Single concentration gradients

Operation m the Simple Mode if there is no concentration gradient within the liquid pool and if there is no coalescence within the rising foam, then the operation shown by the sohd hnes of Fig. 22-42 is truly in the simple mode, i.e., a single theoretical stage of separation. Equations (22-45) and (22-46) will then apply to the steady-flow operation. 

The reflectivity for this simple case can be extended readily to more complex situations where there are concentration gradients in single films or multilayers comprised of different components. Basically the reflectivity can be calculated from a simple recursion relationship that effectively reduces any gradients in composition to a histogram representing small changes in the concentration as a function of depth. Details on this can be found in the literature cited. ... 

In bacteria, accumulation of substrates against a concentration gradient can occur through two main classes of transport systems (see [30] for a summary). The prototype of the first class of transporters is the /3-galactoside permease of Escherichia coli (see [31]). It is a relatively simple system involving only a single membrane-bound protein. It catalyzes a lactose-H symport. Other transporters... 

The CHI can be obtained without preliminary method development direcdy from a single fast-gradient run with a cycle time less than 15 min with a 150-mm column [40] or 5 min with 50-mm column [42]. In this case, the obtained retention time, tr, is expressed within an organic phase concentration (< o) scale using a calibration set of compounds. CHI value can be obtained from ... 

In PAMPA measurements each well is usually a one-point-in-time (single-timepoint) sample. By contrast, in the conventional multitimepoint Caco-2 assay, the acceptor solution is frequently replaced with fresh buffer solution so that the solution in contact with the membrane contains no more than a few percent of the total sample concentration at any time. This condition can be called a physically maintained sink. Under pseudo-steady state (when a practically linear solute concentration gradient is established in the membrane phase see Chapter 2), lipophilic molecules will distribute into the cell monolayer in accordance with the effective membrane-buffer partition coefficient, even when the acceptor solution contains nearly zero sample concentration (due to the physical sink). If the physical sink is maintained indefinitely, then eventually, all of the sample will be depleted from both the donor and membrane compartments, as the flux approaches zero (Chapter 2). In conventional Caco-2 data analysis, a very simple equation [Eq. (7.10) or (7.11)] is used to calculate the permeability coefficient. But when combinatorial (i.e., lipophilic) compounds are screened, this equation is often invalid, since a considerable portion of the molecules partitions into the membrane phase during the multitimepoint measurements. 

A simple case is the diffusion of a single type of ion in a solution containing a sufficient excess of an indifferent electrolyte (see page 116), which then occurs in the same way as in the case of a non-electrolyte. Isotope (tracer) diffusion has the same character, where a concentration gradient of the radioactive isotope of an ion, present in a much lower concentration, is formed in a solution with a much larger, constant salt concentration. 

Evaporation of Atomized Droplets. The prediction of the time to totally evaporate a liquid droplet in an atomized spray is very difficult due to the complex thermal and concentration gradients present in the vicinity of the nozzle. Despite this complexity, it will be beneficial to study what happens to a single droplet of liquid when it is surrounded by a quiescent gas stream. This phenomena has been studied extensively because the time to evaporate a liquid drop has important consequences in a number of different applications e.g., spray drying, fuel injection, and coating. 

Continuous countercurrent operation in a single vertical column where longitudinal concentration gradient could be established... 

With a single release event, the concentration of contaminants at the plume center, and the concentration gradient (i.e., change in concentration per unit distance) between the center of the plume and its leading edge, decrease over time. 

The basic answer to this question is that ions move across the plasma membrane of the neuron. Recall that ions are charged particles, frequently derived from single atoms by the gain or loss of electrons. The ions that are most important to us in understanding nervous system function are sodium ion, Na+, potassium ion, K+, calcium ion, Ca +, and chloride ion, Cl . If we compare the concentrations of these ions on the inside of the neuron and in the extracellular fluid that bathes the neuron, we find the neuron interior has a higher concentration of potassium ion than does the exterior fluid. In contrast, the exterior fluid has higher concentrations of sodium, calcium, and chloride ions than does the neuron interior. These concentration differences are referred to as concentration gradients. 

Colchicine is rapidly absorbed after oral administration and tends to concentrate in the spleen, kidney, liver, and gastrointestinal tract. Leukocytes also avidly accumulate and store colchicine even after a single intravenous injection. Since colchicine can accumulate in cells against a concentration gradient, it is postulated that an active transport process may be involved in its cellular uptake. The drug is metabolized, primarily in the liver, by deacetylation. Fecal excretion plays a major role in colchicine elimination, since it and its metabolites are readily secreted into the bile. Only about 15 to 30% of the drug is eliminated in the urine except in patients with liver disease urinary excretion is more important in these individuals. 

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