Uptake diffusion-limited

Figure 9. Schematic representation of concentration profiles at the biological surface in the case of a diffusion-limited uptake. Note that the ratio of bound metal to free metal is not drawn to scale in reality, the ratio at the biological surface is always larger than that in solution. The figure assumes that the total concentration of ligand is much greater than the total concentration of metal. For further details, refer to [142,331,333]...

If bound first by albumin, heme circulates until it is transferred to hemopexin (52). In vitro in the absence of hemopexin, nonspecific cellular uptake of heme by diffusion is facile (55), but as expected, the presence of hemopexin greatly slows uptake (54), since receptor-mediated uptake is necessarily slower and of lower capacity than diffusion-limited uptake. There is currently no evidence that either receptors for albumin or membrane transporters for heme, like those in prokaryotes, are present in the plasma membrane of mammalian cells, although such transport proteins may be present in the membranes of organelles. 

Thus, 74 is the experimentally measurable diffusion-limited uptake rate, expressed in terms of an effective mass accommodation coefficient. 

If the values of the effective self-diffusion coefficients, D rf [calculated from the complete xit) curves in TD NMR experiments, assuming diffusion-limited uptake (52)] are below the corresponding intracrystalline data, Dintra (measured directly by PFG NMR), the existence of additional mass transfer resistances in a layer near or on the outer surface of the zeolite crystals is indicated. 

Tdifr Time constant of diffusion-limited uptake... 

The actual processes of uptake of chemical species by an organism typically encompass transport in the medium, adsorption at extracellular cell wall components, and internalisation by transfer through the cell membrane. Each of these steps constitutes a broad spectrum of physicochemical aspects, including chemical interactions between relevant components, electrostatic interactions, elementary chemical kinetics (in this volume, as pertains to the interface), diffusion limitations of mass transfer processes, etc. 

In any case, exceptions to the FIAM have been pointed out [2,11,38,44,74,76,78]. For example, the uptake has been shown to depend on the Cj M or rMI (e.g. in the case of siderophores [11] or hydrophobic complexes [43,50]), rather than on the free c M. Several authors [11,12,15] showed that a scheme taking into account the kinetics of parallel transfer of M from several solution complexes to the internalisation transporter ( ligand exchange ) can lead to exceptions to the FIAM, even if there is no diffusion limitation. Adsorption equilibrium has been assumed in all the models discussed so far in this chapter, and the consideration of adsorption kinetics is kept for Section 4. Within the framework of the usual hypotheses in this Section 3, we would expect that the FIAM is less likely to apply for larger radii and smaller diffusion coefficients (perhaps arising from D due to the labile complexation of M with a large macromolecule or a colloid particle, see Section 3.3). 

Four strategies are generally employed to demonstrate mass transfer limitation in aquatic systems. Most commonly, measured uptake rates are simply compared with calculated maximal mass transfer rates (equation (17)) (e.g. [48,49]). Uptake rates can also be compared under different flow conditions (e.g. [52,55,56,84]), or by varying the biomass under identical flow conditions (e.g. [85]). Finally, several recent, innovative experiments have demonstrated diffusion boundary layers using microsensors [50,51]. Of the documented examples of diffusion limitation, three major cases have been identified ... 

In the case of a diffusion limitation, the free metal ion is largely consumed at the surface of the organism such that the concentration gradient of M in the external medium is strongly perturbed by biological uptake. The flux will depend on the concentration gradient of M that occurs between the bulk... 

This final point is important, since it implies that uptake by the same organism might be controlled by a diffusion limitation in one medium (e.g. unpolluted... 

Figure 11. Relative uptake rates as a function of carrier number in the diffusion-limited case. Calculations were performed according to the equations of Berg and Purcell [35] for a carrier radius of 1 nm on a cell with a radius of 1 pm...

For the biological limitation of trace metal internalisation, complex formation will invariably decrease the concentration of free metal ion and thus decrease the biouptake fluxes and carrier-bound metal (FIAM, BLM). In the case of a diffusion-limited internalisation, complex labilities and mobilities become much more pertinent when determining uptake fluxes. As shown earlier, few experiments have been designed to identify diffusion limitation of metal uptake fluxes, despite the fact that such a limitation is possible (Figure 10). Competition experiments that can distinguish a kinetic from a thermodynamic control are rare. In these areas, an important research focus is... 

Koch, A. L. and Wang, C. H. (1982). How close to the theoretical diffusion limit do bacterial uptake systems function Arch. Microbiol., 131, 36-42. 

Mierle, G. (1985). Kinetics of phosphate transport by Synchococcus leopoliensis (Cyanophyta) evidence for diffusion limitation of phosphate uptake, J. Phycol., 21, 177-185. 

Transport of NH4+ to the roots in Kirk and Solivas experiment was mainly by diffusion. The additional transport resulting from mass flow of soil solution in the transpiration stream would have increased the influx across the roots by about QQaVa/0.5bD% where Va is the water flux (Tinker and Nye, 2000, pp. 146-148), or about 4% in Kirk and Solivas experiment. A sensitivity analysis showed that rates of diffusion will generally not limit uptake in well-puddled soils, but they may greatly limit uptake in puddled soils that have been drained and re-flooded and in unpuddled flooded soils. 

Next the effect of cyclic loading on the distribution of a limited solute is investigated. Small and large solutes are considered (glucose, albumin). Both solutes are assumed limited a priori by diffusion and uptake, resulting in equal... 

Due to the limitations of gas diffusion, the uptake of the trace gas is reduced by a factor of n /n. To take this into account, we define a diffusion limited observable mass accommodation coefficient 74 as... 

Pressure in order to minimize this diffusion limitation. At the same time, owever, the ambient water vapor pressure must be maintained at its equilibrium value which at room temperature is about 20 Torr. In our experiment performed with 60 fim radius droplets, gas uptake is dominated by diffusive transport for 7 greater than 0.1. The effect of diffusion on gas uptake is illustrated in Figure 2. 

More sophisticated techniques than the ones used by Haldane have estabhshed that CO binds to hemoglobin with an affinity 200 times greater than that of oxygen (Ernst and Zibrak, 1998 Roughton and Darling, 1944 Sendroy et al, 1930). Carbon monoxide diffuses from the alveoh to the blood in pulmonary capillaries across the alveoh-capillary membrane, which is composed of pulmonary epithelium, the capillary epithelium, and the fused basement membranes of the two. The uptake of CO by Hb is very rapid and the transfer of CO is diffusion limited (Prockop and Chichkoa, 2007). 

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