Transmembrane flux

As expected, ionomycin, a calcium ionophore, causes a sustained rise in the free intracellular concentration to approximately 450 nM (Figure 4). In this system, pardaxin induced an increase in intracellular [Ca ] only in the presence of extracellular Ca (Figure 4). These results indicate that pardaxin mediated a Ca influx but did not release Ca from intracellular stores. This influx is most probably mediated directly by pardaxin channels and possibly also indirectly by activation of the Ca channels of the chromaffin cells by the depolarization produced by the pardaxin channels (data not shown). These observations further substantiate our hypothesis 10) that transmembrane fluxes of Na and Ca are involved in the pathological action of pardaxin. 

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

However this is not dways the case, especially when the two components weakly interact with the surface When using the membrane to separate a H2/ isobutane mixture, the permeation of isobutane, due to its size, is restricted over the entire temperature range and the transmembrane fluxes of the two components of the mixture better follow the permeabilities of the pure gases. Separation factors are here much higher (factors up to 80 have been measured). 

Interference with the transmembrane flux of Na+ and K+ ions and, thus, the electrophysiological activity of the neuron. 

In order to develop a continuous flux maintenance procedure, the present study examined the transmembrane flux values from the cross-flow filtration module with a filtration media area of 0.0198 m2 (0.213 ft2), a slurry density of approximately 0.69 g/cm3 at 200°C, 17 kg of simulated FT wax with a catalyst loading of 0.26 wt%, and a TMP between 0.68 and 1.72 bar (10-25 psig). The filtration process was run in a recycle mode, whereas clean permeate was added back to the slurry mixture, thus allowing the catalyst concentration to remain approximately constant over the course of the run (given minor adjustments for about 5 ml permeate and slurry samples collected throughout the test). 

After initial start-up, it was found that the magnitude of transmembrane flux decreased dramatically, indicating that a mass transfer boundary layer may form on the filter media. This boundary layer appears to remain somewhat constant after about 6 h on-line, whereas the slope of the flux versus time plot (Figure 15.13) becomes fairly linear. This linear decrease in flux was found to continue over the next 43 h. This could be attributed to fouling of the membrane by the small iron/... 

Receptor-effector mechanisms include (1) enzymes with catalytic activities, (2) ion channels that gate the transmembrane flux of ions (ionotropic receptors), (3) G protein-coupled receptors that activate intracellular messengers (metabotropic receptors), and (4) cytosolic receptors that regulate gene transcription. Cytosolic receptors are a specific mechanism of many steroid and thyroid hormones. The ionotropic and metabotropic receptors are discussed in relevance to specific neurotransmitters in chapter 2. 

A. Skeletal muscles depend on the mobilization of intracellular stores of calcium for their contractile responses rather than transmembrane flux of calcium through the calcium channels. Therefore, skeletal muscle weakness is not likely to occur. 

Figure 19.3 schematically describes in more detail the transport phenomena occurring during pervaporation. First, solutes partition into the membrane material according to the thermodynamic equilibrium at the liquid-membrane interface (Fig. 19.3a), followed by diffusion across the membrane material owing to the concentration gradient (Fig. 19.3b). A vacuum or carrier gas stream promotes then continuous desorption of the molecules reaching the permeate side of the membrane (Fig. 19.3c), maintaining in this way a concentration gradient across the membrane and hence a continuous transmembrane flux of compounds.

Smooth muscle responses to calcium influx through receptor-operated calcium channels are also reduced by these drugs but not as markedly. The block can be partially reversed by elevating the concentration of calcium, although the levels of calcium required are not easily attainable. Block can also be partially reversed by the use of drugs that increase the transmembrane flux of calcium, such as sympathomimetics. 

For dilute solutions at a homogeneous, isotropic membrane the transmembrane fluxes of water and solute in the steady state is given by Eqs. (8.69) and (8.70). 

Dense nonporous isotropic membranes are rarely used in membrane separation processes because the transmembrane flux through these relatively thick membranes is too low for practical separation processes. However, they are widely used in laboratory work to characterize membrane properties. In the laboratory, isotropic (dense) membranes are prepared by solution casting or thermal meltpressing. The same techniques can be used on a larger scale to produce, for example, packaging material. 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

A wide variety of polymeric membranes with different barrier properties is already available, many of them in various formats and with various dedicated specifications. The ongoing development in the field is very dynamic and focused on further increasing barrier selectivities (if possible at maximum transmembrane fluxes) and/ or improving membrane stability in order to broaden the applicability. This tailoring of membrane performance is done via various routes controlled macro-molecular synthesis (with a focus on functional polymeric architectures), development of advanced polymer blends or mixed-matrix materials, preparation of novel composite membranes and selective surface modification are the most important trends. Advanced functional polymer membranes such as stimuli-responsive [54] or molecularly imprinted polymer (MIP) membranes [55] are examples of the development of another dimension in that field. On that basis, it is expected that polymeric membranes will play a major role in process intensification in many different fields. 

Since the SF is a ratio of ratios, any measure of composition (mole fraction, mass fraction, concentration, etc.) can be used in Equation 7.1 as long as one consistently uses the same measure for both upstream and downstream phases in contact with the membrane. Locally within a module, the ratio of compositions leaving the downstream face of a membrane equals the ratio of the transmembrane fluxes of A vs. B. Local fluxes of each component are determined by relative transmembrane driving forces and resistances acting on each component. The ratio of the feed compositions in the denominator provides a measure of the ratio of the respective driving forces for the case of a negligible downstream pressure. This form normalizes the SF to provide a measure of efficiency that is ideally independent of the feed composition. 

Because of the high content of suspended solids and pectins in fruit juices, the use of a MF or UF pretreatment before the RO unit is also able to reduce the viscosity of the feed stream, increasing the transmembrane flux. 

Both intracellular release and transmembrane flux contribute to the rise in intracellular Ca2+.14,15 The rise in keratinocyte intracellular Ca2+ in response to raised extracellular Ca2+ has two phases (a) an initial peak, not dependent on extracellular Ca2+ and (b) a later phase that requires extracellular Ca2+.14 An early response of human keratinocytes to increases in extracellular Ca2+ is an acute increase in intracellular Ca2+. Stepwise addition of extracellular Ca2+ to neonatal human keratinocytes is followed by a progressive increase in intracellular Ca2+, where the initial spike of increased intracellular Ca2+ is followed by a prolonged plateau of higher intracellular Ca2+.16 The response of intracellular Ca2+ to increased extracellular Ca2+ in keratinocytes is saturated at 2.0 mM extracellular Ca2+.16,17 The response of intracellular Ca2+ to increased extracellular Ca2+ in keratinocytes resembles the response in parathyroid cells, in that a rapid and transient increase in intracellular Ca2+ is followed by a sustained increase in intracellular Ca2+ above basal level. This multiphasic response is attributed to an initial release of Ca2+ from intracellular stores followed by an increased influx of Ca2+ through voltage-independent cation channels. The keratinocyte and parathyroid cell contains a similar cell membrane calcium receptor thought to mediate this response to extracellular Ca2+. This receptor can activate the phospholipase-C pathway, leading to an increase... 

On stimulation of quiescent cells with growth factors or serum there is a rapid increase in the transmembrane flux of Na+, K+ and H+ and a mobilisation of Ca2+ from intracellular stores. The increase in Ca2+ concentration can be mimicked by treating cells with the Ca2+ ionophore, A23187. There quickly follows a series of events leading to changes in gene expression and cell structure and eventually to DNA replication and cell division. 

Channel activity is best studied electrochemically as charged species cross a cell membrane or artificial lipid bilayer. There is a difference in electrical potential between the interior and exterior of a cell leading to the membrane itself having a resting potential between -50 and -100 mV. This can be determined by placing a microelectrode inside the cell and measuring the potential difference between it and a reference electrode placed in the extracellular solution. Subsequent changes in electrical current or capacitance are indicative of a transmembrane flux of ions. 

Four parameters related to the membrane, feed stream and operating conditions determine the technical as well as economic performance of an inorganic membrane system. They are the transmembrane flux, permselectivity, maintenance of the permeating flux and permselectivity over time and stability toward the applications environment These parameters are the primary considerations for all aspects of the membrane system design, ai lication, and operation. 

Symmetric membranes were the first ones to be produced. Typical symmetric membranes are Vycor glass or solid-electrolyte ones, whereas, in general, an asymmetric structure is preferred for any other material so as to get a proper balance among membrane permselectivity, permeability (the lower the permeability, the lower is the transmembrane flux at a given pressure difference) and mechanical strength [8]. In fact, an inofganic... 

Increases in mesh size will increase the gel diffusivity tending to increase transmembrane flux. 

Increases in swelling ratio will increase the membrane thickness tending to decrease transmembrane flux. 

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