186 research outputs found
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
Gravity-Induced Shape Transformations of Vesicles
We theoretically study the behavior of vesicles filled with a liquid of
higher density than the surrounding medium, a technique frequently used in
experiments. In the presence of gravity, these vesicles sink to the bottom of
the container, and eventually adhere even on non - attractive substrates. The
strong size-dependence of the gravitational energy makes large parts of the
phase diagram accessible to experiments even for small density differences. For
relatively large volume, non-axisymmetric bound shapes are explicitly
calculated and shown to be stable. Osmotic deflation of such a vesicle leads
back to axisymmetric shapes, and, finally, to a collapsed state of the vesicle.Comment: 11 pages, RevTeX, 3 Postscript figures uuencode
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
Vesicles in solutions of hard rods
The surface free energy of ideal hard rods near curved hard surfaces is
determined to second order in curvature for surfaces of general shape. In
accordance with previous results for spherical and cylindrical surfaces it is
found that this quantity is non-analytical when one of the principal curvatures
changes signs. This prohibits writing it in the common Helfrich form. It is
shown that the non-analytical terms are the same for any aspect ratio of the
rods. These results are used to find the equilibrium shape of vesicles immersed
in solutions of rod-like (colloidal) particles. The presence of the particles
induces a change in the equilibrium shape and to a shift of the prolate-oblate
transition in the vesicle phase diagram, which are calculated within the
framework of the spontaneous curvature model. As a consequence of the special
form of the energy contribution due to the rods these changes cannot be
accounted for by a simple rescaling of the elastic constants of the vesicle as
for solutions of spherical colloids or polymers.Comment: 11 pages, 7 figures, submitted to Phys. Rev.
Vesicle shape, molecular tilt, and the suppression of necks
Can the presence of molecular-tilt order significantly affect the shapes of
lipid bilayer membranes, particularly membrane shapes with narrow necks?
Motivated by the propensity for tilt order and the common occurrence of narrow
necks in the intermediate stages of biological processes such as endocytosis
and vesicle trafficking, we examine how tilt order inhibits the formation of
necks in the equilibrium shapes of vesicles. For vesicles with a spherical
topology, point defects in the molecular order with a total strength of
are required. We study axisymmetric shapes and suppose that there is a
unit-strength defect at each pole of the vesicle. The model is further
simplified by the assumption of tilt isotropy: invariance of the energy with
respect to rotations of the molecules about the local membrane normal. This
isotropy condition leads to a minimal coupling of tilt order and curvature,
giving a high energetic cost to regions with Gaussian curvature and tilt order.
Minimizing the elastic free energy with constraints of fixed area and fixed
enclosed volume determines the allowed shapes. Using numerical calculations, we
find several branches of solutions and identify them with the branches
previously known for fluid membranes. We find that tilt order changes the
relative energy of the branches, suppressing thin necks by making them costly,
leading to elongated prolate vesicles as a generic family of tilt-ordered
membrane shapes.Comment: 10 pages, 7 figures, submitted to Phy. Rew.
Tilt Texture Domains on a Membrane and Chirality induced Budding
We study the equilibrium conformations of a lipid domain on a planar fluid
membrane where the domain is decorated by a vector field representing the tilt
of the stiff fatty acid chains of the lipid molecules, while the surrounding
membrane is fluid and structureless. The inclusion of chirality in the bulk of
the domain induces a novel budding of the membrane, which preempts the budding
induced by a decrease in interfacial tension.Comment: 5 pages, 3 figure
Nonlinear competition between asters and stripes in filament-motor-systems
A model for polar filaments interacting via molecular motor complexes is
investigated which exhibits bifurcations to spatial patterns. It is shown that
the homogeneous distribution of filaments, such as actin or microtubules, may
become either unstable with respect to an orientational instability of a finite
wave number or with respect to modulations of the filament density, where long
wavelength modes are amplified as well. Above threshold nonlinear interactions
select either stripe patterns or periodic asters. The existence and stability
ranges of each pattern close to threshold are predicted in terms of a weakly
nonlinear perturbation analysis, which is confirmed by numerical simulations of
the basic model equations. The two relevant parameters determining the
bifurcation scenario of the model can be related to the concentrations of the
active molecular motors and of the filaments respectively, which both could be
easily regulated by the cell.Comment: 13 pages, 7 figure
Phase ordering and shape deformation of two-phase membranes
Within a coupled-field Ginzburg-Landau model we study analytically phase
separation and accompanying shape deformation on a two-phase elastic membrane
in simple geometries such as cylinders, spheres and tori. Using an exact
periodic domain wall solution we solve for the shape and phase ordering field,
and estimate the degree of deformation of the membrane. The results are
pertinent to a preferential phase separation in regions of differing curvature
on a variety of vesicles.Comment: 4 pages, submitted to PR
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