Introduction and Overview

All living matter is built up from cells. This has already been realized during the last century. However, the general principles underlying the structure and dynamics of these cells remained obscure for a long time. Recently, a combination of cell biology, genetics, biochemistry and biophysics has led to a new level of understanding and to the new discipline of molecular cell biology.

One general construction principle which has emerged from molecular cell biology is the use of membranes in order to organize space into different compartments or modules. First of all, each biological cell is enclosed by its outer plasma membrane which controls the interaction between the cell and its environment. This applies both to the relatively small cells of bacteria or prokaryotes, which have no cell nucleus, and to the much larger cells of eucaryotes, which have such a nucleus. The latter class of organisms contains all animals and plants as well as single­celled microorganisms such as amoeba or yeast. In addition to the outer plasma membrane, all eucaryotic cells contain internal membranes which represent the boundaries of the internal organelles such as the nucleus, mitochondria, chloroplasts, etc.

On the molecular level, biomembranes are quite complex: they contain specific mixtures of molecules which reflect their diverse biological functions. However, in spite of this complex composition, all biomembranes exhibit an universal construction principle. Indeed, the basic structural element of all biomembranes appears to be a bilayer of lipid molecules which serves as a two­dimensional solvent for various proteins. Therefore, the simplest model systems for biomembranes are provided by lipid bilayers without any proteins. Such bilayers can be prepared in several ways and can then be studied by physical methods. When dissolved in water, these bilayers form closed vesicles which resemble the compartments formed by biomembranes.

The present volume on "Structure and Dynamics of Membranes" will cover various aspects of biomembranes and lipid bilayers from the biophysical point of view. An alternative title for this volume would be "Biologically inspired physics of membranes". The volume has two parts. Part A with the subtitle "From Cells to Vesicles" starts from cells and biomembranes and proceeds to bilayers and vesicles. Part B about "Generic and Specific interactions" will cover membrane adhesion, membrane fusion and the interaction of biomembranes with the cytoskeleton.

Part A. From Cells to Vesicles

The first two chapters of Part A deal with biomembranes. The first chapter by Sackmann gives an introduction to the membranes of eucaryotes. It contains a general classification of their lipids and proteins and describes the relation between membrane architecture and membrane function. The second chapter by Bloom and Mouritsen on the evolution of membranes compares the properties of prokaryotic and eucaryotic membranes. It is argued that the differences in the composition of these membranes reflect the optimization of their physical properties via evolution.

We then proceed to different aspects of lipids. In the bulk, mixtures of lipids and water exhibit a large variety of thermodynamic phases and phase transitions, see chapter by Seddon and Templer. Typically, a large fraction of the phase diagram is occupied by lamellar phases which represent stacks of lipid bilayers. The structure of these phases can be experimentally studied by X­ray and neutron scattering. Similar techniques have been recently applied to lipid monolayers spread at the water air interface. As explained in the chapter by Möhwald, one now has rather detailed information about the molecular arrangements in the monolayers.

The bilayer contained in biomembranes represents a multicomponent mixture of lipids and proteins. A central question of membrane biophysics is whether lipids play an active role for the self assembly and function of biomembranes. This question is addressed in the chapter by Sackmann which deals with the physical properties of model membranes composed of lipid-protein mixtures. Lateral phase separation of lipids and proteins associated with lipid phase transitions and selective lipid-protein interactions which may control the lateral organization of these membranes are also discussed.

The lipid-protein bilayer usually appears to be in a fluid phase characterized by lateral diffusion of the molecules along the membrane. The diffusion can be measured by several experimental techniques. As as result, one finds that a lipid molecule diffuses rather rapidly along the membrane and covers a distance of the order of 1 µm in 1 sec. This is reviewed in the chapter by Almeida and Vaz with special emphasis on diffusion in heterogeneous membranes.

Fluid bilayers are very flexible since they can relax a shear stress by hydrodynamic flow. The most important elastic modulus is their bending rigidity which can be related to the molecular structure of the membrane as reviewed in the chapter by Ben­Shaul.

One obvious manifestation of their flexibility is the closure of bilayers into vesicles which attain a large variety of different shapes. In addition, these shapes can be transformed by temperature changes, by phase separation within the membrane, or by adhesion towards another surface. This aspect of membrane behavior has recently seen a fruitful interplay between experiment and theory as reviewed in the chapter by Seifert and Lipowsky.

The two final chapters of Part A discuss applications of lipid vesicles and liposomes. The chapter by Cevc focuses on the possibility to use lipid vesicles in order to transport drugs through the skin. The chapter by Lasic gives a general overview over liposome applications in pharmacology, medicine, bioengineering and cosmetics.

Part B. Generic and Specific Interactions

The first two chapters of Part B discuss the generic interactions of membranes from the conceptual point of view. The first chapter by Lipowsky starts with an overview over the different experimental techniques for membrane adhesion and then focuses on the interplay between molecular forces and entropic interactions. The second chapter by Andelman gives a review of the electrostatic interactions of membranes.

The next two chapters summarize the experimental work on two different bilayer systems. In the chapter by Parsegian and Rand, lyotropic liquid crystals consisting of large stacks of interacting bilayers are studied using the osmotic stress method. In this latter approach, one applies an osmotic pressure to the membranes and measures their separation by X­ray diffraction. The second system consists of bunches of lipid bilayers which are observed in the light microscope. The behavior of these bunches is difficult to understand and may indicate that lipid bilayers have a "hidden" reservoir of membrane area, see the chapter by Helfrich.

The adhesion of biomembranes is more complex than the adhesion of lipid bilayers. Plasma membranes, for example, are usually covered by many proteins which lead to specific adhesion mechanisms. The processes of contact formation, focal bounding and macroscopic contacts between cells are discussed in the chapter by Evans. Some representative models for cell adhesion are described in the chapter by Bongrand. In these systems, adhesion is controlled by specific adhesion molecules. For eucaryotic cells, these adhesion molecules are usually connected to the cytoskeleton inside the cells.

The cytoskeleton within eucaryotic cells consists of a network of relatively stiff filaments. Three different types of filaments have been identified: actin filaments, intermediate filaments and microtubuli. As explained in the chapter by Janmey, much has been recently learned about the interaction of these filaments with the cell membrane. This interaction is also crucial for cell shape, cell locomotion, and cell division.

The two final chapters of Part B deal with membrane fusion. Indeed, there are many biological processes in which membrane adhesion is the first step towards membrane fusion. One example is provided by the transport vesicles which shuttle between different compartments of eucaryotic cells. It seems that fusion can be induced in several ways. As discussed in the chapter by Dimitrov, one generic fusion mechanism which has recently become available is electrofusion induced by electroporation. Another more specific mechanism is based on the interactions with cations such as Ca ²⁺ as described in the final chapter by Arnold.

We thank all authors of this handbook for their cooperation and Clarissa Jansen and Gudrun Conrad for their help with the editorial process.

Reinhard Lipowsky and Erich Sackmann