Rate-limiting diffusion carrier requirements

Reagent Dilution/Cairier Gas Requirements Diluted with water to 5-25% concentration. Air is the carrier gas. (Steam is no longer used.) Typically, 4 scfm of plant air per injector is currently used for atomization and for injector cooling. Not diluted with water. Air or steam at 1-2% of the flue gas flow rate is the carrier gas. This gas requirement is reduced by the amount of water vaporized if aqueous ammonia is vaporized. Not diluted with water. Carrier gas is air, flue gas, or possibly steam at about 2 psig. Ammonia/carrier gas volumetric ratio is a minimum of about 1 to 20 for anhydrous ammonia to keep the ammonia concentration in the air below the lower explosive limit. Lower concenuations may be used based on injection diffusion patterns, the ammonia vaporization heat requirement, minimum flow control ranges, and other factors. 

Carrier-mediated passage of a molecular entity across a membrane (or other barrier). Facilitated transport follows saturation kinetics ie, the rate of transport at elevated concentrations of the transportable substrate reaches a maximum that reflects the concentration of carriers/transporters. In this respect, the kinetics resemble the Michaelis-Menten behavior of enzyme-catalyzed reactions. Facilitated diffusion systems are often stereo-specific, and they are subject to competitive inhibition. Facilitated transport systems are also distinguished from active transport systems which work against a concentration barrier and require a source of free energy. Simple diffusion often occurs in parallel to facilitated diffusion, and one must correct facilitated transport for the basal rate. This is usually evident when a plot of transport rate versus substrate concentration reaches a limiting nonzero rate at saturating substrate While the term passive transport has been used synonymously with facilitated transport, others have suggested that this term may be confused with or mistaken for simple diffusion. See Membrane Transport Kinetics... 

The ZLC method offers advantages of speed and simplicity and requires only a very small adsorbent sample thus making it useful for characterization of new materials. The basic experiment using an inert carrier (usually He) measures the limiting transport difiiisivity (Do) at low concentration. A variant of the technique using isotopically labeled tracers (TZLC) yields the tracer diffiisivity and counter diffusion in a binary system may also be studied by this method. To obtain reliable results a number of preliminary experiments are needed, e.g. varying sample quality, nature of the purge gas, the flow rate and, if possible, particle size to confirm intracrystalline diffusion control. 

Low transfection rates are a further problem. Reasons are, among others, insufficient lysosomal release of the therapeutic DNA from its carrier and/or its rapid enzymatic degradation within the lyso-some or in the cytosol. The DNA that was integrated in the Kposomes must be released to become effective (see Fig. 5). Studies on microinjections of Kpid/DNA complexes directly into the nucleus clearly show reduced transfection rates [54]. As the nuclear pores have mean diameters of only 25 to 50 nm and passage for macromolecules is further controlled by the nuclear pore complex, passive diffusion into the karyosol is limited to particle sizes below 45 kDa [51, 55]. Conventional recombinant plasmids normally have a molecular weight ranging from 50 to 100 kDa, and therefore require active transport into the nucleus [55]. 

Whether electron hopping or molecular diffusion takes place will depend on which of these processes is faster. In some cases the centres may diffuse short distances to allow electron hopping to occur. This argument was substantiated by previous workers (65) who measured d-d distances of 2.5-3.4 nm for Fe(CN)5". For electron self-exchange to occur, distances of 0.92 nm are required. Where the response is limited by the rate of charge transfer, a small charge carrier additive may prove useful. For example, a ferro-ethylenediaminetetra-acetic acid additive has been used (66). It is most likely that the mechanism of transport will vary with analyte concentration and also with the morphology of the polymer (67). 

The activity at low moisture levels is due, presumably, to the fact that the substrate (TG) of wheat is present as an oil which can diffuse through the milled materials without the requirement of free water as solvent/carrier. Previous work with model systems consisting of pancreatic lipase on powdered cellulose has shown that liquid, but not solid phase TG are hydrolysed at low moisture levels. The fact that the rate of hydrolysis is inversely related to the particle size distribution of the bran component of wholemeal, provides further evidence for a diffusion-limited process. 

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