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At least 1,531 records · Page 85Linked to original sources
PubMed2000

From breeder reactors to butterflies: risk, culture, and biotechnology.

Social theories of risk suggest that a combination of scientific and cultural perspectives converge to influence risk perception. This article first surveys sociological perspectives suggesting that risks from modern technological development have become predominant concerns in the social consciousness. Particular attention is given to those theses describing how social elements work to create perception of risks in relation to new technologies. The themes that emerge from this survey are then related to contemporary debates concerning biotechnology. Specific attention is given to recent controversies regarding genetically modified crops, and parallels are drawn between debates over nuclear power and biotechnology. A procedural ethic for public discourse and decision making over the diffusion of genetically modified foods is offered. Ethical and social theories are linked with the hope that by recognizing the social dimensions of debates over new technologies a broader framework for conducting risk analysis may emerge.

Animals↗
PubMed2001

The role of short-term memory in semantic priming.

Two theories of priming were compared: spreading activation theories, in particular ACT, and compound-cue theories. Whereas ACT assumes that priming is a result of diffusing activation in long-term memory, compound-cue models suggest that priming results from a formation process of prime and target in short-term memory. Thirty-eight participants took part in a study that combined a digit span task with a double lexical decision task consisting of a prime and a target item. Digit span length (low, medium, and high) and prime type (related or unrelated word or nonword) were both within-subject variables. As expected, results showed significant priming effects. In favor of ACT, no interaction between digit span length and prime type was found. Additionally, a nonword inhibition effect (unrelated versus nonword prime) was found, which was predicted by compound-cue theories. This finding is discussed in terms of the process interference and response competition hypotheses.

Decision Making↗
PubMed1979

Renal and mammary excretion of sulfadimidine in buffaloes.

The renal and mammary excretion of sulfadimidine was investigated in 8 lactating buffaloes after intravenous administration. The results showed that sulfadimidine was bound to the proteins in plasma (39--59 per cent) and milk (5.5 per cent). The renal handling of sulfadimidine was influenced by the variations in the urinary pH and the concentration of non-protein-bound drug. From the results it is concluded that glomerular filtration, back diffusion and active tubular secretion are involved in the renal handling of sulfadimidine in buffaloes. The results of mammary excretion showed that sulfadimidine is excreted into milk of buffaloes in concentration lower than in plasma. The ratio between the concentration of sulfadimidine in milk and plasma increases when the pH of milk increases. The results are consistant with the theory that drugs are excreted through the mammary gland by passive diffusion.

Animals↗
PubMed2004

Rate constant for diffusion-influenced ligand binding to receptors of arbitrary shape on a cell surface.

The theory of ligand binding to receptors on a cell surface suggested by Berg and Purcell and generalized by Zwanzig and Szabo uses the assumption that receptors are circular absorbing disks on an otherwise reflecting sphere. One of the key ingredients of this theory is a solution for the rate constant for ligand binding to a single circular receptor on a reflecting plane. We give an exact solution for the rate constant for binding to a single elliptic receptor and an approximate solution for binding to a single receptor of more general shape. The latter was tested by Brownian dynamics simulations. We found that the approximate formula predicted the rate constant with better than 10% accuracy for all studied receptor shapes. Using our solutions one can find the rate constant for ligand binding to a cell covered by N noncircular receptors by means of the Zwanzig-Szabo formula.

Cell Membrane↗
PubMed2004

Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211).

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.

Journal Article↗
PubMed1999

Pharmacokinetics in organs and the intact body: model validation and reduction.

Physiological pharmacokinetic models are based on the structure of the circulatory system reflecting the convective transport of drug by blood flow to the various organs and tissues. Distribution kinetics at the organ level is mostly simplified as transfer between well-stirred compartments neglecting a priori the effects of intravascular dispersion and diffusion within tissue parenchyma. Recirculatory models based on residence time theory overcome these structural limitations since they allow in a most general way the decomposition of the body into its natural subsystems. Because of the unidentifiability of the global multi-organ model on the basis of plasma concentration-time curves the following methods/experimental designs will be discussed which provide quantitative information regarding the subsystems under in vivo conditions: (i) determination of tissue concentration-time profiles (destructive sampling), (ii) estimation of the organ transit time density from input/output profiles and (iii) application of a recirculatory model with reduced complexity to clinical pharmacokinetic data.

Animals↗
PubMed1994

A new model for bacterial growth in heterogeneous systems.

A general theory of spatially dependent bacterial growth in heterogeneous systems is developed by combining a structured-cell kinetic model of bacterial metabolism with reaction-diffusion equations describing the transport of nutrients in the growth medium. Rate constants in the theory are determined for Listeria monocytogenes by fitting the viable count batch growth data acquired in homogeneous, shake-flask cultures. The rate constants are then used to calculate the form of bacterial growth when it occurs as localized microcolonies in structured systems. The same theory is used to analyse the effects of transient variations in environmental variables.

Bacteriological Techniques↗
PubMed1998

The size of the unstirred layer as a function of the solute diffusion coefficient.

By monitoring the concentration distribution of several solutes that are diffusing at the same time under given mixing conditions, it was established that the unstirred layer (USL) has no clearly defined boundary. For the cases of solute permeation and water movement across planar bilayer lipid membranes, respectively, experiments carried out with double-barreled microelectrodes have shown that the thickness of the USL depends on which species is diffusing. Small molecules with a larger diffusion coefficient encounter an apparently thicker USL than larger molecules with a smaller diffusion coefficient. The ratio of the USL thicknesses of two different substances is equal to the third root of the ratio of the respective diffusion coefficients. This experimental finding is in good agreement with theoretical predictions from the theory of physicochemical hydrodynamics.

Biophysical Phenomena↗
PubMed1999

Anomalous surface diffusion of water compared to aprotic liquids in nanopores.

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.

Diffusion↗
PubMed2006

Stabilization of inhomogeneous patterns in a diffusion-reaction system under structural and parametric uncertainties.

Many phenomena such as neuron firing in the brain, the travelling waves which produce the heartbeat, arrythmia and fibrillation in the heart, catalytic reactions or cellular organization activities, among others, can be described by a unifying paradigm based on a class of nonlinear reaction-diffusion mechanisms. The FitzHugh-Nagumo (FHN) model is a simplified version of such class which is known to capture most of the qualitative dynamic features found in the spatiotemporal signals. In this paper, we take advantage of the dissipative nature of diffusion-reaction systems and results in finite dimensional nonlinear control theory to develop a class of nonlinear feedback controllers which is able to ensure stabilization of moving fronts for the FHN system, despite structural or parametric uncertainty. In the context of heart or neuron activity, this class of control laws is expected to prevent cardiac or neurological disorders connected with spatiotemporal wave disruptions. In the same way, biochemical or cellular organization related with certain functional aspects of life could also be influenced or controlled by the same feedback logic. The stability and robustness properties of the controller will be proved theoretically and illustrated on simulation experiments.

Diffusion↗
PubMed2000

A mechanism on the drug release into a perfect sink from a coated planar matrix with a super-saturation loading in the core.

A comprehensive model is proposed to accurately describe drug release kinetics from a coated plane sheet when drug loading in the core is above its saturation level. The general solutions are acquired in a dimensionless form by the Laplace transform and the solution for the special case-a perfect sink condition, is derived from these general ones. On the basis of the model calculations, the effects of the diffusion ratios and thickness ratios of the coatings to the core, and drug loading entrapped in the core during the release processes have been discussed over wide range of variables. To validate the model equations proposed, the coated systems, 5-fluoracilum/ethylene-vinyl alcohol copolymer (5-Fu/EVAL) core matrix coated with various polymeric materials such as EVAL, cellulose acetate (CA), poly(2-hydroxyethyl methacrylate) (PHEMA) and poly( methyl methacrylate) (PMMA), having different diffusivities, are designed, experimentally investigated and graphically and quantitatively compared with the theoretical values. The results show a good correlation between the theory and experiment.

Algorithms↗
PubMed2006

Optical properties of single semiconductor nanocrystals.

We present an overview of the current progress in the understanding of the (steady state) optical properties of individual II-VI semiconductor nanocrystals. We begin with a presentation of the conceptual development of the theory required to model the electronic structure of these systems. This is followed by an overview of the current experimental results obtained from the spectroscopy of individual semiconductor nanocrystals, and in particular, we focus on the study of photoluminescence intermittency (blinking) and spectral diffusion. Where possible, we link the experimental observations to the predictions of current theories. We conclude that the surface of small semiconductor crystals plays an important role in determining their optical properties.

Anisotropy↗
PubMed1992

A general diffusion model for analyzing the efficacy of synaptic input to threshold neurons.

We describe a general diffusion model for analyzing the efficacy of individual synaptic inputs to threshold neurons. A formal expression is obtained for the system propagator which, when given an arbitrary initial state for the cell, yields the conditional probability distribution for the state at all later times. The propagator for a cell with a finite threshold is written as a series expansion, such that each term in the series depends only on the infinite threshold propagator, which in the diffusion limit reduces to a Gaussian form. This procedure admits a graphical representation in terms of an infinite sequence of diagrams. To connect the theory to experiment, we construct an analytical expression for the primary correlation kernel (PCK) which profiles the change in the instantaneous firing rate produced by a single postsynaptic potential (PSP). Explicit solutions are obtained in the diffusion limit to first order in perturbation theory. Our approximate expression resembles the PCK obtained by computer simulation, with the accuracy depending strongly on the mode of firing. The theory is most accurate when the synaptic input drives the membrane potential to a mean level more than one standard deviation below the firing threshold, making such cells highly sensitive to synchronous synaptic input.

Computer Simulation↗
PubMed2006

Growth dynamics of domains in ternary fluid vesicles.

We have studied the growth dynamics of domains on ternary fluid vesicles composed of saturated (dipalmitoylphosphatidylcholine), unsaturated (dioleoylphosphatidylcholine) phosphatidylcholine lipids, and cholesterol using a fluorescence microscopy. The domain coarsening processes are classified into two types: normal coarsening and trapped coarsening. For the normal coarsening, the domains having flat circular shape grow in a diffusion-and-coalescence manner and phenomenologically the mean size grows as a power law of approximately t(2/3). The observed growth law is not described by a two-dimensional diffusion-and-coalescence growth mechanism following the Saffman and Delbrück theory, which may originate from the two-body hydrodynamic interactions between domains. For trapped coarsening, on the other hand, the domain coarsening is suppressed at a certain domain size because the repulsive interdomain interactions obstruct the coalescence of domains. The two-color imaging of the trapped domains reveals that the repulsive interactions are induced by the budding of domains. The model free energy consisting of the bending energy of domains, the bending energy of matrix, the line energy of domain boundary, and the translation energy of domains can describe the observed trapped coarsening. The trapping of domains is caused by the coupling between the phase separation and the membrane elasticity under the incompressibility constraint.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2006

Vessel enhancing diffusion: a scale space representation of vessel structures.

A method is proposed to enhance vascular structures within the framework of scale space theory. We combine a smooth vessel filter which is based on a geometrical analysis of the Hessian's eigensystem, with a non-linear anisotropic diffusion scheme. The amount and orientation of diffusion depend on the local vessel likeliness. Vessel enhancing diffusion (VED) is applied to patient and phantom data and compared to linear, regularized Perona-Malik, edge and coherence enhancing diffusion. The method performs better than most of the existing techniques in visualizing vessels with varying radii and in enhancing vessel appearance. A diameter study on phantom data shows that VED least affects the accuracy of diameter measurements. It is shown that using VED as a preprocessing step improves level set based segmentation of the cerebral vasculature, in particular segmentation of the smaller vessels of the vasculature.

Anisotropy↗
PubMed1991

Diffusion models for chemotaxis: a statistical analysis of noninteractive unicellular movement.

A program is developed for applying stochastic differential equations to models for chemotaxis. First a few of the experimental and theoretical models for chemotaxis both for swimming bacteria and for cells migrating along a substrate are reviewed. In physical and biological models of deterministic systems, finite difference equations are often replaced by a limiting differential equation in order to take advantage of the ease in the use of calculus. A similar but more intricate methodology is developed here for stochastic models for chemotaxis. This exposition is possible because recent work in probability theory gives ease in the use of the stochastic calculus for diffusions and broad applicability in the convergence of stochastic difference equations to a stochastic differential equation. Stochastic differential equations suggest useful data for the model and provide statistical tests. We begin with phenomenological considerations as we analyze a one-dimensional model proposed by Boyarsky, Noble, and Peterson in their study of human granulocytes. In this context, a theoretical model consists in identifying which diffusion best approximates a model for cell movement based upon theoretical considerations of cell physiology. Such a diffusion approximation theorem is presented along with discussion of the relationship between autocovariance and persistence. Both the stochastic calculus and the diffusion approximation theorem are described in one dimension. Finally, these tools are extended to multidimensional models and applied to a three-dimensional experimental setup of spherical symmetry.

Chemotaxis↗
PubMed2006

Solvent reorganization of electron transitions in viscous solvents.

We develop a model of electron transfer reactions at conditions of nonergodicity when the time of solvent relaxation crosses the observation time window set up by the reaction rate. Solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile is studied by molecular dynamics simulations at varying temperatures. We observe a sharp decrease of the reorganization energy at a temperature identified as the temperature of structural arrest due to cage effect, as discussed by the mode-coupling theory. This temperature also marks the onset of the enhancement of translational diffusion relative to rotational relaxation signaling the breakdown of the Stokes-Einstein relation. The change in the reorganization energy at the transition temperature reflects the dynamical arrest of the slow, collective relaxation of the solvent related to the relaxation of the solvent dipolar polarization. An analytical theory proposed to describe this effect agrees well with both the simulations and experimental Stokes shift data. The theory is applied to the analysis of charge-transfer kinetics in a low-temperature glass former. We show that the reorganization energy is substantially lower than its equilibrium value for the low-temperature portion of the data. The theory predicts the possibility of discontinuous changes in the dependence of the electron transfer rate on the free energy gap when the reaction switches between ergodic and nonergodic regimes.

Journal Article↗