Invagination and Evagination: A Hydromechanical Model

It has been proposed that processes of invagination and evagination be considered in the context of a single, hydromechanical, model. The model is based on the assumption that the structures demonstrating invaginations and evaginations are closed systems capable of changing their intracavitary pressure in an autonomous regime. The mass, which occupies the cavity, should overcome the resistance of the surrounding membrane during its growth. In places of the weakest resistance, the inner mass expands especially quickly: it bulges out. If the pressure under the membrane is less than the exterior one, the process goes in the opposite direction: the outer mass intrudes (invaginates) into the cavity, causing its wall to sag in the weakest regions.     

The dependence of chamber dynamics on chamber dimensions

One can learn something about the determinants of ventricular dimensions and dynamics from a simple spherical model. We have derived equations showing how isometric pressure, compliance, isometric P-V curves and viscous resistance to wall displacement depend on dimensions of a spherical chamber whose fibers adjust for a "normal" stretch at a particular point in the pump cycle. The derivations show: (a) that isometric pressure at this point is proportional to the logarithm of total chamber volume (cavity plus wall) relative to cavity volume; (b) that compliance at this point is proportional to cavity volume and to total chamber volume relative to wall volume; (c) that the rate of wall displacement relative to the disparity between isometric pressure and actual pressure depends on dimensions like compliance depends on dimensions; and (d) since reciprocal compliance does not increase with wall/cavity ratios as much as isometric pressure at the normal-stretch volume, the P-V curves spread out on either side of the normal-stretch volume as the chamber undergoes adaptive thickening, resulting in disproportionate increases of isometric pressure at low cavity volumes. This tends to increase ejection fraction and reduce cavity volumes relative to stroke volume, and it is partly responsible for the "concentric" character of hypertrophy in response to high systolic pressure.     

Water Vapour Diffusion in the Substomatal Cavity

In this paper stomatal pore and substomatal cavity are considered to be elliptic cylinders, A three-dimensional diffusion model is presented, which describes the diffusion of vapour from the surfaces of the cells surrounding the cavity to the outer end of the pore Equations describing vapour diffusion in the model are set up, based on Fick's law and the law of conservation of mass, and are solved by using computer. Quantitative relation between the cavity resistance to water vapour diffusion and: stomatal aperture is obtained and is given more general theoretical explanation. Comparing the formula obtained in this paper with those of Brown and Escombe and of Cooke et al., it is found that the cavity resistance calculated by the latter two formulas are 0.5 to 1 times higher in a large rankle of stomatal aperture values. Besides, it is shown by calculating that the rates of loss from guard cells and subsidiary ceils account for 88%– 93% and 7%–12% respectively of that from epidermic cells, and the litter amounts to 86%–96% of that from all the cells in the cavity in the large range of stomatal change.     

A Mathematical Model of Spatial Self-Organization in a Mechanically Active Cellular Medium

A general continual model of a medium composed of mechanically active cells is proposed. The medium is considered to be formed by three phases: cells, extracellular fluid, and an additional phase that is responsible for active interaction forces between cells and, for instance, may correspond to a system of protrusions that provide the development of active contractile forces. The deformation of the medium, which is identified with the deformation of the cell phase, consists of two components: elastic deformation of individual cells and cell rearrangements. The elastic deformation is associated with stresses in the cell phase. The spherical component of the stress tensor describes the nonlinear resistance of the cellular medium, which leads to the impossibility of its excessive compression. The constitutive equation for pressure in the cell phase is taken in the form of a nonlinear dependence on the volume cell density. The rearrangement of cells is considered as a flow controlled by stresses in the cell phase, active stresses, and fluid pressure. The tensor of active stresses is assumed to be spherical and nonlocally dependent on the cell density. Assuming that the process of biological tissue deformation is slow, we obtained a reduced model that neglects the elastic deformation of cells, compared to the inelastic deformation. A linear stability analysis of a spatially uniform steady-state solution was performed. The hydrostatic pressure of fluid is present among the parameters that are responsible for the loss of stability of the steady-state solution: an increase in it has a destabilizing effect owing to the action of the component of the interphase interaction force that is determined by the fluid pressure. The model we obtained can be used to describe the process of cavity formation in an initially homogeneous cell spheroid. The role of local and nonlocal mechanisms of active stress generation in the formation of cavity is investigated.     

晋西北3个树种抗旱性指数的研究

根据对海红、刺槐和柠条的ＰＶ曲线主要水分参数的测定,进行了抗旱性指数季节变化和种间排序分析。结果表明,树种的抗旱性随着年生长发育节律和环境水分条件而改变,其抗早性生长季初最弱,夏季高温干早阶段增强,秋季降雨峰期减弱,生长季末最强或很强。柠条的抗旱性与海红很接近。这对黄土高原地区果树和饲料的开发与水土保持树种的推广有重要意义。     

Membrane fouling in crossflow filtration: Implications for measurement of the steady state specific cake resistance

Summary When the crossflow specific cake resistance is determined from measurements of the steady state cake mass and filtrate flux, an apparent value is obtained which is equal to the true value only if membrane fouling is negligible. The apparent specific resistance can increase with increasing crossflow velocity and pure-water membrane resistance, and exhibit a minimum with respect to transmembrane pressure.     

Septal fracture in Nautilus: implications for cephalopod paleobathymetry

Analysis of septal geometry, as embodied in the septal strength index model developed by Westermann (1973, Lethaia 6). , has become a prime avenue for estimating living depths of fossil cephalopods. We have examined the fracturing of Nautilus septa, and its bearing on strength index, by inducing septal rupture under the action of hydrostatic pressure. We found that: (1) septa which actually fail under pressure are not generally weakest as defined by their strength indices; (2) septal strength as defined by the strength index is not correlated with rupture pressure; and (3) most instances of septal failure originate in septal sutures, not in the septa. These results indicate that: (1) septal strength index does not yield wholly reliable strength or depth estimates; and (2) the shortcomings of the strength index model stem from its inability to account for complexities of mechanical failure in morphologically complex cephalopod shells.     

A model membrane protein for binding volatile anesthetics

Earlier work demonstrated that a water-soluble four-helix bundle protein designed with a cavity in its nonpolar core is capable of binding the volatile anesthetic halothane with near-physiological affinity (0.7 mM Kd). To create a more relevant, model membrane protein receptor for studying the physicochemical specificity of anesthetic binding, we have synthesized a new protein that builds on the anesthetic-binding, hydrophilic four-helix bundle and incorporates a hydrophobic domain capable of ion-channel activity, resulting in an amphiphilic four-helix bundle that forms stable monolayers at the air/water interface. The affinity of the cavity within the core of the bundle for volatile anesthetic binding is decreased by a factor of 4-3.1 mM Kd as compared to its water-soluble counterpart. Nevertheless, the absence of the cavity within the otherwise identical amphiphilic peptide significantly decreases its affinity for halothane similar to its water-soluble counterpart. Specular x-ray reflectivity shows that the amphiphilic protein orients vectorially in Langmuir monolayers at higher surface pressure with its long axis perpendicular to the interface, and that it possesses a length consistent with its design. This provides a successful starting template for probing the nature of the anesthetic-peptide interaction, as well as a potential model system in structure/function correlation for understanding the anesthetic binding mechanism.     

Mechanistic mathematical model for in vivo aroma release during eating of semiliquid foods

The paper describes a mechanistic mathematical model for aroma release in the oropharynx to the nasal cavity during food consumption. The model is based on the physiology of the swallowing process and is validated with atmospheric pressure chemical ionization coupled with mass spectrometry measurements of aroma concentration in the nasal cavity of subjects eating flavored yogurt. The study is conducted on 3 aroma compounds representative for strawberry flavor (ethyl acetate, ethyl butanoate, and ethyl hexanoate) and 3 panelists. The model provides reasonably accurate time predictions of the relative aroma concentration in the nasal cavity and is able to simulate successive swallowing events as well as imperfect velopharyngeal closure. The most influent parameters are found to be the amount of the residual product in the pharynx and its contact area with the air flux, the volume of the nasal cavity, the equilibrium air/product partition coefficient of the volatile compound, the breath airflow rate, as well as the mass transfer coefficient of the aroma compound in the product, and the amount of product in the mouth. This work constitutes a first step toward computer-aided product formulation by allowing calculation of retronasal aroma intensity as a function of transfer and volatility properties of aroma compounds in food matrices and anatomophysiological characteristics of consumers.     

Role of membrane fluidity in pressure resistance of Escherichia coli NCTC 8164

The relationship among growth temperature, membrane fatty acid composition, and pressure resistance was examined in Escherichia coli NCTC 8164. The pressure resistance of exponential-phase cells was maximal in cells grown at 10 degrees C and decreased with increasing growth temperatures up to 45 degrees C. By contrast, the pressure resistance of stationary-phase cells was lowest in cells grown at 10 degrees C and increased with increasing growth temperature, reaching a maximum at 30 to 37 degrees C before decreasing at 45 degrees C. The proportion of unsaturated fatty acids in the membrane lipids decreased with increasing growth temperature in both exponential- and stationary-phase cells and correlated closely with the melting point of the phospholipids extracted from whole cells examined by differential scanning calorimetry. Therefore, in exponential-phase cells, pressure resistance increased with greater membrane fluidity, whereas in stationary-phase cells, there was apparently no simple relationship between membrane fluidity and pressure resistance. When exponential-phase or stationary-phase cells were pressure treated at different temperatures, resistance in both cell types increased with increasing temperatures of pressurization (between 10 and 30 degrees C). Based on the above observations, we propose that membrane fluidity affects the pressure resistance of exponential- and stationary-phase cells in a similar way, but it is the dominant factor in exponential-phase cells whereas in stationary-phase cells, its effects are superimposed on a separate but larger effect of the physiological stationary-phase response that is itself temperature dependent.     

Importance of the Difference in Surface Pressures of the Cell Membrane in Doxorubicin Resistant Cells That do not Express Pgp and ABCG2

P-glycoprotein (Pgp) represents the archetypal mechanism of drug resistance. But Pgp alone cannot expel drugs. A small but growing body of works has demonstrated that the membrane biophysical properties are central to Pgp-mediated drug resistance. For example, a change in the membrane surface pressure is expected to support drug–Pgp interaction. An interesting aspect from these models is that under specific conditions, the membrane is predicted to take over Pgp concerning the mechanism of drug resistance especially when the surface pressure is high enough, at which point drugs remain physically blocked at the membrane level. However it remains to be determined experimentally whether the membrane itself could, on its own, affect drug entry into cells that have been selected by a low concentration of drug and that do not express transporters. We demonstrate here that in the case of the drug doxorubicin, alteration of the surface pressure of membrane leaflets drive drug resistance.     

Characterization of a continuous supermacroporous monolithic matrix for chromatographic separation of large bioparticles

A continuous supermacroporous monolithic chromatographic matrix has been characterized using a capillary model, experimental breakthrough curves, and pressure drop experiments. The model describes the convective flow and its dispersive mixing effects, mass transfer resistance, pore size distribution, and the adsorption behavior of the monolithic matrix. It is possible to determine an effective pore size distribution by fitting the capillary model to experimental breakthrough curves and pressure drop experiments. The model is able to describe the flow rate dependence of the experimental breakthrough curves. Mass transport resistance was due to: (i) dispersive mixing effects in the convective flow in the pores; and (ii) slow diffusion in the stagnant film covering the surface within each pore, under adsorption conditions. The monolithic matrix can be described by a very narrow pore size distribution, illustrating one of the advantages of the gel. A broader pore size distribution results in increased band broadening. This can be studied easily using the model developed in this investigation.     

Deformations in the cytoplasmic membrane of Escherichia coli direct the synthesis of peptidoglycan. The hernia model.

To explain the growth of the Gram-negative envelope and in particular how it could be strengthened where it is weakest, we propose in the hernia model that local weakening of the peptidoglycan sacculus allows turgor pressure to cause the envelope to bulge outwards in a hernia; the consequent local alteration in the radius of curvature of the cytoplasmic membrane causes local alterations in phospholipid structure and composition that determine both the synthesis and hydrolysis of peptidoglycan. This proposal is supported by evidence that phospholipid composition determines the activity of phospho-N-acetylmuramic acid pentapeptide translocase, UDP-N-acetylglucosamine:N-acetylmuramic acid-(pentapeptide)-P-P-bactoprenyl-N-acetylglucosamine transferase, bactoprenyl phosphate phosphokinase, and N-acetylmuramyl-L-alanine amidase. We also propose that the shape of Escherichia coli is maintained by contractile proteins acting at the hernia. Given the universal importance of membranes, these proposals have implications for the determination of shape in eukaryotic cells.     

A Two Compartment Model Method for Continuous Determination of Hydraulic Parameters of Higher Plant Cells by Pressure Probe Technique

Zhu, G-L. and Lou, C-H. 1988. A two compartment model methodfor continuous determination of hydraulic parameters of higherplant cells by pressure probe technique.—J. exp. BoL 39:961–971. The new model treats the whole measuring system as two compartments:the vacuole and the cavity of the probe, which are connectedby a microcapillary. Instead of a short injection, a continuousinjection of distilled water is used in this model. The mainprinciple of the calculation method is to estimate cell turgorpressure and hydraulic conductivity continuously from the resistanceof the microcapillary and the probe pressure as well as thecurrent flow through the microcapillary. The model avoids theeffect of membrane potential changes on the measurement of hydraulicparameters in pressure steps. In this model, microcapillarieswith a tip diameter smaller than flow rate-limiting dimensionscan be employed. The present method provides a way to monitorcontinuously cell hydraulic conductivity during the pressurerelaxation process with a time resolution in seconds. The calculationis performed automatically using a microcomputer. Key words: Two compartment model, cell pressure probe, computer     

The nature of fluctuations in spontaneous otoacoustic emission

The nature of fluctuations of sound pressure during spontaneous otoacoustic emission is studied. The phenomenological mechanism of realization of positive feedback in cochlear is proposed. It is based on usage of hair cells ionic channels. By means of this model it is shown that spontaneous emission fluctuations are determined with thermal oscillations. The mass of the vibrating part of the basilar membrane, its dimensions and elasticity are calculated.     

Effects of mass loading on the upper airway

To learn how increased cervical adipose tissue might affect upper airway function, we studied effects of mass loading on upper airway dimensions, stability, and resistance. Eight rabbits were studied (anesthetized and postmortem) using lard-filled bags to simulate cervical fat accumulation. Additionally, a handheld device was used to apply measured loads at localized sites along the airway. Upper airway resistance and closing pressure (a reflection of airway stability) were determined before and after loading. Endoscopy revealed concentric narrowing of the pharynx during loading in anesthetized and postmortem preparations. Upper airway resistance was increased by mass loads, with larger loads having greater effects. Loading caused decreased airway stability as reflected by closing pressures. The area over the thyrohyoid membrane was more vulnerable to mass loading than adjacent areas. Because mass loading of the upper airway causes changes in its configuration and function similar to those seen in obstructive sleep apnea syndrome (OSA), we speculate that such loading may contribute to the pathogenesis of OSA associated with obesity.     

Reaction of cholinoreceptors from an isolated molluskan neuron with analogs of acetylcholine and tetramethylammonium

The effect of 13 acetylcholine and tetramethylammonium derivatives on cholinoreceptors of isolated neurone on the pond snail has been investigated by the analysis of membrane current fluctuations at 10 and 20 degrees C. The elementary current was independent from the agonist structure. The channel open time and its Q10 coefficient were found to be maximum for acetylcholine. At low concentrations, acetylcholine derivative with four groups in its methylene chain exhibited the weakest activity. Agonist activity at higher concentrations was evaluated by maximum increase in the membrane current. With respect to this character, the derivative with two ethyl radicals in its cation group was found to be the weakest one. Possible causes of low activities of the agonists studied in comparison with that of acetylcholine are discussed.     

DIFFUSION RESISTANCE IN LEAVES AS RELATED TO THEIR STOMATAL ANATOMY AND MICRO-STRUCTURE

Transpiration from a plant leaf depends upon the water vapor pressure gradient between the substomatal cavity and the free air beyond the leaf. Transpiration also depends inversely on the resistance of the diffusion pathway through the substomatal cavity, stomate, and surface boundary layer. The value of the diffusion resistance is derived mathematically for Zebrina pendula, Medicago sativa, and Pinus resinosa. The vapor pressure gradient depends on the leaf temperature and therefore is related to the energy budget of the leaf. The exact solution of the diffusion equation is described and limiting examples discussed. The so-called “diameter law” is a special case which is distinctly limited in its application.     

Snoring source identification and snoring noise prediction

This paper investigates the snoring mechanism of humans by applying the concept of structural intensity to a three-dimensional (3D) finite element model of a human head, which includes: the upper part of the head, neck, soft palate, hard palate, tongue, nasal cavity and the surrounding walls of the pharynx. Results show that for 20, 40 and 60Hz pressure loads, tissue vibration is mainly in the areas of the soft palate, the tongue and the nasal cavity. For predicting the snoring noise level, a 3D boundary element cavity model of the upper airway in the nasal cavity is generated. The snoring noise level is predicted for a prescribed airflow loading, and its range agrees with published measurements. These models may be further developed to study the various snoring mechanisms for different groups of patients.     

Water transport through plant tissue: the apoplasm and symplasm pathways

A detailed quantitative analysis of water flow through the apoplasm and symplasm of plant tissue is presented. The analysis results in two coupled diffusion equations which describe water transport in the two pathways. Various parameters entering the analysis identify the physical properties of the tissue which control the transport process as the resistance to water flow per cell in the two parallel pathways, the resistance per cell between pathways, and the water capacity per cell in the two pathways. Values for the several resistances and water capacities are estimated from available data, and a model problem is solved wherein a sheet of tissue at an initial water potential of — δ bars is immersed in a container of water. The resulting solutions show that depending on the values assigned to the controlling parameters, local water potential equilibrium between each cell and its cell wall may or may not obtain. In the special case of local equilibrium (water potential in the symplasm and apoplasm pathways essentially equal), the transport process can be described by a single diffusion equation which is derived along with an expression for the tissue diffusivity. It is concluded that the weakest link in the analysis is the estimated value for the permeability of the plasmodesma membrane, and that a logical extension of the theory would be to include the effects of a diffusable solute.