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...

Configurations of fluid membranes and vesicles

The aim of this mini-review is to relate membrane physical properties to the adaptation and resistance of microorganisms to environmental stresses. In the first part, the effects of various stresses on the structure and dynamic properties of phospholipid and biological membranes are presented. The compensation of these effects, i.e., change in membrane fluidity, phase transitions, by the active cellular control of the membrane chemical composition, is then described. In this natural process, the change in membrane fluidity is viewed as the detecting "input" signal that initiates the regulation, activating proteic effectors that in turn may influence the chemical composition of the membrane (feedback). This adaptation system allows the maintenance of the physical characteristics of membranes and, thereby, of their functionality. When environmental stresses are extreme and occur abruptly, the regulation process may not compensate for the changes in the membrane physical characteristics. In such cases, important variations in the membrane fluidity and structure may induce cellular damages and cell death. However, the lethal consequences are not systematically observed because protective effects of changes in the membrane physical state on the resistance to stresses are also reported.

Influence of the fluidity of the membrane on the response of microorganisms to environmental stresses.

Simulating the deformation of vesicle membranes under elastic bending energy in three dimensions

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.

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Mapping vesicle shapes into the phase diagram: A comparison of experiment and theory

We use a simple and efficient computer model to investigate the physical properties of bilayer membranes. The amphiphilic molecules are modeled as short rigid trimers with finite range pair interactions between them. The pair potentials have been designed to mimic the hydrophobic interactions, and to allow the simulation of the membranes without the embedding solvent as if the membrane is in vacuum. We find that upon decreasing the area density of the molecules the membrane undergoes a solid–fluid phase transition, where in the fluid phase the molecules can diffuse within the membrane plane. The surface tension and the bending modulus of the fluid membranes are extracted from the analysis of the spectrum of thermal undulations. At low area densities we observe the formation of pores in the membrane through which molecules can diffuse from one layer to the other. The appearance of the pores is explained using a simple model relating it to the area dependence of the free energy.

“Water-free” computer model for fluid bilayer membranes

The equilibrium shapes of fluid-phase phospholipid vesicles in an aqueous solution are controlled by bending elasticity. The regime of nonvesiculated shapes at reduced volume v\ensuremath{\ge}1/ \ensuremath{\surd}2 involves the interplay of five branches of distinct stationary shapes: pears, prolates, oblates, stomatocytes, plus a branch of nonaxisymmetric shapes with the symmetry ${\mathit{D}}_{2\mathit{h}}$. We exploit a method for calculating explicitly the stability of arbitrary axisymmetric shapes to map out in a numerically exact way both the stable phases and the metastability of the low-lying shape branches. To obtain additional required information about nonaxisymmetric shapes, we calculate these by numerical minimization of the curvature energy on a triangulated surface. Combining these two methods allows us to construct the full (shape) phase diagram and the full stability diagram in this region. We provide explicit results for values of the bending constants appropriate to stearoyl-oleoyl-phosphatidylcholine; generalization to other values is straightforward. (c) 1995 The American Physical Society

Marija Jarić

Papers

Vesicular instabilities: The prolate-to-oblate transition and other shape instabilities of fluid bilayer membranes

Thermal shape fluctuations of fluid-phase phospholipid-bilayer membranes and vesicles