Osmotic dehydration in plant tissues

The primary aim of the thesis is to provide a comprehensive investigation of the osmotic dehydration processes in plant tissue. Effort has been concentrated on the modelling for simulating the processes. Two mathematical models for simulating the mass transfer during osmotic dehydration processes in plant tissues are developed and verified using existing experimental data. Both models are based on the mechanism of diffusion and convection of any mobile material that can transport in plant tissues. The mass balance equation for the transport of each constituent is established separately for intracellular and extra-cellular volumes with taking into account the mass transfer across the cell membrane the intracellular and extra-cellular volumes and the shrinkage of the whole tissue. The contribution from turgor pressure is considered in both models. Model two uses Darcy’s law to build the relation between shrinkage velocity and hydrostatic pressure in each volume because the plant tissue can be considered as the porous medium. Moreover, it has been extended to solve the multi-dimensional problems. A lot of efforts have been made to the parameter study and the sensitivity analyses. The parameters investigated including the concentration of the osmotic solution, diffusion coefficient, permeability of the cell membrane, elastic modulus of the cell wall, critical cell volume etc. The models allow us to quantitatively simulate the time evolution of intracellular and extra-cellular volumes as well as the time evolution of concentrations in each cross-section.

Numerical simulation of mass transfer during the osmotic dehydration of biological tissues

In this paper a mathematical model based on mass transfer in plant tissues is developed. The model takes into account the diffusion and convection of each constituent within the tissue. The driving force for the convection is assumed to be the gradient of hydrostatic pressure. The mass balance equation for the transport of each constituent is established separately for intracellular and extracellular volumes but taking into account the mass exchange across the cell membrane between the intracellular and extracellular volumes. The mass transfer results in not only the change of intracellular and extracellular volumes but also the shrinkage of whole tissue. The model allows us to quantitatively simulate the time evolution of intracellular and extracellular volumes, which was observed in histological sections under the microscope. © 2005 Elsevier B.V. All rights reserved.

Polyol transport in yeast: studying transport proteins and manipulating osmotic responses

The Saccharomyces cerevisiae MIP channel Fps1p plays an important role in yeast osmoregulation by exporting glycerol. Glycerol accumulates in the cell as a compatible osmolyte during hyperosmotic conditions and is exported once conditions become hypotonic. A gpd1 gpd2 mutant is unable to produce glycerol and is therefore very sensitive to high concentrations of polyols in the growth medium. The sensitivity to C3, C4 and C5, but not C6 polyols, is suppressed by expression of truncated, hyperactive Fps1p. This is because the polyols can then equilibrate over the membrane and hence the concentration gradient collapses. This experiments reveals the substrate spectrum of Fps1p. The system can be used in different ways. For instance, growth assays on different polyols elucidate the substrate range of heterologous channels such as that of the rat aquaglyceroporin AQP9. In addition, the same system is used to search for novel hyperactive mutants of Fps1p, which provide additional information on the mechanism underlying channel regulation. Finally we illustrate that the gpd1 gpd2 double mutant expressing hyperactive Fps1p can be used to manipulate activation and deactivation of the HOG pathway, contributing to our understanding of the control of this osmoregulatory system.

Effect of osmotic and matrix potential on Saprolegnia diclina and S. ferax

Growth of biomass and sporulation of pathogenic and non-pathogenic Saprolegnia species was markedly decreased at reduced water potentials. Oogonium and zoosporangium formation were more sensitive to reduced osmotic and matrix potentials than growth in biomass. Although little difference was observed between the effects of matrix and osmotic potentials, the Saprolegnia species investigated responded differently to those solutes utilized in control of osmotic potential. Biomas, oogonium and zoosporangium formation were greater in the presence of reduced osmotic potentials mediated by mannitol than equivalent potentials mediated by potassium chloride. Endogenous potassium levels varied little with reduced matrix or osmotic potentials. Conversly, mannitol content of colonies exposed to reduced osmotic potentials mediated by mannitol initailly increased while endogenous amino acid levels were observed to rise in response to moderately reduced water potentials. Sensitivity of Saprolegnia species to reduced potantials and effects on substrate colonization are discussed in the light of these observations.

CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors: modelling the impact of biomass shrinkage

The fluid–particle interaction and the impact of shrinkage on pyrolysis of biomass inside a 150 g/h fluidised bed reactor is modelled. Two 500 View the MathML sourcem in diameter biomass particles are injected into the fluidised bed with different shrinkage conditions. The two different conditions consist of (1) shrinkage equal to the volume left by the solid devolatilization, and (2) shrinkage parameters equal to approximately half of particle volume. The effect of shrinkage is analysed in terms of heat and momentum transfer as well as product yields, pyrolysis time and particle size considering spherical geometries. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user defined function (UDF).

Osmotic flow in porous membranes:effects of electric charge

The electrostatic model for osmotic flow across a porous membrane in our previous study (Akinaga et al. 2008)" was extended to include the streaming potential, for solutes and pores of like charge and fixed surface charge densities. The magnitude of the streaming potential was determined to satisfy zero current condition along the pore axis. It was found that the streaming potential affects the velocity profiles of the pressure driven flow as well as the osmotic flow through the pore, and decreases their flow rates, particularly in the case of large Debye length relative to the pore radius, whereas it has little effect on the reflection coefficients of spherical solutes through cylindrical pores.

Fluid mechanical and electrostatic study on the osmotic flow through cylindrical pores

When two solutions differing in solute concentration are separated by a porous membrane, the osmotic pressure will generate a net volume flux of the suspending fluid across the membrane; this is termed osmotic flow. We consider the osmotic flow across a membrane with circular cylindrical pores when the solute and the pore walls are electrically charged, and the suspending fluid is an electrolytic solution containing small cations and anions. Under the condition in which the radius of the pores and that of the solute molecules greatly exceed those of the solvent as well as the ions, a fluid mechanical and electrostatic theory is introduced to describe the osmotic flow in the presence of electric charge. The interaction energy, including the electrostatic interaction between the solute and the pore wall, plays a key role in determining the osmotic flow. We examine the electrostatic effect on the osmotic flow and discuss the difference in the interaction energy determined from the nonlinear Poisson-Boltzmann equation and from its linearized equation (the Debye-Hückel equation).

Effects of electric charge on osmotic flow across periodically arranged circular cylinders

An electrostatic model is developed for osmotic flow across a layer consisting of identical circular cylinders with a fixed surface charge, aligned parallel to each other so as to form an ordered hexagonal arrangement. The expression of the osmotic reflection coefficient is derived for spherical solutes with a fixed surface charge suspended in an electrolyte, based on low-Reynolds-number hydrodynamics and a continuum, point-charge description of the electric double layers. The repulsive electrostatic interaction between the surface charges with the same sign on the solute and the cylinders is shown to increase the exclusion region of solute from the cylinder surface, which enhances the osmotic flow. Applying the present model to the study of osmotic flow across the endothelial surface glycocalyx of capillary walls has revealed that this electrostatic model could account well for the reflection coefficients measured for charged macromolecules, such as albumin, in the physiological range of charge density and ion concentration.

Electrical charge effect on osmotic flow through pores

An electrostatic model for osmotic flow through circular cylindrical pores is developed to describe the reflection coefficient for the membrane transport in the presence of surface charges on the pore wall and the solute. For a spherical solute placed at an arbitrary radial position in the pore, the electrical potential was computed by a spectral element method applied to the Poisson-Boltzmann equation together with the condition of electrical neutrality. The interaction energy between the surface charges was used to estimate the osmotic reflection coefficient. The proposed model predicts that even for a small Debye length compared to the pore radius, the repulsive electrostatic interaction between the surface charges could significantly increase the osmotic flow through the pore.

Conditional osmotic stress in yeast:a system to study transport through aquaglyceroporins and osmostress signaling

The accumulation and transport of solutes are hallmarks of osmoadaptation. In this study we have employed the inability of the Saccharomyces cerevisiae gpd1Δ gpd2Δ mutant both to produce glycerol and to adapt to high osmolarity to study solute transport through aquaglyceroporins and the control of osmostress-induced signaling. High levels of different polyols, including glycerol, inhibited growth of the gpd1Δ gpd2Δ mutant. This growth inhibition was suppressed by expression of the hyperactive allele Fps1-AΔ of the osmogated yeast aquaglyceroporin, Fps1. The degree of suppression correlated with the relative rate of transport of the different polyols tested. Transport studies in secretory vesicles confirmed that Fps1-Δ1 transports polyols at increased rates compared with wild type Fps1. Importantly, wild type Fps1 and Fps1-Δ1 showed similarly low permeability for water. The growth defect on polyols in the gpd1Δ gpd2Δ mutant was also suppressed by expression of a heterologous aquaglyceroporin, rat AQP9. We surmised that this suppression was due to polyol influx, causing the cells to passively adapt to the stress. Indeed, when aquaglyceroporin-expressing gpd1Δ gpd2Δ mutants were treated with glycerol, xylitol, or sorbitol, the osmosensing HOG pathway was activated, and the period of activation correlated with the apparent rate of polyol uptake. This observation supports the notion that deactivation of the HOG pathway is closely coupled to osmotic adaptation. Taken together, our "conditional" osmotic stress system facilitates studies on aquaglyceroporin function and reveals features of the osmosensing and signaling system. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.

Design and evaluation of compound metformin/glipizide elementary osmotic pump tablets

A simple elementary osmotic pump (EOP) system that could deliver metformin hydrochloride (MT) and glipizide (GZ) simultaneously for extended periods of time was developed in order to reduce the problems associated with multidrug therapy of type 2 non-insulin-dependent diabetes mellitus. In general, both highly and poorly water-soluble drugs are not good candidates for elementary osmotic delivery. However, MT is a highly soluble drug with a high dose (500 mg) while GZ is a water-insoluble drug with a low dose (5 mg) so it is a great challenge to pharmacists to provide satisfactory extended release of MT and GZ. In this paper sodium carbonate was used to modulate the solubility of GZ within the core and MT was not only one of the active ingredients but also the osmotic agent. The optimal EOP was found to deliver both drugs at a rate of approximately zero order for up to 10 h in pH 6.8, independent of environment media. In-vivo evaluation was performed relative to the equivalent dose of conventional MT tablet and GZ tablet by a cross-study in six Beagle dogs. The EOP had a good sustained effect in comparison with the conventional product. The prototype design of the system could be applied to other combinations of drugs used for cardiovascular diseases, diabetes, etc.