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At least 271 records · Page 15Linked to original sources
PubMed2000

Random walks and cell size.

For many years, it has been believed that diffusion is the principle motive force for distributing molecules within the cell. Yet, our current information about the cell makes this improbable. Furthermore, the argument that limitations responsible for the relative constancy of cell size--which seldom varies by more than a factor of 2, whereas organisms can vary in mass by up to 10(24)--are based on the limits of diffusion is questionable. This essay seeks to develop an alternative explanation based on transport of molecules along structural elements in the cytoplasm and nucleus. This mechanism can better account for cell size constancy, in light of modern biological knowledge of the complex microstructure of the cell, than simple diffusion.

Animals↗
PubMedDate not supplied

Infant mortality time series are random walks with drift: are they cointegrated with socioeconomic variables?

Previous time series analyses of infant mortality have failed to provide evidence to support their implicit assumption that infant mortality data used behaved as a stationary time series. The present study applies the augmented Dickey Fuller Test to infant mortality time series for Sweden (1800-1989), United Kingdom (1839-1989) and United States (1915-1989). The null hypothesis that each of these series is non-stationary is accepted at standard levels of significance. A conceptual framework of infant mortality which uses a combination of physical and social overhead capital as factors in a production function is developed to explain the finding of non-stationarity as derivative from the non-stationarity of a stock of health-enhancing capital. Estimation of econometric models of the socioeconomic determinants of infant mortality using differenced data with ARIMA estimation is inconclusive. Estimation of a bivariate cointegration model supports the hypothesis that infant survival and GNP/Capita are cointegrated for 19th century Sweden but not for 19th century UK. Bivariate analysis of 20th century Sweden, UK, and US data demonstrated no cointegration. This may be due to the onset of disequilibrium in the economic determination of infant mortality in the present era as technological advances and demographic shifts began to play a larger role. Supplementing the bivariate analysis with measures of unemployment, and crude birth rate in the 20th century permitted the detection of cointegration in US and UK. The multivariate results may suggest that improvements in 20th century UK GNP/capita have had greater impact on infant survival relative to US GNP/capita.

Birth Rate↗
PubMed1997

Random-Walk Aggregation Phenomena in Solid Bimodal Liquid Dispersions: Transition to Nondeterminism from Si3 N4 to Si3 N4 + Al2 O3 Aqueous Systems

This paper, which is based on another recent work, (Mezzasalma, S. A., Phys. Rev. E 55 (4), (1997)) deals with experiments and theory concerning an aqueous dispersed system formed from silicon nitride (Si3 N4 ), alumina (Al2 O3 ), and mixed silicon nitride + alumina (Si3 N4 + Al2 O3 ) solid agglomerates. From titration data applied to a thermodynamic equilibrium condition, the minimum number of each agglomerate species and their maximal average dimensions have been derived as functions of the aqueous solution pH. These parameters are of the order of, respectively, (1-2) &mgr;m for Si3 N4 and Al2 O3 agglomerates and (20-50) &mgr;m for the mixed agglomerates. The numbers of solid particles of all species are poorly correlated with changes in pH of the liquid phase. This behavior has been interpreted as intrinsically related to the complexity of the system which, due to the many interactions among the different species, probably becomes nondeterministic. In order to describe such behavior a probabilistic approach has been developed. The probability of finding a given solid agglomerate number within a scatter band varies with the suspension pH. Furthermore, the scatter band amplitude becomes negligible near the isoelectric point. Accordingly, only the numbers of aggregates derived in the neighborhood of the isoelectric point are predictable.

Journal Article↗
PubMed1997

Random walk models for DNA synapsis by resolvase.

During site-specific recombination by resolvase, the protein binds to two sites on a supercoiled DNA molecule and the loaded sites then interact with each other to form a synaptic complex. The kinetics of synapsis show non-exponential behaviour extending over five log units of time and are independent of the length of the DNA molecule and the length of DNA between the sites. In this study, numerical models were developed in order to account for how fluctuations in the structure of supercoiled DNA might lead to the juxtaposition of distant sites in a manner consistent with the experimental data on synapsis by resolvase. Models where the juxtaposition arises from fluctuations around branch points in the superhelix failed to match the data: they yielded non-exponential kinetics but only over two log units of time and they predicted longer synapsis times for both larger DNA molecules and larger inter-site spacings. In another model, one fraction of the juxtaposition events gives rise directly to the productive complex while the remaining fraction initially yields a non-productive complex: the latter molecules undergo no further fluctuations until the abortive synapse dissociates at the end of a delay period. This model again failed to match the experimental data. However, the inclusion of three sorts of non-productive complexes, each with a different delay constant, led to progress curves that concurred with the data. Schemes were also developed to account for the juxtaposition of three sites at a branch point in supercoiled DNA.

Algorithms↗
PubMed1989

Random walking.

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Crime↗
PubMed1990

Random walking.

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Animals↗
PubMed1990

Random walking.

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Animals↗
PubMed1990

Random walking.

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Animals↗
PubMed1991

Random walking.

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Altitude↗
PubMed1989

Weak reflection of ultrasound by elements arranged in the steps of a one-dimensional random walk, with reference to backscatter by blood.

Blood backscatters ultrasound primarily because the erythrocytes (red blood corpuscles) have a specific acoustic impedance slightly different from that of the surrounding plasma. In an attempt to explain some of the main results concerning such scatter, a one-dimensional model is set up and analysed. The results computed from it are in qualitative agreement with, and contribute a partial explanation of, the variation of reflection coefficient with erythrocyte concentration and the noise in the backscattered signal.

Erythrocytes↗
PubMed2005

A new test of random walks in heterogeneous environments.

Environmental heterogeneities can change animal movement in two different manners. First, they can modify movement characteristics (move lengths or turning angles), in which case the movement remains of the diffusive kind. Second, they can bias displacement towards a particular direction in which case it becomes non-diffusive. We propose in this paper a simple method that only requires computing the mean length of a sample of trajectories in some bounded area to distinguish between these two kinds of movement. We show through simulations that the method allows to detect the presence of heterogeneities that orient animal movement. We apply it to experimental trajectories of Messor sancta ants engaged in corpse aggregation to show that their displacement is oriented at the contact of the formed corpse piles and that their trajectories become non-diffusive.

Animals↗