862 research outputs found
Inclusions induced phase separation in mixed lipid film
The effect of rigid inclusions on the phase behavior of a film containing a
mixture of lipid molecules is investigated. In the proposed model, the
inclusion-induced deformation of the film, and the resulting energy cost are
strongly dependent upon the spontaneous curvature of the mixed film. The
spontaneous curvature is in turn strongly influenced by the composition of
film. This coupling between the film composition and the energy per inclusion
leads to a lateral modulation of the composition, which follows the local
curvature of the membrane. In particular, it is shown that the inclusion may
induce a global phase separation in a film which would otherwise be
homogeneously mixed. The mixed film is then composed of patches of different
average composition, separated by the inclusions. This process may be of
relevance to explain some aspects of lipid-protein association in biological
membranes.Comment: 19 pages, 5 figure
Comment on ``Adsorption of Polyelectrolyte onto a Colloid of Opposite Charge''
In a recent Letter, Gurovitch and Sens studied the adsorption of a weakly
charged polyelectrolyte chain onto an oppositely charged colloidal particle. By
using a variational technique they found that the colloidal particle can adsorb
a polymer of higher charge than its own, and thus be ``overcharged.'' I argue
that the observed overcharging by a factor of 16/5 is indeed an artifact of the
approximations involved in the study. Moreover, I show that the existence of
overcharging depends crucially on the choice of the trial wave function,
contrary to their claim.Comment: A comment on: E. Gurovitch and P. Sens, Phys. Rev. Lett. 82, 339
(1999
Cooperative protein transport in cellular organelles
Compartmentalization into biochemically distinct organelles constantly
exchanging material is one of the hallmarks of eukaryotic cells. In the most
naive picture of inter-organelle transport driven by concentration gradients,
concentration differences between organelles should relax. We determine the
conditions under which cooperative transport, i.e. based on molecular
recognition, allows for the existence and maintenance of distinct organelle
identities. Cooperative transport is also shown to control the flux of material
transiting through a compartmentalized system, dramatically increasing the
transit time under high incoming flux. By including chemical processing of the
transported species, we show that this property provides a strong functional
advantage to a system responsible for protein maturation and sorting.Comment: 9 pages, 5 figure
Reversibility of Red blood Cell deformation
The ability of cells to undergo reversible shape changes is often crucial to
their survival. For Red Blood Cells (RBCs), irreversible alteration of the cell
shape and flexibility often causes anemia. Here we show theoretically that RBCs
may react irreversibly to mechanical perturbations because of tensile stress in
their cytoskeleton. The transient polymerization of protein fibers inside the
cell seen in sickle cell anemia or a transient external force can trigger the
formation of a cytoskeleton-free membrane protrusion of micrometer dimensions.
The complex relaxation kinetics of the cell shape is shown to be responsible
for selecting the final state once the perturbation is removed, thereby
controlling the reversibility of the deformation. In some case, tubular
protrusion are expected to relax via a peculiar "pearling instability".Comment: 4 pages, 3 figure
VCube-PS: A Causal Broadcast Topic-based Publish/Subscribe System
In this work we present VCube-PS, a topic-based Publish/Subscribe system
built on the top of a virtual hypercube-like topology. Membership information
and published messages are broadcast to subscribers (members) of a topic group
over dynamically built spanning trees rooted at the publisher. For a given
topic, the delivery of published messages respects the causal order. VCube-PS
was implemented on the PeerSim simulator, and experiments are reported
including a comparison with the traditional Publish/Subscribe approach that
employs a single rooted static spanning-tree for message distribution. Results
confirm the efficiency of VCube-PS in terms of scalability, latency, number and
size of messages.Comment: Improved text and performance evaluation. Added proof for the
algorithms (Section 3.4
Chromosome Oscillations in Mitosis
Successful cell division requires a tight regulation of chromosome motion via
the activity of molecular motors. Many of the key players at the origin of the
forces generating the movement have been identified, but their spatial and
temporal organization remains elusive. The protein complex Kinetochore on the
chromosome associates with microtubules emanating from one of the spindle poles
and drives the chromosome toward the pole. Chromokinesin motors on the
chromosome arms also interact with microtubules, ejecting the chromosome away
from the pole. In animal cells, a monooriented chromosome (associated to a
single pole) periodically switches between phases of poleward and away from the
pole movement[, a behavior tentatively explained so far by the existence of a
complex switching mechanism within the kinetochore itself. Here we show that
the interplay between the morphology of the mitotic spindle and the collective
kinetics of chromokinesins can account for the highly non-linear periodic
chromosome motion. Our analysis provides a natural explanation for the origin
of chromosome directional instability and for the mechanism by which
chromosomes feel their position in space.Comment: http://hogarth.pct.espci.fr/~pierre
Non-equilibrium raft-like membrane domains under continuous recycling
We present a model for the kinetics of spontaneous membrane domain (raft)
assembly that includes the effect of membrane recycling ubiquitous in living
cells. We show that the domains have a broad power-law distribution with an
average radius that scales with the 1/4 power of the domain lifetime when the
line tension at the domain edges is large. For biologically reasonable
recycling and diffusion rates the average domain radius is in the tens of nm
range, consistent with observations. This represents one possible link between
signaling (involving rafts) and traffic (recycling) in cells. Finally, we
present evidence that suggests that the average raft size may be the same for
all scale-free recycling schemes.Comment: 8 pages, 5 figure
The Effect of Thermal Fluctuations on Schulman Area Elasticity
We study the elastic properties of a two-dimensional fluctuating surface
whose area density is allowed to deviate from its optimal (Schulman) value. The
behavior of such a surface is determined by an interplay between the
area-dependent elastic energy, the curvature elasticity, and the entropy. We
identify three different elastic regimes depending on the ratio
between the projected (frame) and the saturated areas. We show that thermal
fluctuations modify the elastic energy of stretched surfaces (),
and dominate the elastic energy of compressed surfaces (). When
the elastic energy is not much affected by the fluctuations; the
frame area at which the surface tension vanishes becomes smaller than and
the area elasticity modulus increases.Comment: 12 pages, to appear in Euro. Phys. J.
Force balance and membrane shedding at the Red Blood Cell surface
During the aging of the red-blood cell, or under conditions of extreme
echinocytosis, membrane is shed from the cell plasma membrane in the form of
nano-vesicles. We propose that this process is the result of the
self-adaptation of the membrane surface area to the elastic stress imposed by
the spectrin cytoskeleton, via the local buckling of membrane under increasing
cytoskeleton stiffness. This model introduces the concept of force balance as a
regulatory process at the cell membrane, and quantitatively reproduces the rate
of area loss in aging red-blood cells.Comment: 4 pages, 3 figure
Micellization of Sliding Polymer Surfactants
Following up a recent paper on grafted sliding polymer layers (Macromolecules
2005, 38, 1434-1441), we investigated the influence of the sliding degree of
freedom on the self-assembly of sliding polymeric surfactants that can be
obtained by complexation of polymers with cyclodextrins. In contrast to the
micelles of quenched block copolymer surfactants, the free energy of micelles
of sliding surfactants can have two minima: the first corresponding to small
micelles with symmetric arm lengths, and the second corresponding to large
micelles with asymmetric arm lengths. The relative sizes and concentrations of
small and large micelles in the solution depend on the molecular parameters of
the system. The appearance of small micelles drastically reduces the kinetic
barrier signifying the fast formation of equilibrium micelles.Comment: Submitted to Macromolecule
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