers or proteins residing within membranes—ion channels, for instance—organize the movement of ions from one location to another. This diffusion type is a kinetic, not thermodynamic, effect in which a for the transfer is lowered and the rate of diffusion is accelerated. Facilitated diffusion channels organize ion movements in both directions, and the process can be inhibited both competitively and noncompetitively. It is known that most cells maintain open channels for K+ most of the time and closed channels for other ions. Potassium-ion-dependent enzymes include NaVK+ ATPases (to be discussed in Section 5.4.1), pyruvate kinases, and dioldehydratases (not to be discussed further). 

The parameters D and Dk > whether for macro (denoted by subscript m) or for micro (denoted by subscript ju) regions, are normal bulk and Knudsen diffusion coefficients, respectively, and can be estimated from kinetic theory, provided the mean radii of the diffusion channels are known. Mean radii, of course, are obtainable from pore volume and surface area measurements, as pointed out in Sect. 3.1. For a bidisperse system, two peaks (corresponding to macro and micro) would be expected in a differential pore size distribution curve and this therefore provides the necessary information. Macro and micro voidages can also be determined experimentally. 

The first process for which, by direct experiment, the electron donor and the electron acceptor particles were proved to be at a large distance from each other at the moment of reaction and for which, in addition, any possibility of a diffusion channel for the decay of the reagents was excluded, appeared to be the reaction between the trapped electron, et , and the anion radical, O, in a vitreous 10 M NaOH water-alkaline solution. Let us examine the results of research on this reaction in more detail. 

At still higher temperatures (above 160 K), close to that of matrix devitrification, the decay of etr appears to proceed along the diffusion channel. This conclusion can be made on the basis of the results obtained from the kinetic studies of the reactions of et with additives, electron acceptors in the same water-alkaline matrices (see Sect.4). 

Let us use the obtained values of the diffusion coefficients D in water-alkaline glasses to estimate the contribution of diffusion to the decay of etr by reaction with acceptors at low temperatures. Let us estimate, for example, the temperature at which, for a typical concentration of acceptor additive N = 10 2M and for a maximal time of observation t = 106s, the condition 4nRDDNt = 0.01 [or, which is the same thing, exp( —4nRDDNt) = 0.99] will be fulfilled, i.e. the decay of et by the diffusion channel will amount to 1%. Taking into account the abnormally high mobility of solvated electrons [114] it is reasonable to assume that the main contribution to D is made by the diffusion of elr rather than by that of the acceptor. In this case, all the values of D obtained above must be related to the same process, the diffusion of e. ... 

Additional functionalities at electrodes appear in connection with an opportunity of formation on a surface of a bottom the gas diffusion channel a Pd-film (inset 8 on Fig. 2). Formation a Pd-film is made before removal of mask Si02 (a position e , Fig. 3). But for all that, the height of prominent hollow-pins should not exceed necessary thickness a Pd-film. Presence of this film allows applying as fuel not only pure hydrogen, but also hydrogen containing mixes. 

In a general context, suppose a given volume V contains a spatially homogeneous mixture of Ni particles from m different populations of initial size noi (i = 1,..., m). Suppose further that these m populations can interact through to0 specified reaction or diffusion channels Ri (1 = 1,..., m0). These processes... 

Intercellular Volume and Penetration. The surface coats of epidermal cells occupy intercellular space and these gelatinous layers probably act as watery diffusion channels for nutrients (35). Surface coats are quite different from keratinized intercellular cement (32, 43) kera-tinization modifies desmosomes and intercellular materials to resist diffusion and premature separation in SC (32, 43, 44). 

Examples. Interactions between solvents and SC are shown in Figure 20 (106). The rapid and effective transfer of nonionized scopolamine from chloroform into SC suggests that cell membranes and the 80-A coatings are saturated with the nonpolar solvent to become diffusion channels for the drug. Both the base and the salt of scopolamine are absorbed into SC much jnore slowly from water or N,iV-dimethylformam-ide than from chloroform. These polar solvents are unlikely to dissolve in the 80-A layer or to form diffusion channels for the lipid-soluble scopolamine base. 

Fig. 5(0).— The orientation of diffusion channels in chabazite, in relation to hexagonal and rhombic unit cells.

It thus seems clear that tracer diffusion in zeolites involves the sticking of water to certain preferential positions—e.g., in clusters around cations—and to variations of this stickiness, largely of electrostatic origin, along the diffusion channels. There is an approximate in-... 

One of the main obstacles in the migration of large cations such as Na, K+, or NH4 + (not Li + or H +) in rigid lattices is a too-small size of bottlenecks, or diffusion channels, between the cation sites, as shown above for the perovskite type (Figure 7.4). This factor is most critical for high-connectivity structures having D = 3 or 2. For D = 1 no data are available, while for D = 0 this factor may sometimes be of minor importance as small isolated ions (e.g., SO4 ) may rotate in the crystal structures, providing the so-called paddle-wheel mechanism [1, 2, 5, 61]. 

Figure 19.3 An idealization of a material where interdiffusion of components can occur with limited facility at all points but with greater facility along identifiable high-diffusion channels. In such a material, nonhydrostatic stress can produce a difference in composition at the channel wall between channels of one direction and channels of another direction.

Variations of the size distribution of aerosol particles as a function of relative humidity can be observed in situ in the air. Changes in particle size distributions are detected by optical counters (Laktionov and Bogomolov, 1972), by nephelometers (Charlson et al., 1969) or by diffusion channels (Sinclair et al., 1974). 

Channel proteins transport water or specific types of Ions and hydrophilic small molecules down their concentration or electric potential gradients. Such protein-assisted transport sometimes is referred to as facilitated diffusion. Channel proteins form a hydrophilic passageway across the membrane through which multiple water molecules or ions move simultaneously, single file at a very rapid rate. Some ion chan-... 

Slow rerarrangement of molecules can be due to long diffusional paths, constricted diffusion channels in a solid matrix or diffusion through a dense or highly viscous material. Diffusion of KCl and chlorinated benzenes through immobile fluid for the unmodified versus organic-modified porous... 

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