Passive flux

The concentration of iron present in the permeate wax was found to be consistently less than 35 ppm, with over 85% below the 16 ppm level. Following an active flux maintenance procedure results in short-term recovery of flux, which declines to base value within 24 h. The passive flux maintenance procedure of interrupting the permeate flow for 30 or 60 s per 30 min was effective in recovering the initial membrane fouling temporarily. Better flux stability was attained only after increasing the permeate off-cycle to 1 h per day in addition to 30 s off per half-hour cycle. Variation of flux magnitude with TMP was found to follow a linear relationship within the range studied. 

The passive flux of molecules down a concentration gradient is given by Fick s law ... 

Drug [Applied Concentration] Skin Type Passive Flux (pg/h/cm2) Iontophoretic Flux (pg/h/mA)... 

This model has been used to predict flux, flux enhancement, and the reduction in the lag time when a voltage is applied [55,56]. An enhancement factor (EF) has been defined as the ratio of the steady-state flux with an applied voltage (/ion) to the corresponding passive flux (7pas) ... 

As shown by ERs (Table 15.1), both pulsing protocols significantly increased the penetration of estradiol and L-glutamic acid in relation to their respective steady-state passive fluxes... 

The ratio of the apparent flux during electrical period to the apparent steady-state passive flux for estradiol and L-glutamic acid, respectively. bThe ratio of the apparent flux during second passive stage to the apparent steady state passive flux for estradiol and L-glutamic acid, respectively. 

A passive flux of water continually flows across the endothelial layer toward the stroma, which has a tendency to swell. An active pump mechanism pulls an aqueous flux in the opposite direction which controls corneal turgescence [13]. Corneal deturgescence is an ATP-dependent process of the endothelial cells and as such any disruption of the endothelium may result in corneal oedema, thereby affecting corneal transparency. The specific distribution of different proteoglycans across the cornea has recently been implicated in water gradients across the cornea. This water gradient serves to diminish dehydration of the front of the cornea, which is exposed to the atmosphere. 

Diffusive samplers have also been developed to determine SVOCs but there have been relatively few studies to date. An example is the passive flux sampler developed by Fujii et al. (2003) to determine the rate of emission of phthalate esters from materials. The sampler consisted of a circular metal disc containing activated carbon particles held within an inert matrix of PTFE. The sampler was placed on the material under test giving a diffusion length of 0.5 or 2 mm depending upon the design and adsorbed phthalate esters were extracted from the sampler with toluene and determined by GC-MS. 

Fujii, M., Shinohara, N., Lim, A., Otake, T., Kumagai, K. and Yanagisawa, Y. (2003) A study of the emission of phthalate esters from plastic materials using a passive flux sampler. Atmospheric Environment, 37, 5495-504. 

Figure 7.36. Passive influx of Na+ (upper panel) and passive efflux of K+ (lower panel) in liver slices of the rat and a reptile (Amphibolurus vitticeps). Measurements were made in the presence of ouabain, an inhibitor of the sodium pump (Na+-K+-ATPase), at 37°C. Passive fluxes are higher in the leakier membranes of the mammal. (Data from Else and Hulbert, 1987.)...

Equation (3.50) is used below to develop an expression for the passive flux of solute across a thin homogeneous membrane. In addition, diffusion-driven processes will appear in our study of spatially distributed systems in Chapter 8. 

Equation (3.63) is known as the the Goldman-Hodgkin-Katz equation for passive flux of an ion through a membrane [108, 123],... 

Examine the model of passive flux through a membrane introduced in Section 3.2.4. How does the flux expression change if it is assumed that the transported solute (for example, oxygen) is consumed in the membrane ... 

Fluxes of many different solutes occur across biological membranes. Inward fluxes move mineral nutrients into cells, while certain products of metabolism flow out of cells. The primary concern in this section is the passive fluxes of ions toward lower chemical potentials. First, we indicate that the passive flux density of a solute is directly proportional to the driving force causing the movement. Next, the driving force is expressed in terms of the relevant components of the chemical potential. We then examine the consequences of electroneutrality when there are simultaneous passive fluxes of more than one type of ion. This leads to an expression describing the electrical potential difference across a membrane in terms of the properties of the ions penetrating it. 

The initiation of an electrogenic process causes an adjustment of the passive ionic fluxes across the membrane. In particular, the net charge actively brought in is soon electrically compensated by appropriate passive movements of that ion and other ions into or out of the cell. The actual electrical potential difference across the membrane then results from the diffusion potential caused by these new passive fluxes plus a steady-state contribution from the electrogenic process involving the active transport of various charged species. We can represent the electrical potential difference generated by the active transport of species/, atj, by... 

Once the radioactivity has built up inside to a substantial level, we may remove the radioisotope from the external solution. The flux of the radioisotope is then from inside the cells to the external solution. In this case, the specific activity for c° equals zero, and cj determines the net flux of the radioisotope. By Equation 3.16, this efflux of the radioisotope of species j differs in magnitude from the initial influx only by having the factor c° replaced by c ezJFEM RT, the quantities in the first two parentheses remaining the same. The ratio of these two passive flux densities —each of which can be separately measured—takes the following relatively simple form ... 

The ratio of the influx of species) to its efflux, as given by Equation 3.25, can be related to the difference in its chemical potential across a membrane. This difference causes the passive flux ratio to differ from 1. Moreover, we will use the chemical potential difference to estimate the minimum amount... 

A difference in chemical potential of species j across a membrane causes the ratio of the passive flux densities to differ from 1 (Fig. 3-12), a conclusion that follows directly from Equation 3.26. When fi° is equal to fip the influx balances the efflux, so no net passive flux density of species j occurs across the membrane (Jj = JJ1 - J°ut by Eq. 3.16). This condition (fi° = fij) is also described by Equation 3.5, which was used to derive the Nernst equation (Eq. 3.6). In fact, the electrical potential difference across a membrane when... 

Both active and passive fluxes across the cellular membranes can occur simultaneously, but these movements depend on concentrations in different ways (Fig. 3-17). For passive diffusion, the unidirectional component 7jn is proportional to c°, as is indicated by Equation 1.8 for neutral solutes [Jj = Pj(cJ — cj)] and by Equation 3.16 for ions. This proportionality strictly applies only over the range of external concentrations for which the permeability coefficient is essentially independent of concentration, and the membrane potential must not change in the case of charged solutes. Nevertheless, ordinary passive influxes do tend to be proportional to the external concentration, whereas an active influx or the special passive influx known as facilitated diffusion—either of which can be described by a Michaelis-Menten type of formalism—shows saturation effects at higher concentrations. Moreover, facilitated diffusion and active transport exhibit selectivity and competition, whereas ordinary diffusion does not (Fig. 3-17). 

B. If the passive flux density of the previous substance into the cell at 25°C is 1 nmol m-2 s 1, what is the difference in its chemical potential across the cellular membrane ... 

The passive flux of sodium ions was shown to be highly correlated (r = 0.98) to the inverse of the skin s impedance [16]. Note that since this impedance was measured at 0.2 Hz, it represents mainly the skin s electrical resistance. A weaker correlation was obtained for the passive flux of tritiated water [19]. This was found to be true both before and after the application of iontophoresis [16]. Since iontophoresis decreases the skin s impedance, the passive flux was greater after iontophoresis than before iontophoresis [16]. Inada et al. [27] have also demonstrated for tetraethylammonium ion and mannitol that their passive and iontophoretic fluxes are related to the reciprocal of the skin s resistance. In addition, Inada et al. [27] showed that the higher... 

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