Gradient secondary

The resorption process is facilitated by the large inner surface of the intestine, with its brush-border cells. Lipophilic molecules penetrate the plasma membrane of the mucosal cells by simple diffusion, whereas polar molecules require transporters (facilitated diffusion see p. 218). In many cases, carrier-mediated cotransport with Na"" ions can be observed. In this case, the difference in the concentration of the sodium ions (high in the intestinal lumen and low in the mucosal cells) drives the import of nutrients against a concentration gradient (secondary active transport see p. 220). Failure of carrier systems in the gastrointestinal tract can result in diseases. 

The separation of different species in a centrifiigal force field (the imposed primary force field) having a radial density gradient (secondary physical property based field) is identified as isopycnic sedimentation or density-gradient sedimentation (Giddings and Dahlgren, 1971). Each species... 

The work of Thiele (1939) and Zeldovich (1939) called attention to the fact that reaction rates can be influenced by diffusion in the pores of particulate catalysts. For industrial, high-performance catalysts, where reaction rates are high, the pore diffusion limitation can reduce both productivity and selectivity. The latter problem emerges because 80% of the processes for the production of basic intermediates are oxidations and hydrogenations. In these processes the reactive intermediates are the valuable products, but because of their reactivity are subject to secondary degradations. In addition both oxidations and hydrogenation are exothermic processes and inside temperature gradients further complicate secondary processes inside the pores. 

In most situations the eluent composition is chosen to minimize the effects of hydrophobic interaction, but these secondary effects can be used to advantage. By careful selection of a salt and its concentration, specific selectivities for analytes can be achieved without the use of organic solvents. Therefore, many separations usually run by solvent gradient reversed-phase methods can be completed with a purely aqueous isocratic eluent (13,14). 

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

The density change on polymerization is typically about 20%, and this density gradient can cause significant secondary flows and natural convection effects. The experiments cited above for vinyl acetate polymerization were performed in a helical reactor. The centrifugal force in helical reactors induces secondary flows as well. The effects of helical flow have been analyzed, but were found to be less significant than the effects of natural convection [14]. 

As described above, because MAO is bound to mitochondrial outer membranes, MAOIs first increase the concentration of monoamines in the neuronal cytosol, followed by a secondary increase in the vesicle-bound transmitter. The enlarged vesicular pool will increase exocytotic release of transmitter, while an increase in cytoplasmic monoamines will both reduce carrier-mediated removal of transmitter from the synapse (because the favourable concentration gradient is reduced) and could even lead to net export of transmitter by the membrane transporter. That MAOIs increase the concentration of extracellular monoamines has been confirmed using intracranial microdialysis (Ferrer and Artigas 1994). 

Passivation phenomena on the whole are highly multifarious and complex. One must distinguish between the primal onset of the passive state and the secondary phenomena that arise when passivation has already occurred (i.e., as a result of passivation). It has been demonstrated for many systems by now that passivation is caused by adsorbed layers, and that the phase layers are formed when passivation has already been initiated. In other cases, passivation may be produced by the formation of thin phase layers on the electrode surface. Relatively thick porous layers can form both before and after the start of passivation. Their effects, as a rule, amount to an increase in true current density and to higher concentration gradients in the solution layer next to the electrode. Therefore, they do not themselves passivate the electrode but are conducive to the onset of a passive state having different origins. 

As the electric field always points in the direction of the electrode, the densities of the electrons and negative ions are set equal to zero at the electrode. It is assumed that the ion flux at the electrodes has only a drift component, i.e., the density gradient is set equal to zero. The conditions in the sheath, which depend on pressure, voltage drop, and sheath thickness, are generally such that secondary electrons (created at the electrodes as a result of ion impact) will ionize at most a few molecules, so no ionization avalanches will occur. Therefore, secondary electrons can be neglected. 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

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