Transport concentration gradient-dependent

The relationship between the flux and the concentration gradient depends on an experimental observation A one-dimensional gradient in the particle concentration is set up in a fluid by fixing the concentration at two parallel planes. The fluid is isothermal and stationary. It is observed that the rate at which particles are transported from the high to the low concentration (particlcs/cm- sec) is proportional to the local concentration gradient, Bn/dx ... 

Often, advective and diffusive process act together, in series, in parallel, or in opposition. Solute and colloid bound chemicals diffuse in the open pores of bed-sediment driven by concentration gradients. Depending on the relative positions of the hydraulic heads in ground water compared to the water body, a positive or negative Darcian velocity exists in the bed, resulting in an in-or-out advective chemical transport superimposed on the diffusive transport. Both transport processes must be appropriately combined for assessing the overall chemical mobility (see Section 4.4). 

Concentration gradients for the analyte in the absence of convection, showing the time-dependent change in diffusion as a method of mass transport. 

Back-diffusion is the transport of co-ions, and an equivalent number of counterions, under the influence of the concentration gradients developed between enriched and depleted compartments during ED. Such back-diffusion counteracts the electrical transport of ions and hence causes a decrease in process efficiency. Back-diffusion depends on the concentration difference across the membrane and the selectivity of the membrane the greater the concentration difference and the lower the selectivity, the greater the back-diffusion. Designers of ED apparatus, therefore, try to minimize concentration differences across membranes and utilize highly selective membranes. Back-diffusion between sodium chloride solutions of zero and one normal is generally [Pg.173]

The kinetics of transport depends on the nature and concentration of the penetrant and on whether the plastic is in the glassy or rubbery state. The simplest situation is found when the penetrant is a gas and the polymer is above its glass transition. Under these conditions Fick s law, with a concentration independent diffusion coefficient, D, and Henry s law are obeyed. Differences in concentration, C, are related to the flux of matter passing through the unit area in unit time, Jx, and to the concentration gradient by,... 

A combination of continuum transport theory and the Poisson distribution of solution charges has been popular in interpreting transport of ions or conductivity of electrolytes. Assuming zero gradient in pressure and concentration of other species, the flux of an ion depends on the concentration gradient, the electrical potential gradient, and a convection... 

Sodium SGLTl -dependent unidirectionai transporter Small intestine and kidney Active uptake of glucose from lumen of intestine and reabsorption of glucose in proximal tubule of kidney against a concentration gradient... 

Historically, the absorption of lipid-soluble nutrients has been considered to be carrier-independent, with solutes diffusing into enterocytes down concentration gradients. This is true for some lipid-soluble components of plants (e.g. the hydroxytyrosol in olive oil Manna et al., 2000). However, transporters have been reported for several lipid-soluble nutrients. For example, absorption of cholesterol is partly dependent on a carrier-mediated process that is inhibited by tea polyphenols (Dawson and Rudel, 1999) and other phytochemicals (Park et al., 2002). A portion of the decreased absorption caused by tea polyphenols may be due to precipitation of the cholesterol associated with micelles (Ikeda et al., 1992). Alternatively, plant stanols and other phytochemicals may compete with cholesterol for transporter sites (Plat and Mensink, 2002). It is likely that transporters for other lipid-soluble nutrients are also affected by phytochemicals, although this has not been adequately investigated. 

Diffnsion finxes develop as a result of these concentration gradients. The layer of electrolyte where the concentration changes occur and within which the substances are transported by diffnsion is called the diffusion layer. Its thickness, 5 (the diffusion path length), depends on cell design features and on the intensity of convechve... 

In electrochemical cells we often find convective transport of reaction components toward (or away from) the electrode surface. In this case the balance equation describing the supply and escape of the components should be written in the general form (1.38). However, this equation needs further explanation. At any current density during current flow, the migration and diffusion fluxes (or field strength and concentration gradients) will spontaneously settle at values such that condition (4.14) is satisfied. The convective flux, on the other hand, depends on the arbitrary values selected for the flow velocity v and for the component concentrations (i.e., is determined by factors independent of the values selected for the current density). Hence, in the balance equation (1.38), it is not the total convective flux that should appear, only the part that corresponds to the true consumption of reactants from the flux or true product release into the flux. This fraction is defined as tfie difference between the fluxes away from and to the electrode ... 

In eukaryotes there is also evidence that Met(O) is actively transported. It has been reported that Met(O) is transported into purified rabbit intestinal and renal brush border membrane vesicles by a Met-dependent mechanism and accumulates inside the vesicles against a concentration gradient . In both types of vesicles the rate of transport is increased with increasing concentrations of Na" in the incubation medium. The effect of the Na" is to increase the affinity of Met(O) for the carrier. Similar to that found in the bacterial system, the presence of Met and other amino acids in the incubation medium decreased the transport of Met(O). These results suggest that Met(O) is not transported by a unique carrier. 

Facilitated transport combines some properties of both mechanisms discussed above. This type of transport is carrier mediated so that there is substrate specificity, a transport maximum, and competitive inhibition. However, facilitated transport is not energy-dependent and is unable to transport a substrate against a concentration gradient. 

Anandamide is inactivated in two steps, first by transport inside the cell and subsequently by intracellular enzymatic hydrolysis. The transport of anandamide inside the cell is a carrier-mediated activity, having been shown to be a saturable, time- and temperature-dependent process that involves some protein with high affinity and specificity for anandamide (Beltramo, 1997). This transport process, unlike that of classical neurotransmitters, is Na+-independent and driven only by the concentration gradient of anandamide (Piomelli, 1998). Although the anandamide transporter protein has not been cloned yet, its well characterized activity is known to be inhibited by specific transporter inhibitors. Reuptake of 2-AG is probably mediated by the same facilitating mechanism (Di Marzo, 1999a,b Piomelli, 1999). 

Both active and passive transport occur simultaneously, and their quantitative roles differ at different concentration gradients. At low substrate concentrations, active transport plays a major role, whilst above the concentration of saturation passive diffusion is the major transport process. This very simple rule can be studied in an experimental system using cell culture-based models, and the concentration dependency of the transport of a compound as well as asymmetric transport over the membrane are two factors used to evaluate the presence and influence of transporters. Previous data have indicated that the permeability of actively absorbed compounds may be underestimated in the Caco-2 model due to a lack of (or low) expression of some uptake transporters. However, many data which show a lack of influence of transporters are usually derived from experiments... 

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