Effect of intraluminal thrombus thickness and bulge diameter on the oxygen diffusion in abdominal aortic aneurysm.

The intraluminal thrombus (ILT) commonly found within abdominal aortic aneurysm (AAA) may serve as a barrier to oxygen diffusion from the lumen to the inner layers of the aortic wall. The purpose of this work was to address this hypothesis and to assess the effects of AAA bulge diameter (dAAA) and ILT thickness (delta) on the oxygen flow. A hypothetical, three-dimensional, axisymmetric model of AAA containing ILT was created for computational analysis. Commercial software was utilized to estimate the volume flow of O2 per cell, which resulted in zero oxygen tension at the AAA wall. Solutions were generated by holding one of the two parameters fixed while varying the other. The supply of O2 to the AAA wall increases slightly and linearly with dAAA for a fixed delta. This slight increase is due to the enlarged area through which diffusion of O2 may take place. The supply of O2 was found to decrease quickly with increasing delta for a fixed dAAA due to the increased resistance to O2 transport by the ILT layer. The presence of even a thin, 3 mm ILT layer causes a diminished O2 supply (less than 4 x 10(-10) mumol/min/cell). Normally functioning smooth muscle cells require a supply of 21 x 10(-10) mumol/min/cell. Thus, our analysis serves to support our hypothesis that the presence of ILT alters the normal pattern of O2 supply to the AAA wall. This may lead to hypoxic cell dysfunction in the AAA wall, which may further lead to wall weakening and increased potential for rupture.     

Investigations of reaction kinetics for immobilized enzymes--identification of parameters in the presence of diffusion limitation

A method is proposed for identification of kinetic parameters when diffusion of substrates is limiting in reactions catalyzed by immobilized enzymes. This method overcomes conventional sequential procedures, which assume immobilization does not affect the conformation of the enzyme and, thus, consider intrinsic and inherent kinetics to be the same. The coupled equations describing intraparticle mass transport are solved simultaneously using numerical methods and are used for direct estimation of kinetic parameters by fitting modeling results to time-course measurements in a stirred tank reactor. While most traditional procedures were based on Michaelis-Menten kinetics, the method presented here is applicable to more complex kinetic mechanisms involving multiple state variables, such as ping-pong bi-bi. The method is applied to the kinetic resolution of (R/S)-1-methoxy-2-propanol with vinyl acetate catalyzed by Candida antarctica lipase B. A mathematical model is developed consisting of irreversible ping-pong bi-bi kinetics, including competitive inhibition of both enantiomers. The kinetic model, which fits to experimental data over a wide range of both substrates (5-95%) and temperatures (5-56 degrees C), is used for simulations to study typical behavior of immobilized enzyme systems.     

Persistence in reaction diffusion models with weak allee effect

We study the positive steady state distributions and dynamical behavior of reaction-diffusion equation with weak Allee effect type growth, in which the growth rate per capita is not monotonic as in logistic type, and the habitat is assumed to be a heterogeneous bounded region. The existence of multiple steady states is shown, and the global bifurcation diagrams are obtained. Results are applied to a reaction-diffusion model with type II functional response, and also a model with density-dependent diffusion of animal aggregation. J. S. is partially supported by United States NSF grants DMS-0314736 and EF-0436318, College of William and Mary summer grants, and a grant from Science Council of Heilongjiang Province, China.     

Bizygomatic diameter: the thickness of the soft tissues over the zygions

When comparisons are made between the bizygomatic diameter of different subjects, or when measurements made on the living are compared with those on skulls, the thickness of the soft tissue overlying the zygions is of considerable importance. The thickness of these tissues over each of 208 zygions of cadavera has been studied and has been found to differ considerably. The range was from 1.4 mm to 21.4 mm, and the thickness was related to the general body-build — fat, medium or thin. Statements in the literature assume that 6 mm is a suitable estimate of the total thickness of the soft tissues over both zygions. However the present findings indicate that these assumptions are not justified, for in 92% of these specimens the thickness exceeded 6 mm.     

Modelling of the enzymatic kinetically controlled synthesis of cephalexin: influence of diffusion limitation

In this study the influence of diffusion limitation on enzymatic kinetically controlled cephalexin synthesis from phenylglycine amide and 7-aminodeacetoxycephalosporinic acid (7-ADCA) was investigated systematically. It was found that if diffusion limitation occurred, both the synthesis/hydrolysis ratio (S/H ratio) and the yield decreased, resulting in lower product and higher by-product concentrations. The effect of pH, enzyme loading, and temperature was investigated, their influence on the course of the reaction was evaluated, and eventually diffusion limitation was minimised. It was found that at pH >or=7 the effect of diffusion limitation was eminent; the difference in S/H ratio and yield between free and immobilised enzyme was considerable. At lower pH, the influence of diffusion limitation was minimal. At low temperature, high yields and S/H ratios were found for all enzymes tested because the hydrolysis reactions were suppressed and the synthesis reaction was hardly influenced by temperature. The enzyme loading influenced the S/H ratio and yield, as expected for diffusion-limited particles. For Assemblase 3750 (the number refers to the degree of enzyme loading), it was proven that both cephalexin synthesis and hydrolysis were diffusion limited. For Assemblase 7500, which carries double the enzyme load of Assemblase 3750, these reactions were also proven to be diffusion limited, together with the binding-step of the substrate phenylglycine amide to the enzyme. For an actual process, the effects of diffusion limitation should preferably be minimised. This can be achieved at low temperature, low pH, and high substrate concentrations. An optimum in S/H ratio and yield was found at pH 7.5 and low temperature, where a relatively low reaction pH can be combined with a relatively high solubility of 7-ADCA. When comparing the different enzymes at these conditions, the free enzyme gave slightly better results than both immobilised biocatalysts, but the effect of diffusion limitation was minimal.     

The effect of dopamine on the pupillary diameter in mice

Dopamine and adrenaline injected into mice produce dose-related mydriasis. The effects of both dopamine and adrenaline are antagonized similarly by the alpha-adrenergic blocking agents, phentolamine and thymoxamine, as well as by haloperidol, but are not prevented by pretreatment with reserpine. These results suggest that in mice dopamine produces mydriasis by direct stimulation of alpha-adrenergic receptors in the dilator iridis.     

The deacetylation reaction in Eucalyptus wood: Kinetics and effects on the effective diffusion

The removal of native acetyl groups from hardwood O-acetyl-glucuronoxylan has a strong effect on physical characteristics, accessibility and structure of this polymer. The removal also has effects on the swelling and ion transport capacity of the cell wall of hardwoods. In this work, a kinetic expression for Eucalyptus wood deacetylation is determined. Two liquid mediums are considered: a simple alkaline one and another with a higher sodium concentration. The kinetic expression is a power law for the acetyl content and the concentrations in the liquid medium dependence, and is an Arrhenius type expression for temperature dependence. The kinetic expression can be useful to predict the physical properties of wood since the analysis of deacetylation effects on effective capillarity (ECCSA) shows that the acetyl content is a determining factor of wood ionic transport capacity.     

Dyes as fungal inhibitors: effect on colony diameter.

The effects of a wide range of concentrations of 13 dyes on the colony diameters of nine fungal strains (including members of the Deuteromycetes and Zygomycetes) were evaluated. Auramine at a concentration of 50 ppm (50 micrograms/ml), methylene blue at a concentration of 500 ppm, gentian violet at a concentration of 5 ppm, and phenol red at a concentration of 50 ppm performed as well as the commonly used dyes dichloran at a concentration of 2 ppm and rose bengal at a concentration of 50 ppm in that they allowed adequate colony development of the Deuteromycetes strains tested and controlled rapidly spreading fungi.     

Dyes as fungal inhibitors: effect on colony diameter.

The effects of a wide range of concentrations of 13 dyes on the colony diameters of nine fungal strains (including members of the Deuteromycetes and Zygomycetes) were evaluated. Auramine at a concentration of 50 ppm (50 micrograms/ml), methylene blue at a concentration of 500 ppm, gentian violet at a concentration of 5 ppm, and phenol red at a concentration of 50 ppm performed as well as the commonly used dyes dichloran at a concentration of 2 ppm and rose bengal at a concentration of 50 ppm in that they allowed adequate colony development of the Deuteromycetes strains tested and controlled rapidly spreading fungi.     

Biofilm development in a membrane-aerated biofilm reactor: effect of flow velocity on performance

The effect of liquid flow velocity on biofilm development in a membrane-aerated biofilm reactor was investigated both by mathematical modeling and by experiment, using Vibrio natriegens as a test organism and acetate as carbon substrate. It was shown that velocity influenced mass transfer in the diffusion boundary layer, the biomass detachment rate from the biofilm, and the maximum biofilm thickness attained. Values of the overall mass transfer coefficient of a tracer through the diffusion boundary layer, the biofilm, and the membrane were shown to be identical during different experiments at the maximum biofilm thickness. Comparison of the results with published values of this parameter in membrane attached biofilms showed a similar trend. Therefore, it was postulated that this result might indicate the mechanism that determines the maximum biofilm thickness in membrane attached biofilms. In a series of experiments, where conditions were set so that the active layer of the membrane attached biofilm was located close to the membrane biofilm interface, it was shown that the most critical effect on process performance was the effect of velocity on biofilm structure. Biofilm thickness and effective diffusivity influenced reaction and diffusion in a complex manner such that the yield of biomass on acetate was highly variable. Consideration of endogenous respiration in the mathematical model was validated by direct experimental measurements of yield coefficients. Good agreement between experimental measurements of acetate and oxygen uptake rates and their prediction by the mathematical model was achieved.     

Immobilized enzymes: Electrokinetic effects on reaction rates under external diffusion

The rates of reactions catalyzed by enzymes immobilized on a nonporous solid surface have been computed employing a Nernst film model. The Nernst-Planck equations for the transport of the charged substrate and product species in the film and the Poisson equation for the distribution of electrical potential are solved numerically with the appropriate boundary conditions. The electrical charge at the surface is assumed to arise from the dissociation equilibria of the acidic and basic surface groups of the enzyme. The pH at the surface affects both the surface charge as well as the intrinsic kinetics of the enzyme-catalyzed reaction. Factors which determine the pH at the surface include the pH in the bulk solution and the release of H(+) ions in the enzyme-catalyzed reaction. The latter causes a lowering of pH at the surface, causing the reaction rate to differ from that computed assuming an equilibrium distribution of electrical potential. Another kind of nonequilibrium contribution is caused by unequal charges or diffusivities of the substrate and products, which results in a diffusion potential being set up. Two moduli are introduced to evaluate the significance of the reaction-generated lowering of pH and the diffusion potential effect. The effect of changing various parameters, e.g., reaction rate constant, substrate concentration, enzyme concentration, pH, etc., on the overall reaction rate are studied.     

Microwave effect on diffusion: a possible mechanism for non-thermal effect

Abstract

In this study, we assume that microwave radiation affects hydrogen bonding between dipolar water molecules and through that diffusion in water at constant temperature. The experimental study was performed on the setup of two identical reservoirs filled with pure water and 0.9% NaCl solution and connected by a thin tube. Alterations of NaCl concentration in the reservoir initially filled with pure water were measured using the resistance of the solution as an indicator. The applied 450?MHz continuous-wave microwave field had the maximal specific absorption rate of 0.4?W/kg on the connecting tube. The standard deviation of water temperature in the setup was 0.02?°C during an experiment. Our experimental data demonstrated that microwave exposure makes faster the process of diffusion in water. The time required for reduction of initial resistance of the solution by 10% was 1.7 times shorter with microwave. This result is consistent with the proposed mechanism of low-level microwave effect: microwave radiation, rotating dipolar water molecules, causes high-frequency alterations of hydrogen bonds between water molecules, thereby affects its viscosity and makes faster diffusion.     

氮限制对硅藻三角褐指藻光系统Ⅱ光化学反应的影响

本文以硅藻三角褐指藻为实验材料,设置氮充足(882.35μmol·L~(-1))和氮限制(40μmol·L~(-1))两种营养盐供应水平进行培养,以探讨氮限制条件下三角褐指藻光系统Ⅱ(PSII)光化学反应及其对高光强的耐受性变化。结果表明:氮限制抑制三角褐指藻的生长,降低其叶绿素a含量,并加剧其受光抑制程度;通过分析快速叶绿素荧光诱导动力学曲线发现,氮限制使得电子从Q_A~-向QB的传递受阻,且高光强作用后,Q_A~-的瞬时积累进一步增加;此外,氮限制条件下,活性反应中心部分关闭,藻细胞单位反应中心吸收的能量(ABS/RC)、捕获的用于还原Q_A的能量(TRo/RC)以及耗散的能量(DI_o/RC)均明显升高;而高光的耦合作用使得PSII反应中心进一步失活;氮限制影响三角褐指藻PSII光化学反应,进而降低其对高光的耐受能力。     

Biofilm development in a membrane-aerated biofilm reactor: effect of intra-membrane oxygen pressure on performance

The effect on intra-membrane oxygen pressure at a constant carbon substrate loading rate on the development of biofilms of Vibrio natrigens in a membrane aerated biofilm reactor (MABR) was investigated experimentally and by mathematical modelling. A recently reported technique (Zhang et al., 1998. Biotechnol. Bioeng. 59: 80-89) for the in situ measurement of the substrate diffusion coefficients in a growing biofilm and the mass transfer coefficients in the boundary layer at the biofilm liquid interface was used. This aided the study of the effect of the heterogeneous biofilm structure and also improved the reliability of the model predictions. The different intra-membrane oxygen pressures used, 12.5, 25 and 50 kPa, with acetate as the carbon substrate, showed a marked effect on the initial biofilm growth rate, on acetate removal rate, particularly in thick biofilms and on biofilm structure. The model predicted the substrate limitation regimes, the location of the active biomass layer within the biofilms and the trends in oxygen uptake rate through the membrane into the biofilms. During the development of the biofilms, the biofilm thickness and the intra-membrane oxygen pressure were found to be the most important parameters influencing the MABR performance while the effect of biofilm structure was less marked.     

Insect eggs at a transition between diffusion and reaction limitation: temperature, oxygen, and water. art.woods@mso.umt.edu

In diverse animal taxa, eggs and embryos are incapable of transporting oxygen by convection. In such cases, internal oxygen distributions are determined jointly by rates of oxygen consumption and diffusive transport. Here we develop a mathematical model of oxygen consumption and transport in insect eggs, with the goal of understanding-for eggs in variable-temperature environments-the interactive effects of the two processes on development. We fit the model to previously published data on development time of eggs of a sphingid moth, Manduca sexta. The fitted coefficients suggest that eggs develop at a transition point between reaction- and diffusion-limitation. We test then this conclusion with independent data on development times of eggs distributed across a set of temperatures generated by a thermal gradient bar. Finally, we develop an extension of the model that considers tradeoffs between oxygen transfer to eggs versus water loss from them. The model results provide both a rationale for why development is often mass-transfer limited and a set of new predictions about oxygen-water tradeoffs.     

The effect of predator limitation on the dynamics of simple food chains

We investigate the influence of competition between predators on the dynamics of bitrophic predator–prey systems and of tritrophic food chains. Competition between predators is implemented either as interference competition, or as a density-dependent mortality rate. With interference competition, the paradox of enrichment is reduced or completely suppressed, but otherwise, the dynamical behavior of the systems is not fundamentally different from that of the Rosenzweig–MacArthur model, which contains no predator competition and shows only continuous transitions between fixed points or periodic oscillations. In contrast, with density-dependent predator mortality, the system shows a surprisingly rich dynamical behavior. In particular, decreasing the density regulation of the predator can induce catastrophic shifts from a stable fixed point to a large oscillation where the predator chases the prey through a cycle that brings both species close to the threshold of extinction. Other catastrophic bifurcations, such as subcritical Hopf bifurcations and saddle-node bifurcations of limit cycles, do also occur. In tritrophic food chains, we find again that fixed points in the model with predator interference become unstable only through Hopf bifurcations, which can also be subcritical, in contrast to the bitrophic situation. The model with a density limitation shows again catastrophic destabilization of fixed points and various nonlocal bifurcations. In addition, chaos occurs for both models in appropriate parameter ranges.     

The effect of fertility limitation on intergenerational social mobility: the quality-quantity trade-off during the demographic transition

The hypothesis that family size limitation by parents enhances the upward mobility chances of their children in (post)industrial populations has a long-standing record in many disciplines, including sociology and economics, as well as evolutionary anthropology and social biology. Yet the empirical record supporting or contradicting the theory is surprisingly limited. The aim of this contribution is to develop a test of the effect of family size limitation on children's intergenerational mobility. This test is applied to an urban population in Belgium that was in the process of experiencing its demographic transition, including the decline of fertility, at the end of the 19th century. The results indicate that the effect of family size was strong, even after controlling for parental social status as well as birth order. Surprisingly, the effects of birth order and family size appear to be largely independent.     

Factors that influence elastomeric coating performance: the effect of coating thickness on basal plate morphology, growth and critical removal stress of the barnacle Balanus amphitrite

Silicone coatings are currently the most effective non-toxic fouling release surfaces. Understanding the mechanisms that contribute to the performance of silicone coatings is necessary to further improve their design. The objective of this study was to examine the effect of coating thickness on basal plate morphology, growth, and critical removal stress of the barnacle Balanus amphitrite. Barnacles were grown on silicone coatings of three thicknesses (0.2, 0.5 and 2 mm). Atypical ("cupped") basal plate morphology was observed on all surfaces, although there was no relationship between coating thickness and i) the proportion of individuals with the atypical morphology, or ii) the growth rate of individuals. Critical removal stress was inversely proportional to coating thickness. Furthermore, individuals with atypical basal plate morphology had a significantly lower critical removal stress than individuals with the typical ("flat") morphology. The data demonstrate that coating thickness is a fundamental factor governing removal of barnacles from silicone coatings.