2,290 research outputs found
The switching dynamics of the bacterial flagellar motor
Many swimming bacteria are propelled by flagellar motors that stochastically
switch between the clockwise and counterclockwise rotation direction. While the
switching dynamics are one of the most important characteristics of flagellar
motors, the mechanisms that control switching are poorly understood. We present
a statistical-mechanical model of the flagellar rotary motor, which consists of
a number of stator proteins that drive the rotation of a ring of rotor
proteins, which in turn drives the rotation of a flagellar filament. At the
heart of our model is the assumption that the rotor protein complex can exist
in two conformational states corresponding to the two respective rotation
directions, and that switching between these states depends on interactions
with the stator proteins. This naturally couples the switching dynamics to the
rotation dynamics, making the switch sensitive to torque and speed. Another key
element of our model is that after a switching event, it takes time for the
load to build up, due to polymorphic transitions of the filament. Our model
predicts that this slow relaxation dynamics of the filament, in combination
with the load dependence of the switching frequency, leads to a characteristic
switching time, in agreement with recent observations.Comment: 7 pages, 6 figures, RevTeX
Enzyme localization can drastically affect signal amplification in signal transduction pathways
Push-pull networks are ubiquitous in signal transduction pathways in both
prokaryotic and eukaryotic cells. They allow cells to strongly amplify signals
via the mechanism of zero-order ultrasensitivity. In a push-pull network, two
antagonistic enzymes control the activity of a protein by covalent
modification. These enzymes are often uniformly distributed in the cytoplasm.
They can, however, also be colocalized in space, for instance, near the pole of
the cell. Moreover, it is increasingly recognized that these enzymes can also
be spatially separated, leading to gradients of the active form of the
messenger protein. Here, we investigate the consequences of the spatial
distributions of the enzymes for the amplification properties of push-pull
networks. Our calculations reveal that enzyme localization by itself can have a
dramatic effect on the gain. The gain is maximized when the two enzymes are
either uniformly distributed or colocalized in one region in the cell.
Depending on the diffusion constants, however, the sharpness of the response
can be strongly reduced when the enzymes are spatially separated. We discuss
how our predictions could be tested experimentally.Comment: PLoS Comp Biol, in press. 32 pages including 6 figures and supporting
informatio
Long-time asymptotic of temporal-spatial coherence function for light propagation through time dependent disorder
Long-time asymptotic of field-field correlator for radiation propagated
through a medium composed of random point-like scatterers is studied using
Bete-Salpeter equation. It is shown that for plane source the fluctuation
intensity (zero spatial moment of the correlator) obeys a power-logarithmic
stretched exponential decay law, the exponent and preexponent being dependent
on the scattering angle. Spatial center of gravity and dispersion of the
correlator (normalized first and second spatial moments, respectively) prove to
weakly diverge as time tends to infinity. A spin analogy of this problem is
discussed.Comment: 12 pages, Latex, no figures, to be publication in Phys. Lett.
The Universality of Initial Conditions for Globular Cluster Formation
We investigate a simple model for globular cluster (GC) formation. We
simulate the violent relaxation of initially homogeneous isothermal stellar
spheres and show that it leads to the formation of clusters with radial density
profiles that match the observed profiles of GCs. The best match is achieved
for dynamically unevolved clusters. In this model, all the observed
correlations between global GC parameters are accurately reproduced if one
assumes that all the clusters initially had the same value of the stellar
density and the velocity dispersion. This suggests that the gas which formed
GCs had the same values of density and temperature throughout the universe.Comment: 12 pages, 4 figures, 1 table. Accepted for publication in ApJ Letter
Global HI profiles of spiral galaxies
In this paper we present short HI synthesis observations of 57 galaxies
without HI information in the RC3. These are a by-product of a large survey
with the WSRT of the neutral hydrogen gas in spiral and irregular galaxies.
Global profiles and related quantities are given for the 42 detected galaxies
and upper limits for the remaining 15. A number of galaxies have low values of
HI mass-to-blue luminosity ratio.Comment: A LATEX file without figures. The postscript version including all
the figures can be retrieved from http://www.astro.rug.nl:80/~secr/ Accepted
for publication in Astronomy & Astrophysics Suppl. Serie
Bose-Einstein Condensates in Strongly Disordered Traps
A Bose-Einstein condensate in an external potential consisting of a
superposition of a harmonic and a random potential is considered theoretically.
From a semi-quantitative analysis we find the size, shape and excitation
energy as a function of the disorder strength. For positive scattering length
and sufficiently strong disorder the condensate decays into fragments each of
the size of the Larkin length . This state is stable over a large
range of particle numbers. The frequency of the breathing mode scales as
. For negative scattering length a condensate of size
may exist as a metastable state. These finding are generalized to anisotropic
traps
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