Showing papers by "Udo Seifert published in 1997"
TL;DR: In this article, the authors describe the systematic physical theory developed to understand the static and dynamic aspects of membrane and vesicle configurations, and the preferred shapes arise from a competition between curvature energy which derives from the bending elasticity of the membrane, geometrical constraints such as fixed surface area and fixed enclosed volume, and a signature of the bilayer aspect.
Abstract: Vesicles consisting of a bilayer membrane of amphiphilic lipid molecules are remarkably flexible surfaces that show an amazing variety of shapes of different symmetry and topology. Owing to the fluidity of the membrane, shape transitions such as budding can be induced by temperature changes or the action of optical tweezers. Thermally excited shape fluctuations are both strong and slow enough to be visible by video microscopy. Depending on the physical conditions, vesicles adhere to and unbind from each other or a substrate. This article describes the systematic physical theory developed to understand the static and dynamic aspects of membrane and vesicle configurations. The preferred shapes arise from a competition between curvature energy, which derives from the bending elasticity of the membrane, geometrical constraints such as fixed surface area and fixed enclosed volume, and a signature of the bilayer aspect. These shapes of lowest energy are arranged into phase diagrams, which separate regi...
1,555 citations
TL;DR: This paper presents a large-scale simulation of the response of the immune system to carbon dioxide in the context of a high-pressure environment and shows clear patterns of decline in the immune systems of Response to CO2.
Abstract: Phase-contrast microscopy is used to monitor the shapes of micron-scale fluid-phase phospholipid-bilayer vesicles in an aqueous solution. At fixed temperature, each vesicle undergoes thermal shape fluctuations. We are able, experimentally, to characterize the thermal shape ensemble by digitizing the vesicle outline in real time and storing the time sequence of images. Analysis of this ensemble using the area-difference-elasticity (ADE) model of vesicle shapes allows us to associate (map) each time sequence to a point in the zero-temperature (shape) phase diagram. Changing the laboratory temperature modifies the control parameters (area, volume, etc.) of each vesicle, so it sweeps out a trajectory across the theoretical phase diagram. It is a nontrivial test of the ADE model to check that these trajectories remain confined to regions of the phase diagram where the corresponding shapes are locally stable. In particular, we study the thermal trajectories of three prolate vesicles which, upon heating, experienced a mechanical instability leading to budding. We verify that the position of the observed instability and the geometry of the budded shape are in reasonable accord with the theoretical predictions. The inability of previous experiments to detect the ``hidden'' control parameters (relaxed area difference and spontaneous curvature) make this the first direct quantitative confrontation between vesicle-shape theory and experiment.
213 citations
TL;DR: The modification of the bending elastic modulus kc of lipid bilayers (here DMPC) by small amounts of small amphiphiles which exchange between the bilayer and the aqueous phase to establish correlations between the effective tension and the excess area of the quasi-spherical vesicles was analysed.
Abstract: We report the modification of the bending elastic modulus kc of lipid bilayers (here DMPC) by small amounts (c ≤5 mol %) of (i) small amphiphiles which exchange between the bilayer and the aqueous phase (e.g. the ion carrier valinomycin and the Ca++ carrier A23187) and (ii) amphiphiles solubilized in the membrane (cholanic acid). Large reductions of the bending stiffness may be induced by a few percent of the solutes, e.g. 1 mol % of valinomycin reduce kc by a factor of two. The effect is rationalised in terms of local thinning of the bilayer. The strong effect of solutes on kc contrasts with its weak dependence on the lipid structure since the C18:0/C18:1-lipid stearoyl-oleoyl-phosphatidyl-choline (SOPC) exhibits only a 15% higher value of kc than DMPC. The effect of temperature on the flicker behaviour was analysed in order to establish correlations between the effective tension and the excess area of the quasi-spherical vesicles. The temperature dependence of the bilayer excess area for a DMPC vesicle leads to the thermal expansion coefficient, β, for which a value of β= 10.4 ×10-3 K-1 is obtained. A much stronger tendency for budding on μm-scale (micro budding) during thermal area expansion of POPC and SOPC compared to DMPC was observed.
57 citations
TL;DR: In this article, the authors discuss the origins of the area-difference elasticity model, a Landau-like theory which describes the mesoscale shape mechanics of bilayer sheets and vesicles.
19 citations
TL;DR: In this article, the authors consider two-dimensional triangular networks of beads connected by Hookean tethers under isotropic compression and determine both the compression and the shear modulus as a function of temperature and compression within simple approximations and by a Monte Carlo simulation.
Abstract: We consider two-dimensional triangular networks of beads connected by Hookean tethers under isotropic compression. We determine both the compression and the shear modulus as a function of temperature and compression within simple approximations and by a Monte Carlo simulation. At low temperature, this network undergoes a collapse transition with increasing compression. In the two phase region, collapsed and non-collapsed triangles coexist. While the compression modulus vanishes in the two phase region, the shear modulus shows only a small anomaly at the transition. With increasing temperature, this transition disappears in our simulation. Anharmonic shear fluctuations invalidate a harmonic analysis in large regions of the phase space. In application to the red blood cell membrane, we obtain good agreement with more microscopic models for the shear modulus. Our results also indicate that strong compression will lead to non-trivial elastic behavior of the cell membrane.
13 citations
TL;DR: In this article, the dynamics of vesicle shape transformations are governed by the competition between curvature energy, geometrical constraints and viscous dissipation in the surrounding liquid.
Abstract: The dynamics of vesicle shape transformations are governed by the competition between curvature energy, geometrical constraints and viscous dissipation in the surrounding liquid. After a discussion of the general principles, three examples illustrate these concepts, (i) Slow conformal diffusion of vesicles of higher genus arises from the invanance of the curvature elasticity with respect to conformal transformations, (ii) spinodal fluctuations indicate the onset of the budding instability, and (iii) the dynamic pearling instability of cylindrical vesicles is caused by the action of laser tweezers.
4 citations
TL;DR: The combination of length-xales of several microns, time scales for dynamic fluctuations accessible to video frequency, and energy scales of a few kbT all contribute to make vesicles a unique model system to study the statistical physics of soft surfaces in real space as discussed by the authors.
Abstract: The combination of length-xales of several microns, time scales for dynamic fluctuations accessible to video frequency, and energy scales of a few kbT all contribute to make vesicles a unique model system to study the statistical physics of soft surfaces in real space.