44 research outputs found
Lattice Boltzmann method for viscoelastic fluids
Lattice Boltzmann model for viscoelastic flow simulation is proposed; elastic
effects are taken into account in the framework of Maxwell model. The following
three examples are studied using the proposed approach: a transverse velocity
autocorrelation function for free evolving system with random initial
velocities, a boundary-driven propagating shear waves, and a resonant
enhancement of oscillations in a periodically driven fluid in a capillary. The
measured shear wave dispersion relation is found to be in a good agreement with
the theoretical one derived for the Navier-Stokes equation with the Maxwell
viscoelastic term.Comment: 4 pages, 5 figure
A model for the self-organization of vesicular flux and protein distributions in the Golgi apparatus
The generation of two non-identical membrane compartments via exchange of
vesicles is considered to require two types of vesicles specified by distinct
cytosolic coats that selectively recruit cargo and two membrane-bound SNARE
pairs that specify fusion and differ in their affinities for each type of
vesicles. The mammalian Golgi complex is composed of 6-8 non-identical
cisternae that undergo gradual maturation and replacement yet features only two
SNARE pairs. We present a model that explains how the distinct composition of
Golgi cisternae can be generated with two and even a single SNARE pair and one
vesicle coat. A decay of active SNARE concentration in aging cisternae provides
the seed for a cis > trans SNARE gradient that generates the predominantly
retrograde vesicle flux which further enhances the gradient. This flux in turn
yields the observed inhomogeneous steady-state distribution of Golgi enzymes,
which compete with each other and with the SNAREs for incorporation into
transport vesicles. We show analytically that the steady state SNARE
concentration decays exponentially with the cisterna number. Numerical
solutions of rate equations reproduce the experimentally observed SNARE
gradients, overlapping enzyme peaks in cis, medial and trans and the reported
change in vesicle nature across Golgi: Vesicles originating from younger
cisternae mostly contain Golgi enzymes and SNAREs enriched in these cisternae
and extensively recycle through the Endoplasmic Reticulum (ER), while the other
subpopulation of vesicles contains Golgi proteins prevalent in older cisternae
and hardly reaches the ER.Comment: 15 pages, 6 figure
Competition-driven evolution of organismal complexity
Non-uniform rates of morphological evolution and evolutionary increases in
organismal complexity, captured in metaphors like "adaptive zones", "punctuated
equilibrium" and "blunderbuss patterns", require more elaborate explanations
than a simple gradual accumulation of mutations. Here we argue that non-uniform
evolutionary increases in phenotypic complexity can be caused by a
threshold-like response to growing ecological pressures resulting from
evolutionary diversification at a given level of complexity. Acquisition of a
new phenotypic feature allows an evolving species to escape this pressure but
can typically be expected to carry significant physiological costs. Therefore,
the ecological pressure should exceed a certain level to make such an
acquisition evolutionarily successful. We present a detailed quantitative
description of this process using a microevolutionary competition model as an
example. The model exhibits sequential increases in phenotypic complexity
driven by diversification at existing levels of complexity and the resulting
increase in competitive pressure, which can push an evolving species over the
barrier of physiological costs of new phenotypic features.Comment: Open PDF with Acrobat to see movies, 22 pages, 9 figure