Uptake, concentrative

Molybdenum is efficiently absorbed over a wide range of dietary intakes mainly as molybdate, although competitive inhibition of absorption by sulfate reduces intestinal uptake. Concentrations in whole blood are about l.Opg/L (10 nmol/L) and some 80% to 90% or more of molybdenum in whole blood is bound to red cell proteins. Transport of the smaller amount in blood plasma may involve a2-macroglobulin. Urine output directly reflects the... 

Since V)o is found in the media spectra obtained from cells exposed to both pH 6.5 and pH 4.0 media, after formation of the cell-associated Vio it is most likely that V,o has been extruded into the medium. After exposure to cells overnight, the pH 6.5 medium did not drop below pH 6.2, making it unlikely that the V,o formed spontaneously. Since the formation of V,o requires an acidic environment and the cytoplasmic pH values are nearly neutral, it is likely that Vio is formed in acidic vesicles prior to extrusion. There is precedence for the concentration of vanadium from the ocean by ascidians." These invertebrate animals are believed to concentrate the vanadium in vesicles that are acidic and/or contain the appropriate chelating agents." During the uptake/concentration the vanadium(V) is reduced to vanadium(IV) and/or vanadium(III) in the ascidians. It is possible that our observations in yeast may use an analogous mode of action. 

Caccia S, Gobbi M (2009) St. John s wort components and the brain uptake, concentrations reached and the mechanisms underlying pharmacological effects. Curr Drug Metab 10 1055-1065... 

The uptake and elimination coefficients k and k can in principle be calculated by first extracting k from Equation 5.18c using measmed-dearance histories, and then substituting it into Equation 5.17c and performing a similar analysis of uptake-concentration histories. In actual practice, it is more common to perform an independent evaluation of k by using the initial portion of the uptake process, which is imaffected by the relatively slow elimination process. Equation 5.17b then assumes the reduced form... 

One procedure makes use of a box on whose silk screen bottom powdered desiccant has been placed, usually lithium chloride. The box is positioned 1-2 mm above the surface, and the rate of gain in weight is measured for the film-free and the film-covered surface. The rate of water uptake is reported as u = m/fA, or in g/sec cm. This is taken to be proportional to - Cd)/R, where Ch, and Cd are the concentrations of water vapor in equilibrium with water and with the desiccant, respectively, and R is the diffusional resistance across the gap between the surface and the screen. Qualitatively, R can be regarded as actually being the sum of a series of resistances corresponding to the various diffusion gradients present ... 

The unexpected preference for the interfacial region at lower concentrations of benzene has prompted speculation. It has been demonstrated that aromatic compounds are capable of forming weak hydrogen bonds with water. This ability favours uptake in the aqueous interface over solubilisation in the interior. Alternatively, some authors have attributed the binding behaviour of benzene to its... 

The foregoing equation reveals that essentially the concentration distribution ratio for trace concentrations of an exchanging ion is independent of the respective solution of that ion and that the uptake of each trace ion by the resin is directly proportional to its solution concentration. However, the... 

Figure 19.7 shows a typical construction of a concentric-tube nebulizer. The sample (analyte) solution is placed in the innermost of two concentric capillary tubes and a flow of argon is forced down the annular space between the two tubes. As it emerges, the fast-flowing gas stream causes a partial vacuum at the end of the inner tube (Figure 19.4), and the sample solution lifts out (Figure 19.5). Where the emerging solution meets the fast-flowing gas, it is broken into an aerosol (Figure 19.7), which is swept along with the gas and eventually reaches the plasma flame. Uptake of sample solution is commonly a few milliliters per minute.

The dimensions of concentric-tube nebulizers have been reduced to give microconcentric nebulizers (MCN), which can also be made from acid-resistant material. Sample uptake with these microbore sprayers is only about 50 xl/min, yet they provide such good sample-transfer efficiencies that they have a performance comparable with other pneumatic nebulizers, which consume about 1 ml/min of sample. Careful alignment of the ends of the concentric capillary tubes (the nozzle)... 

For a step change in sorbate concentration at the particle surface (r = R) at time 2ero, assuming isothermal conditions and diffusion control, the expression for the uptake curve maybe derived from the appropriate solution of this differential equation ... 

In certain adsorbents, notably partially coked 2eohtes and some carbon molecular sieves, the resistance to mass transfer may be concentrated at the surface of the particle, lea ding to an uptake expression of the form... 

Fig. 1. The relationship between rate of oxygen uptake and dissolved oxygen, concentration where is the critical oxygen concentration.

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

A variety of methods have been devised to stabilize shales. The most successful method uses an oil or synthetic mud that avoids direct contact between the shale and the emulsified water. However, preventing direct contact does not prevent water uptake by the shale, because the organic phase forms a semipermeable membrane on the surface of the wellbore between the emulsified water in the mud and the water in the shale. Depending on the activity of the water, it can be drawn into the shale (activity lower in the shale) or into the mud (activity higher in the shale) (95—97). This osmotic effect is favorable when water is drawn out of the shale thus the aqueous phase of the oil or synthetic mud is maintained at a low water activity by a dding a salt, either sodium chloride or more commonly, calcium chloride. The salt concentration is carried somewhat above the concentration required to balance the water activity in the shale to ensure water movement into the mud. 

Saponins. Although the hypocholesterolemic activity of saponins has been known since the 1950s, thek low potency and difficult purification sparked Htde interest in natural saponins as hypolipidemic agents. Synthetic steroids (292, 293) that are structurally related to saponins have been shown to lower plasma cholesterol in a variety of different species (252). Steroid (292) is designated CP-88,818 [99759-19-0]. The hypocholesterolemic agent CP-148,623 [150332-35-7] (293) is not absorbed into the systemic ckculation and does not inhibit enzymes involved in cholesterol synthesis, release, or uptake. Rather, (293) specifically inhibits cholesterol absorption into the intestinal mucosa (253). As of late 1996, CP-148,623 is in clinical trials as an agent that lowers blood concentrations of cholesterol (254). 

As the oxygen uptake or F/M iacreases, the dissolved oxygen must be iacreased to provide sufficient driving force to penetrate the floe. Minimum concentrations of nitrogen and phosphoms are necessary in the effluent. 

In these equations, [L = specific growth rate coefficient, v = specific rate of substrate uptake, t = time, x = biomass concentration, Sj = intracellular substrate, and Cj = concentration of intracellular substrate. Several examples where these equations can be applied include nitrogen hmitations in M. citrifolia cultures and phosphate hmited growth in C. roseus, N. tahacum, a.nd Papaver. somniferum. 

A useful index of process performance is the oxygen uptake rate, OUR, that is calculated from the difference in oxygen concentration of the inlet air and the exiting gas. Also important is the respiration ratio defined as the carbon dioxide evolved divided by the oxygen consumed. 

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