418 research outputs found
Shape Changes of Self-Assembled Actin Bilayer Composite Membranes
We report the self-assembly of thin actin shells beneath the membranes of
giant vesicles. Ion-carrier mediated influx of Mg2+ induces actin
polymerization in the initially spherical vesicles. Buckling of the vesicles
and the formation of blisters after thermally induced bilayer expansion is
demonstrated. Bilayer flickering is dominated by tension generated by its
coupling to the actin cortex. Quantitative flicker analysis suggests the
bilayer and the actin cortex are separated by 0.4 \mum to 0.5 \mum due to
undulation forces.Comment: pdf-file, has been accepted by PR
Non-equilibrium hydrodynamics of a rotating filament
The nonlinear dynamics of an elastic filament that is forced to rotate at its
base is studied by hydrodynamic simulation techniques; coupling between
stretch, bend, twist elasticity and thermal fluctuations is included. The
twirling-overwhirling transition is located and found to be strongly
discontinuous. For finite bend and twist persistence length, thermal
fluctuations lower the threshold rotational frequency, for infinite persistence
length the threshold agrees with previous analytical predictions
On Shape Transformations and Shape Fluctuations of Cellular Compartments and Vesicles
We discuss the shape formation and shape transitions of simple bilayer vesicles in context with their role in biology. In the first part several classes of shape changes of vesicles of one lipid component are described and it is shown that these can be explained in terms of the bending energy concept in particular augmented by the bilayer coupling hypothesis. In the second
part shape changes and vesicle fission of vesicles composed of membranes of lipid mixtures are reported. These are explained in terms of coupling between local curvature and phase separation
The origin of stiffening in cross-linked semiflexible networks
Strain stiffening of protein networks is explored by means of a finite strain
analysis of a two-dimensional network model of cross-linked semiflexible
filaments. The results show that stiffening is caused by non-affine network
rearrangements that govern a transition from a bending dominated response at
small strains to a stretching dominated response at large strains.
Thermally-induced filament undulations only have a minor effect; they merely
postpone the transition.Comment: 5 pages, 5 figure
The impact of jamming on boundaries of collectively moving weak-interacting cells
Collective cell migration is an important feature of wound healing,
as well as embryonic and tumor development. The origin of collective cell
migration is mainly intercellular interactions through effects such as a line
tension preventing cells from detaching from the boundary. In contrast, in this
study, we show for the first time that the formation of a constant cell front of a
monolayer can also be maintained by the dynamics of the underlying migrating
single cells. Ballistic motion enables the maintenance of the integrity of the
sheet, while a slowed down dynamics and glass-like behavior cause jamming of
cells at the front when two monolayers—even of the same cell type—meet. By
employing a velocity autocorrelation function to investigate the cell dynamics
in detail, we found a compressed exponential decay as described by the
Kohlrausch–William–Watts function of the form C(δx)t ∼ exp (−(x/x0(t))β(t)),
with 1.5 6 β(t) 6 1.8. This clearly shows that although migrating cells are an
active, non-equilibrium system, the cell monolayer behaves in a glass-like way,
which requires jamming as a part of intercellular interactions. Since it is the
dynamics which determine the integrity of the cell sheet and its front for weakly
interacting cells, it becomes evident why changes of the migratory behavior
during epithelial to mesenchymal transition can result in the escape of single
cells and metastasis
Finite Size Polyelectrolyte Bundles at Thermodynamic Equilibrium
We present the results of extensive computer simulations performed on
solutions of monodisperse charged rod-like polyelectrolytes in the presence of
trivalent counterions. To overcome energy barriers we used a combination of
parallel tempering and hybrid Monte Carlo techniques. Our results show that for
small values of the electrostatic interaction the solution mostly consists of
dispersed single rods. The potential of mean force between the polyelectrolyte
monomers yields an attractive interaction at short distances. For a range of
larger values of the Bjerrum length, we find finite size polyelectrolyte
bundles at thermodynamic equilibrium. Further increase of the Bjerrum length
eventually leads to phase separation and precipitation. We discuss the origin
of the observed thermodynamic stability of the finite size aggregates
Elasticity of Semiflexible Biopolymer Networks
We develop a model for gels and entangled solutions of semiflexible
biopolymers such as F-actin. Such networks play a crucial structural role in
the cytoskeleton of cells. We show that the rheologic properties of these
networks can result from nonclassical rubber elasticity. This model can explain
a number of elastic properties of such networks {\em in vitro}, including the
concentration dependence of the storage modulus and yield strain.Comment: Uses RevTeX, full postscript with figures available at
http://www.umich.edu/~fcm/preprints/agel/agel.htm
Giant vesicles at the prolate-oblate transition: A macroscopic bistable system
Giant phospholipid vesicles are shown to exhibit thermally activated
transitions between a prolate and an oblate shape on a time scale of several
seconds. From the fluctuating contour of such a vesicle we extract ellipticity
as an effective reaction coordinate whose temporal probability distribution is
bimodal. We then reconstruct the effective potential from which we derive an
activation energy of the order of in agreement with theoretical
calculations. The dynamics of this transition is well described within a
Kramers model of overdamped diffusion in a bistable potential. Thus, this
system can serve as a model for macroscopic bistability.Comment: 10 pages, LaTeX, epsfig, 4 eps figures included, to appear in
Europhys. Let
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