Model For Facilitated Transport Research Articles

Computational modeling and analysis of iron release from macrophages.

A major process of iron homeostasis in whole-body iron metabolism is the release of iron from the macrophages of the reticuloendothelial system. Macrophages recognize and phagocytose senescent or damaged erythrocytes. Then, they process the heme iron, which is returned to the circulation for reutilization by red blood cell precursors during erythropoiesis. The amount of iron released, compared to the amount shunted for storage as ferritin, is greater during iron deficiency. A currently accepted model of iron release assumes a passive-gradient with free diffusion of intracellular labile iron (Fe2+) through ferroportin (FPN), the transporter on the plasma membrane. Outside the cell, a multi-copper ferroxidase, ceruloplasmin (Cp), oxidizes ferrous to ferric ion. Apo-transferrin (Tf), the primary carrier of soluble iron in the plasma, binds ferric ion to form mono-ferric and di-ferric transferrin. According to the passive-gradient model, the removal of ferrous ion from the site of release sustains the gradient that maintains the iron release. Subcellular localization of FPN, however, indicates that the role of FPN may be more complex. By experiments and mathematical modeling, we have investigated the detailed mechanism of iron release from macrophages focusing on the roles of the Cp, FPN and apo-Tf. The passive-gradient model is quantitatively analyzed using a mathematical model for the first time. A comparison of experimental data with model simulations shows that the passive-gradient model cannot explain macrophage iron release. However, a facilitated-transport model associated with FPN can explain the iron release mechanism. According to the facilitated-transport model, intracellular FPN carries labile iron to the macrophage membrane. Extracellular Cp accelerates the oxidation of ferrous ion bound to FPN. Apo-Tf in the extracellular environment binds to the oxidized ferrous ion, completing the release process. Facilitated-transport model can correctly predict cellular iron efflux and is essential for physiologically relevant whole-body model of iron metabolism.

Effects of Minor SO2 on the Transport Properties of Fixed Carrier Membranes for CO2 Capture

A key concern for application of fixed carrier membranes in CO2 capture is the effects of a trace amount of SO2 present in the flue gas. To eliminate this concern, the effects of SO2 in the feed gas on CO2/N2 permeance and selectivity of fixed carrier membranes were investigated by combining experimental and modeling study. The experimental results reveal that the separation performance of the membrane slightly decreases with increasing SO2 concentration in the feed gas due to the competitive sorption and competitive reaction caused by SO2. The decrease in separation performance is tolerable at even 5000 ppm of SO2 present in the feed gas. The facilitated transport model was modified by considering the competitive reaction to investigate the transport and interactions of CO2 and SO2. The modeling results suggest that the SO2 partial pressure at the membrane surface is much lower than that in the bulk feed gas due to the serious concentration polarization of SO2 under the mixed gas condition, and the decre...

A kinetics analysis applied to the recovery of Zn(II) content from mine drainage by using a surfactant liquid membrane

A kinetics analysis was conducted for the recovery of zinc (II) ions from a mine drainage sample by a liquid surfactant membrane containing di-(2-ethylhexyl)phosphoric acid as a carrier extractant and Span-80 as the surfactant. The extraction of metal was proportional to the concentration of carrier in the organic phase, inversely proportional to the zinc content in the feed solution, and was enhanced as the pH of the feed phase was increased. A minimum sulfuric acid content of 150 g/L in the stripping solution was necessary to favor the transport of metals from the external aqueous solution to the metal-receiving strip liquor. The experimentally observed results were analyzed by a metal extraction kinetics mechanism based on a facilitated transport model, which considers the interfacial chemical reaction between the metal and the carrier at the external interface of the liquid membrane as kinetic controlling step. The experimental results were fairly well explained by the model that takes into account th...

Liquid membrane emulsion process for recovering the copper content of a mine drainage

A study of the treatment of an acidic mine drainage to extract its Cu(II) content by emulsion liquid membranes is made. Copper was removed using 5-nonylsalicylaldoxime (LIX-860 N-IC) dissolved in aviation kerosene. The effects of various factors, such as pH and metal content in the feed solution, carrier concentration and surfactant content in the membrane, acidity of the stripping solution, and stirring speed to prepare the primary and the double emulsion, are reviewed. The experimental data were analyzed in terms of a metal transport mechanism based on a facilitated transport model, which takes account the chemical reaction between the metal and the oximic extractant and the diffusion processes in the aqueous phase close to the external interface of membrane. The results indicate that with adequate control and adjustment of these experimental conditions the extraction extent of metal through the membrane as well their degree of enrichment in the stripping liquor can be favored.

Transport mechanism of carbon dioxide through perfluorosulfonate ionomer membranes containing an amine carrier

A new facilitated transport model for CO 2 through ion-exchange membranes containing a diamine complexing agent was developed. The diamine ion behaves as a mobile carrier for CO 2. Although the morphology of the ion-exchange membrane affects carrier transport, the effect of morphology on ionic carrier transport is not clear. The Nernst-Planck equation and the penetration model were employed in this modeling study. The electrical double layer effect and friction effect in the ion-exchange membrane was also considered. In the membrane, there are two kinds of counter ions (NH 3 +-R-NH 2 and NH 3 +-R-NH 3 +), CO 2 and NH 3 +-R-NHCOO − (carbamate ion). The carbamate ion can be treated as a neutral molecule because it has both plus and minus charge. Commercial Nafion 117 (N117) and heat treated Nafion117 (HN117) were used as ion-exchange membranes. The water content of N117 and HN117 was 16 and 45%, respectively. Nafion has cluster channels which were filled with water, and HN117 has a larger cluster channel size than N117. Monoprotonated ethylenediamine was used as a carrier. Mobile counter ion diffusivities were measured by membrane conductivity. Carbon dioxide diffusivity was determined from transport measurements in a nonreactive Nafion membrane. The diffusivity ratio of carbamate ion to CO 2 was estimated by the group contribution method which is effective in aqueous solutions. We estimated a friction effect for the carbamate ion which reduces the carbamate ion diffusivity ratio in the cluster channel. For the HN117 membrane case, experimental results and simulations were in good agreement when we used the diffusivity ratio which was estimated from the group contribution method. The counter ion diffusivities, which are restricted by electrical forces, are the rate limiting step for CO 2 transport through large clusters. For the N117 case, we must consider the friction effect, and when we use a small carbamate diffusivity ratio, simulations and experimental results agreed well. The diffusivity of the carbamate ion, which is the largest molecule in the membrane, is the rate limiting step for transport through small cluster channels. This model can explain the permeate-side CO 2 pressure effect as the permeate-side CO 2 pressure seriously reduces the facilitation effect.

Voltage dependence of facilitated arginine flux mediated by the system y+ basic amino acid transporter

Two-microelectrode voltage clamp was used to measure membrane currents resulting from flux of cationic amino acids in Xenopus oocytes expressing the cloned dual-function ecotropic murine leukemia virus receptor/system y+ transporter. At membrane potentials ranging from +20 mV to -120 mV, arginine influx obeyed Michaelis-Menten kinetics. At concentrations from 0.01 to 1 mM, influx increased exponentially with membrane hyperpolarization (e-fold increase/-59 mV). Efflux from oocytes preloaded with arginine increased exponentially with depolarization (e-fold increase/+52 mV). Kinetic analysis based on an iso uni uni facilitated transport model suggests that the effect of voltage on steady-state flux arises largely from charge movement across the membrane field during the conformational transition of the unliganded transporter which switches the substrate binding site from one membrane face to the other. This charge movement would facilitate rapid increases in intracellular arginine concentrations in response to hyperpolarization, a property which could play a role in modulating nitric oxide synthesis in some types of cells.

Emulsion liquid membrane extraction of organic acids—I. A theoretical model for lactic acid extraction with emulsion swelling

A new quantitative model is developed which describes the transport mechanisms and kinetics of organic acid extraction using emulsion liquid membranes. The model includes the contributions from facilitated transport of the acid by a tertiary amine carrier, and convective acid transport resulting from water transport to the emulsions. The facilitated transport model considers mass transfer in the external phase, the solute-carrier chemical reaction, diffusion in the globule as described by a shrinking-core model, and the stripping reaction. Convective transport is accounted for in terms of solute dissolved in the transported water. The effects of swelling on globule size and diffusion distance are also considered.

Molten salt facilitated transport membranes. Part 2. Separation of ammonia from nitrogen and hydrogen at high temperatures

Immobilized molten salt (IMS) membranes have been prepared and shown to be effective for the separation of NH 3 from mixtures with N 2 and H 2 at the high temperature encountered in the recycle loop of ammonia synthesis plants. The membranes were prepared by immobilizing molten LiNO 3 and ZnCl 2, salts which react reversibly with NH 3 but not with N 2 or H 2, in thin porous supports. Ammonia permeabilities of 10 3 to 10 5 barrer and NH 3/N 2 and NH 3/H 2 selectivities of at least 1000 were observed at 250–300°C. The extraordinary NH 3 permeability through the ZnCl 2 IMS membrane is explained by a carrier-mediated facilitated transport model wherein zinc ammoniate complexes function as mobile carriers.

Analysis of Transport Rate of Zinc Extraction through Liquid Surfactant Membrane

Abstract Analysis of the transport rate of a metal ion through a liquid surfactant membrane is important for understanding an extraction system. A facilitated transport model for zinc extraction through a liquid surfactant membrane is proposed for the analysis of the transport rate. Based on the model, the transport rates, including the interfacial reaction rates and the diffusion rate of the zinc ion, are analyzed. From an analysis of the model, it is shown that the reactions at both interfaces are not in equilibrium before extraction is complete. It is also shown that the transport rates are not equal value in the early stage of a run. Parametric effects on the transport rates are also illustrated.

Nucleocytoplasmic transport of ribosomes in a eukaryotic system: is there a facilitated transport process?

We have examined the kinetics of the process by which ribosomes are exported from the nucleus to the cytoplasm using Xenopus laevis oocytes microinjected into the germinal vesicle with radiolabeled ribosomes or ribosomal subunits from X. laevis, Tetrahymena thermophila, or Escherichia coli. Microinjected eukaryotic mature ribosomes are redistributed into the oocyte cytoplasm by an apparent carrier-mediated transport process that exhibits saturation kinetics as increasing amounts of ribosomes are injected. T. thermophila ribosomes are competent to traverse the Xenopus nuclear envelope, suggesting that the basic mechanism underlying ribosome transport is evolutionarily conserved. Microinjected E. coli ribosomes are not transported in this system, indicating that prokaryotic ribosomes lack the "signals" required for transport. Surprisingly, coinjected small (40S) and large (60S) subunits from T. thermophila are transported significantly faster than individual subunits. These observations support a facilitated transport model for the translocation of ribosomal subunits as separate units across the nuclear envelope whereby the transport rate of 60S or 40S subunits is enhanced by the presence of the partner subunit. Although the basic features of the transport mechanism have been preserved through evolution, other aspects of the process may be mediated through species-specific interactions. We hypothesize that a species-specific nuclear 40S-60S subunit association may expedite the transport of individual subunits across the nuclear envelope.