Mass transfer effects on the reaction rate for heterogeneously distributed immobilized yeast cells

Here we examine the efficiency of different immobilized cell gradients applied to immobilized Saccharomyces cerevisiae fermenting glucose to ethanol. We developed a simulation model to fully study the competing effects of mass transfer hindrance and kinetics. It is based on a diffusion-reaction model and can be used to analyze the different cell concentration profiles inside an immobilized gel bead, in terms of effectiveness factors, productivity, and mass flux. The internal diffusion coefficient, which varies with the local cell concentration, as well as the external mass transfer, is taken into account when describing the efficiency. Although the diffusion hindrance is greater at higher cell concentrations, high cell concentration is still advantageous in the present case because the increase in reaction rate outweighs the diffusion hindrance. Thus, high cell concentrations contribute to increased productivity. The influence of the cell concentration gradient on the efficiency of the beads is negligible. Within the range of cell profiles studied it has been established that the location of the cells within the bead is of lesser importance. However, a steep cell gradient increases the importance of the external mass transfer.     

Phosphorus absorption by intact maize plants from flowing solutions influenced by 2.4-dinitrophenol and viscosity of solution

The investigation of phosphorus absorption by intact plants during a short period has above all confirmed the validity of the results obtained in the foregoing study of the kinetics of absorption by excised roots. Further, the results show the unquestionably important role of mass flow in transporting ions to plant roots, mainly at lower and medium concentrations, that is, from about 0.1 to 10 mM. Under conditions of growth close to the optimum, the supply by means of mass flow can be sufficient even at lower concentrations of phosphorus, such as 1.47 mM KH₂PO₄, or the absorption of phosphorus by plants can be higher than in the case of ions being transported to roots by diffusion. With a higher absorption the phosphorus distribution somewhat changes as well, relatively more of it being accumulated in the roots. 2.4-DNP applied to the nutrient solution at a concentration of 10^-5 M reduces the phosphorus absorption.     

Comparative study on gastric absorption of albendazole and mebendazole in rats

1. A study was carried out to determine the kinetics of the gastric absorption of two wide spectrum benzimidazole anthelmintics, albendazole and mebendazole.2. The method used was gastric recirculation of solutions containing the drugs in concentrations ranging from 0.5 to 100 mM.3. The results obtained showed that absorption corresponds to first order kinetics, with diffusion constants of 0.0087 min⁻¹ for albendazole and 0.0077 min⁻¹ for mebendazole.4. Blood levels of the drugs for the whole range of concentrations were always higher in the case of albendazole.     

A kinetic study of phosphorus absorption by excised maize roots from flowing solutions with different phosphorus concentrations

The effect of two different mechanisms of phosphorus ion transport from the nutrient solution volume to the surface areas of excised maize roots was studied under concentrations ranging from 0.01 mM to 50.0 mM KH₂PO₄. A modified technique of study of kinetic ion absorption was used. In the control series, the roots were placed in absorption solution without flow (the dominant mechanism of ion transport to the roots being diffusion), while in the experimental series the absorption solution was flowing round the roots at a rate of 0.162 cm s^?1 (the dominant mechanism of ion transport to the roots being mass flow). The rate of phosphorus absorption by the roots from flowing solutions was highly significantly increased at all concentrations of absorption solution except for the 50.0 mM KH₂PO₄ concentration. The increase in phosphorus absorption in the case of 50.0 mM KH₂PO₄ concentration was non-significant due to the fact that the high concentration of phosphorus together with the diffusion of phosphorus ions ensured a sufficient supply of phosphorus to the roots, covering the requirement for their uptake. The results point to the need for an analysis of environmental factors to be carried out in studying ion absorption kinetics, and reveal the inadequacy of methods usually employed in such investigations, in particular with respect to the homogeneity of the nutrient solution in the whole of its volume and especially round the roots.     

Comparative study on gastric absorption of albendazole and mebendazole in rats.

1. A study was carried out to determine the kinetics of the gastric absorption of two wide spectrum benzimidazole anthelmintics, albendazole and mebendazole. 2. The method used was gastric recirculation of solutions containing the drugs in concentrations ranging from 0.5 to 100 mM. 3. The results obtained showed that absorption corresponds to first order kinetics, with diffusion constants of 0.0087 min-1 for albendazole and 0.0077 min-1 for mebendazole. 4. Blood levels of the drugs for the whole range of concentrations were always higher in the case of albendazole.     

Putrescine uptake in saintpaulia petals

Putrescine uptake and the kinetics of this uptake were studied in petals of Saintpaulia ionantha Wendl. Uptake experiments of [³H] or [¹⁴C] putrescine were done on single petals at room temperature at various pH values. The results show that putrescine uptake occurs against a concentration gradient at low external putrescine concentration (0.5-100 micromolar) and follows a concentration gradient at higher external putrescine concentrations (100 micromolar to 100 millimolar). 2,4-Dinitrophenol and carbonylcyanide-m-chlorophenylhydrazone, two uncouplers, had no effect on putrescine uptake. Uptake rates were constant for 2 hours, reaching a maximum after 3 to 4 hours. Putrescine uptake depended markedly on the external pH and two maxima were observed: at low external concentrations of putrescine, the optimum was at pH 5 to 5.5; at higher concentrations the optimum was at pH 8.     

Transmitter concentration profiles in the synaptic cleft: an analytical model of release and diffusion.

A three-dimensional model for release and diffusion of glutamate in the synaptic cleft was developed and solved analytically. The model consists of a source function describing transmitter release from the vesicle and a diffusion function describing the spread of transmitter in the cleft. Concentration profiles of transmitter at the postsynaptic side were calculated for different transmitter concentrations in a vesicle, release scenarios, and diffusion coefficients. From the concentration profiles the receptor occupancy could be determined using alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor kinetics. It turned out that saturation of receptors and sufficiently fast currents could only be obtained if the diffusion coefficient was one order of magnitude lower than generally assumed, and if the postsynaptic receptors formed clusters with a diameter of roughly 100 nm directly opposite the release sites. Under these circumstances the gradient of the transmitter concentration at the postsynaptic membrane outside the receptor clusters was steep, with minimal cross-talk among neighboring receptor clusters. These findings suggest that for each release site a corresponding receptor aggregate exists, subdividing an individual synapse into independent functional subunits without the need for specific lateral diffusion barriers.     

Effect of luminal Na+ on the kinetics of intestinal absorption of sugars in vivo

The effect of sodium concentration on the absorption kinetics of glucose, galactose and 3-o-methyl-glucose in rat and hamster jejunum in vivo has been studied. In consecutive 1 min periods the total absorption and absorption in presence of 0.5 mM phlorizin were measured. The difference between them was taken as the active transport rate. The perfusion rate value was 5.6 ml X min-1 and sugar concentrations in the perfusion solution ranged from 1 to 10 mM. The results for the different sodium concentrations show a nearly common Vmax for the same sugar and animal species, while the apparent KT values increase when the sodium concentration in the lumen decreases, mimicking a pure affinity-type activation system. The absorption of sugar when solutions without Na+ are perfused, is greater than that entering passively in the presence of phlorizin. An explanation may be that appreciable amounts of endogenous Na+ find their way to the intestinal lumen in favour of the gradient, making Na+-sugar cotransport possible.     

A dual, concentration-dependent absorption mechanism of linoleic acid by rat jejunum in vitro.

Linoleic acid absorption was studied using everted rat jejunal sacs. At low concentrations (42-1260 microM), the relationship between linoleic acid concentration and its absorption rate fitted best to a rectangular hyperbola. At high concentrations (2.5-4.2 mM) the relationship between the two parameters was linear. The separate additions of 2,4-dinitrophenol, cyanide, or azide, or decrease in the incubation temperature from 37 to 20 degrees C did not change the absorption rate of linoleic acid. Absorption rate of linoleic acid at low concentrations increased as the hydrogen ion and taurocholate concentrations were increased or as the unstirred water layer thickness was decreased. Linoleic acid absorption rate was decreased after the additions of lecithin, oleic, linolenic, and arachidonic acids or the substitution of taurocholate with the nonionic surfactant Pluronic F 68. These observations indicate that a concentration-dependent, dual mechanism of transport is operative in linoleic acid absorption. Facilitated diffusion is the predominant mechanism of absorption at low concentrations, while at high concentrations, simple diffusion is predominant. At low concentrations, the absorption rate of linoleic acid is influenced by the pH, surfactant type and concentration, the simultaneous presence of other polyunsaturated fatty acids, and the thickness of the unstirred water layer.     

Effect of chelating agents on the kinetics of diffusion of zinc to a simulated root system and its uptake by wheat

Summary The kinetics of Zn diffusion to a simulated root system was investigated at a controlled rate of exudation of mobile chelating agents through porous ceramic tubes into a soil tagged with ⁶⁵Zn tracer. The chelating agents enhanced the rate of Zn diffusion from the soil into the simulated root to varying extents depending upon their relative efficiency in increasing the concentration gradient of diffusible Zn. The rat eof Zn diffusion from the soil into the simulated root conformed to pseudo-first order kinetics. The uptake of Zn by wheat plants was significantly increased when a constant flux of DTPA, EDTA and FA (fulvic acid) into the ⁶⁵Zn labelled soil was maintained during a 60 days growth period. A significant linear regression between the concentration of soluble Zn in soil and its uptake by wheat was observed. The calculated diffusive flux of Zn based on the assumption of a constant concentration of Zn at the root surface bore a curvilinear relationship with Zn uptake by wheat.     

Nickel absorption and distribution from rat small intestine In situ

The aim of this work was to study the absorption of nickel chloride in rats by means of the intestinal perfusion in situ technique at nickel concentrations of 1, 5, 10, 25, and 100 mg/L. Active transport and facilitated diffusion seem to play an important role in the intestinal absorption of nickel at concentrations≤10 mg/L. At higher concentrations, the absorption rate would be limited by saturation of the carriers. The distribution of the absorbed nickel was studied by intestinal perfusion of a 10-mg Ni/L solution for 30 or 60 min. Both in concentration and amount, the jejunum showed the higher values of absorbed nickel, followed by the kidneys and liver. When all of the collected organs (brain, heart, liver, lungs, spleen, kidneys, and testicles) and blood, but not the small intestine, are analyzed following a 60-min perfusion, it was found that 1% of the initial concentration had passed through the intestinal barrier.     

A model for glucose-induced wave propagation in pancreatic islets of Langerhans

A reaction-diffusion type model is constructed, describing the spatio-temporal dynamics of the basic intracellular variables assumed to be involved in the initiation of the insulin secretion process by beta -cells in the pancreatic islets of Langerhans. The model includes equations for the electric membrane potential of the cells, with respective kinetics for ionic currents, for concentrations of both free and stored intracellular calcium, and for the intra- and extracellular concentrations of glucose. An empirical expression connecting the equation for the intracellular glucose concentration to the electrical equation is introduced. The model reproduces the events observed in experiments in vitro upon external glucose application to the islets of Langerhans, such as usual bursting oscillations of the membrane potential and corresponding oscillations of the intracellular calcium concentration. It also allows simulation of electric wave propagation through the islet, initiated by the spatial gradient of glucose concentration within the islet. The gradient emerges due to glucose diffusing into the islets from the external medium, being high at the edges. The latter results show that glucose diffusion presents a means for wave initiation in the islets, which supports our previous assumption (Aslanidi et al., 2001).     

Mass transfer kinetics in the chromatography of insulin variants under nonlinear conditions

Experimental data regarding the thermodynamics and kinetics of adsorption of lispro, an insulin variant, onto a YMC ODS-A column, from an aqueous solution of acetonitrile (31%) and TFA are reinterpreted, using a more complex model of the mass transfer kinetics. The adsorption behavior follows the Toth isotherm model, suggesting either a strongly heterogeneous surface or, rather, that when insulin molecules adsorb they contact the surface along different areas of the molecule. The lumped pore diffusion (POR) model of chromatography accounts well for the band profiles. The internal mass transfer resistances are higher than expected, which suggests that intraparticle diffusion is slower. Furthermore, the pore diffusion coefficient increases with decreasing sample size. That surface diffusion accounts for the mass transfer kinetics inside particles explains these results. Assuming that the gradient of the surface concentration is the driving force of surface diffusion, it is possible to account very well for the band profiles of samples of widely different sizes, using a single value of the surface diffusivity.     

双指数方程在青藤碱脂质体贴剂体外透皮特性拟合中的应用研究

为研究青藤碱脂质体贴剂的体外透皮特性,采用Franz扩散池进行体外透皮扩散,对不同时间点的接收液进行采样1 mL,并随即更换1 mL新鲜的接收液,HPLC法测定接收液浓度,并进行模型拟合。以AIC及相关系数为判断标准,结果表明双指数双相动力学方程为最佳拟合。     

Variation of intracellular Ca2+ following Ca2+ current in heart. A theoretical study of ionic diffusion inside a cylindrical cell.

The variation in the concentration of a diffusing substance inside a cylindrical cell submitted to a time-dependent flux at the sarcolemmal membrane was studied theoretically. An application was derived to estimate the local modifications of intracellular free Ca2+ concentration ([CA2+]i) induced by the slow inward Ca2+ current (ICa) in frog heart. During a O mV voltage clamp depolarization, [Ca2+]i at the inner side of the membrane rises earlier and faster than [Ca2+]i at the center of the cell. The binding of intracellular Ca2+ to specific sites enhances the deviation between the two concentrations and may generate an accumulation-depletion process of Ca2+ near the membrane. However, it also decreases the overall [Ca2+]i. The relatively slow diffusion of sarcoplasmic Ca2+ does not significantly affect the kinetics of ICa through a modification in the Ca2+ gradient across the membrane.     

Systemic delivery of enkephalin peptide through eyes

Leu-enkephalin is one of the important peptides which could become useful drugs in the clinics because of its analgesic action and its availability in mass quantity through biotechnology production. It is found in this study that enkephalin can be effectively absorbed systemically through eyes without using a surfactant as absorption enhancer. Enkephalin at 0.125% (31.25 micrograms/25 microliter) reached a plateau of blood concentration at 11.5 ng/ml in 3-4 hrs and stayed high for 8-9 hrs. In contrast, the blood concentration of enkephalin declined rapidly after i.v. administration with a T1/2 of less than 30 min and reached the lowest point at 22 ng/ml in 5 hrs. With higher concentrations at 1% (.25 mg/25 microliter) and 5% (1.25 mg/25 microliter) similar absorption kinetics was observed except that they reached higher plateau of blood concentration at 72 ng/ml and 233 ng/ml, respectively.     

Electrogenic L-malate transport by Lactobacillus plantarum: a basis for energy derivation from malolactic fermentation.

L-Malate transport in Lactobacillus plantarum was inducible, and the pH optimum was 4.5. Malate uptake could be driven by an artificial proton gradient (delta pH) or an electroneutral lactate efflux. Because L-lactate efflux was unable to drive L-malate transport in the absence of a delta pH, it did not appear that the carrier was a malate-lactate exchanger. The kinetics of malate transport were, however, biphasic, suggesting that the external malate concentration was also serving as a driving force for low-affinity malate uptake. Because the electrical potential (delta psi, inside negative) inhibited malate transport, it appeared that the malate transport-lactate efflux couple was electrogenic (net negative) at high concentrations of malate. De-energized cells that were provided with malate only generated a large proton motive force (greater than 100 mV) when the malate concentration was greater than 5 mM, and malate only caused an increase in cell yield (glucose-limited chemostats) when malate accumulated in the culture vessel. The use of the malate gradient to drive malate transport (facilitated diffusion) explains how L. plantarum derives energy from malolactic fermentation, a process which does not involve substrate-level phosphorylation.     

Kinetics of glycerol uptake by the perfused rat liver. Membrane transport, phosphorylation and effect on NAD redox level.

The kinetics of glycerol uptake by the perfused rat liver were determined according to a model which includes membrane transport, intracellular phosphorylation and competitive inhibition of glycerol phosphorylation by L-glycerol 3-phosphate. The membrane transport obeys first-order kinetics at concentrations below 10 mM in the affluent medium. The K-m of the glycerol phosphorylation was 10 muM and the K-i of the L-glycerol 3-phosphate inhibition was 50 muM. The maximum activity (V) was 3.70 mumoles/min per g liver wet wt. These results are similar to in vitro kinetics of the glycerol kinase, except that K-i was found to be somewhat lower in the intact organ. At low glycerol concentrations, a steep concentration gradient exists across the liver cell membrane. The increase in the lactate to pyruvate concentration ratio during glycerol metabolism is related to the actual concentration of L-glycerol 3-phosphate, not to the rate of glycerol uptake.     

Effect of Na+ and K+ ions on the luminescence of intact Vibrio harveyi cells at different pH values

The bioluminescent activity of intact Vibrio harveyi cells loaded with different concentrations of NaCl and KCl at different pH values was studied. In the pH range of 6.5-8.5, the effect of Na+ was significantly higher than that of K+ at all concentrations studied. Maximum luminescent activity was observed in cells loaded with 0.68 M NaCl. When Na+ was nonuniformly distributed on the plasma membrane, the cell luminescence kinetics was nonstationary in the 20-min range: during incubation, the luminescence intensity increased at pH 6.5 and decreased at pH 8.5. The activation and damping rate constants depended on the Na+ gradient value. The maximum of luminescent activity shifted during incubation from pH 8.5 to 6.5-7.0. The luminescence kinetics in the systems with KCl was stationary; the maximum level of luminescence was observed in the pH range of 7.0-7.5. Under Na(+)-controlled conditions, the cell respiration and luminescence changed in synchronism. The protonophore CCP at a concentration of 20 microM completely inhibited luminescence at pH 6.5 and was ineffective at pH 8.5.     

Transport of monosaccharides in kidney-cortex cells

1. The aerobic transport of d-glucose and d-galactose in rabbit kidney tissue at 25 degrees was studied. 2. In slices forming glucose from added substrates an accumulation of glucose against its concentration gradient was found. The apparent ratio of intracellular ([S](i)) and extracellular ([S](o)) glucose concentrations was increased by 0.4mm-phlorrhizin and 0.3mm-ouabain. 3. Slices and isolated renal tubules actively accumulated glucose from the saline; the apparent [S](i)/[S](o) fell below 1.0 only at [S](o) higher than 0.5mm. 4. The rate of glucose oxidation by slices was characterized by the following parameters: K(m) 1.16mm; V(max.) 4.5mumoles/g. wet wt./hr. 5. The active accumulation of glucose from the saline was decreased by 0.1mm-2,4-dinitrophenol, 0.4mm-phlorrhizin and by the absence of external Na(+). 6. The kinetic parameters of galactose entry into the cells were: K(m) 1.5mm; V(max) 10mumoles/g. wet wt./hr. 7. The efflux kinetics from slices indicated two intracellular compartments for d-galactose. The galactose efflux was greatly diminished at 0 degrees , was inhibited by 0.4mm-phlorrhizin, but was insensitive to ouabain. 8. The following mechanism of glucose and galactose transport in renal tubular cells is suggested: (a) at the tubular membrane, these sugars are actively transported into the cells by a metabolically- and Na(+)-dependent phlorrhizin-sensitive mechanism; (b) at the basal cell membrane, these sugars are transported in accordance with their concentration gradient by a phlorrhizin-sensitive Na(+)-independent facilitated diffusion. The steady-state intracellular sugar concentration is determined by the kinetic parameters of active entry, passive outflow and intracellular utilization.