28,788 research outputs found
The influence of geometry, surface character and flexibility on the permeation of ions and water through biological pores
A hydrophobic constriction site can act as an efficient barrier to ion and
water permeation if its diameter is less than the diameter of an ion's first
hydration shell. This hydrophobic gating mechanism is thought to operate in a
number of ion channels, e.g. the nicotinic receptor, bacterial mechanosensitive
channels (MscL and MscS) and perhaps in some potassium channels (e.g. KcsA,
MthK, and KvAP). Simplified pore models allow one to investigate the primary
characteristics of a conduction pathway, namely its geometry (shape, pore
length, and radius), the chemical character of the pore wall surface, and its
local flexibility and surface roughness. Our extended (ca. 0.1 \mu s) molecular
dynamic simulations show that a short hydrophobic pore is closed to water for
radii smaller than 0.45 nm. By increasing the polarity of the pore wall (and
thus reducing its hydrophobicity) the transition radius can be decreased until
for hydrophilic pores liquid water is stable down to a radius comparable to a
water molecule's radius. Ions behave similarly but the transition from
conducting to non-conducting pores is even steeper and occurs at a radius of
0.65 nm for hydrophobic pores. The presence of water vapour in a constriction
zone indicates a barrier for ion permeation. A thermodynamic model can explain
the behaviour of water in nanopores in terms of the surface tensions, which
leads to a simple measure of "hydrophobicity" in this context. Furthermore,
increased local flexibility decreases the permeability of polar species. An
increase in temperature has the same effect, and we hypothesise that both
effects can be explained by a decrease in the effective solvent-surface
attraction which in turn leads to an increase in the solvent-wall surface free
energy.Comment: Peer reviewed article appeared in Physical Biology
http://www.iop.org/EJ/abstract/1478-3975/1/1/005
Nonuniqueness in a minimal model for cell motility
Twoâphase flow models have been used previously to model cell motility, however these have rapidly become very complicated, including many physical processes, and are opaque. Here we demonstrate that even the simplest oneâdimensional, twoâphase, poroviscous, reactive flow model displays a number of behaviours relevant to cell crawling. We present stability analyses that show that an asymmetric perturbation is required to cause a spatially uniform, stationary strip of cytoplasm to move, which is relevant to cell polarization. Our numerical simulations identify qualitatively distinct families of travellingâwave solution that coâexist at certain parameter values. Within each family, the crawling speed of the strip has a bellâshaped dependence on the adhesion strength. The model captures the experimentally observed behaviour that cells crawl quickest at intermediate adhesion strengths, when the substrate is neither too sticky nor too slippy
A CFH12k lensing survey of X-ray luminous galaxy clusters. II. Weak lensing analysis and global correlations
Aims. We present a wide-field multi-color survey of a homogeneous sample of eleven clusters of galaxies for which we measure total masses and mass distributions from weak lensing. This sample, spanning a small range in both X-ray luminosity and redshift, is ideally suited to determining the normalisation of scaling relations between X-ray properties of clusters and their masses (the M â T_X and the M â L_X relations) and also estimating the scatter in these relations at a fixed luminosity.
Methods. The eleven clusters in our sample are all X-ray luminous and span a narrow redshift range at z = 0.21 ± 0.04. The weak lensing analysis of the sample is based on ground-based wide-field imaging obtained with the CFH12k camera on CFHT. We use the methodology developed and applied previously on the massive cluster Abell 1689. A Bayesian method, implemented in the Im2shape software, is used to fit the shape parameters of the faint background galaxies and to correct for PSF smearing. A multi-color selection of the background galaxies is applied to retrieve the weak lensing signal, resulting in a background density of sources of ~10 galaxies per square arc minute. With the present data, shear profiles are measured in all clusters out to at least 2 Mpc (more than 15 from the center) with high confidence. The radial shear profiles are fitted with different parametric mass profiles and the virial mass M_(200) is estimated for each cluster and then compared to other physical properties.
Results. Scaling relations between mass and optical luminosity indicate an increase of the M/L ratio with luminosity (M/L â L^(0.8)) and a LXâM_(200) relation scaling as L_X â M^(0.83±0.11)_(200) while the normalization of the M_(200) â T^(3/2)_X relation is close to the one expected from hydrodynamical simulations of cluster formation as well as previous X-ray analyses. We suggest that the dispersion in the M_(200) â T_X and M_(200) â L_X relations reflects the different merging and dynamical histories for clusters of similar X-ray luminosities and intrinsic variations in their measured masses. Improved statistics of clusters over a wider mass range are required for a better control of the intrinsic scatter in scaling relations
Superconducting d-wave stripes in cuprates: Valence bond order coexisting with nodal quasiparticles
We point out that unidirectional bond-centered charge-density-wave states in
cuprates involve electronic order in both s- and d-wave channels, with
non-local Coulomb repulsion suppressing the s-wave component. The resulting
bond-charge-density wave, coexisting with superconductivity, is compatible with
recent photoemission and tunneling data and as well as neutron-scattering
measurements, once long-range order is destroyed by slow fluctuations or glassy
disorder. In particular, the real-space structure of d-wave stripes is
consistent with the scanning-tunneling-microscopy measurements on both
underdoped Bi2Sr2CaCu2O8+x and Ca2-xNaxCuO2Cl2 of Kohsaka et al. [Science 315,
1380 (2007), arXiv:cond-mat/0703309].Comment: 5 pages, 3 figs, (v2) final version to be published in PR
The European Large Area ISO Survey - ISOPHOT results using the MPIA-pipeline
The European Large Area ISO Survey (ELAIS) will provide Infrared observations
of 4 regions in the sky with ISO. Around 2000 Infrared sources have been
detected at 7 and 15 microns (with ISOCAM), 90 and 175 microns (with ISOPHOT))
over 13 square degrees of the sky. We present the source extraction pipeline of
the 90 microns ISOPHOT observations, describe and discuss the results obtained
and derive the limits of the ELAIS observational strategy.Comment: 4 pages, 5 figures, to appear in the ISO conference "The Universe as
seen by ISO", 1998, UNESCO, Pari
Galaxy formation in the Planck cosmology - II. Star-formation histories and post-processing magnitude reconstruction
We adapt the L-Galaxies semi-analytic model to follow the star-formation
histories (SFH) of galaxies -- by which we mean a record of the formation time
and metallicities of the stars that are present in each galaxy at a given time.
We use these to construct stellar spectra in post-processing, which offers
large efficiency savings and allows user-defined spectral bands and dust models
to be applied to data stored in the Millennium data repository.
We contrast model SFHs from the Millennium Simulation with observed ones from
the VESPA algorithm as applied to the SDSS-7 catalogue. The overall agreement
is good, with both simulated and SDSS galaxies showing a steeper SFH with
increased stellar mass. The SFHs of blue and red galaxies, however, show poor
agreement between data and simulations, which may indicate that the termination
of star formation is too abrupt in the models.
The mean star-formation rate (SFR) of model galaxies is well-defined and is
accurately modelled by a double power law at all redshifts: SFR proportional to
, where Gyr, is the age of the
stars and is the loopback time to the onset of galaxy formation; above a
redshift of unity, this is well approximated by a gamma function: SFR
proportional to , where Gyr. Individual
galaxies, however, show a wide dispersion about this mean. When split by mass,
the SFR peaks earlier for high-mass galaxies than for lower-mass ones, and we
interpret this downsizing as a mass-dependence in the evolution of the quenched
fraction: the SFHs of star-forming galaxies show only a weak mass dependence.Comment: Accepted version of the paper, to appear in MNRAS. Compared to the
original version, contains more detail on the post-processing of magnitudes,
including a table of rms magnitude errors. SFHs available on Millennium
database http://gavo.mpa-garching.mpg.de/MyMillennium
Antibiotic resistance in community-acquired urinary tract infections
The International Society for Burns Injuries (ISBI) has published guidelines for the management of multiple or mass burns casualties, and recommends that 'each country has or should have a disaster planning system that addresses its own particular needs.' The need for a national burns disaster plan integrated with national and provincial disaster planning was discussed at the South African Burns Society Congress in 2009, but there was no real involvement in the disaster planning prior to the 2010 World Cup; the country would have been poorly prepared had there been a burns disaster during the event. This article identifies some of the lessons learnt and strategies derived from major burns disasters and burns disaster planning from other regions. Members of the South African Burns Society are undertaking an audit of burns care in South Africa to investigate the feasibility of a national burns disaster plan. This audit (which is still under way) also aims to identify weaknesses of burns care in South Africa and implement improvements where necessary
- âŠ