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Volume 1. Structure and Dynamics of Membranes

Chapter 1. Biological membranes architecture and function

E. Sackmann
Technische Universität München,
James-Franck-Strasse, D-85748 Garching, Germany

1. Introduction

1.1. Why are biomembranes a playground for physicists?

Life in all its diversity became possible after nature had found the trick with the membrane. It enabled the separation of living entities from the lifeless and hostile environment under preservation of selective material exchange between the two worlds. It led to the evolution of cells the function of which depends on the well controlled interplay and material exchange between compartments performing different functions. Simultaneously, the biomembranes developed into sites of essential biochemical functions, such as protein biosynthesis or oxidative phosphorylation. The reduction to two dimensions increased the efficiency drastically and opened the possibility for irreversible charge separation (as in the charge transfer chains of mitochondria or photosynthetic membranes) and transient storage of energy in the form of electrochemical potential gradients.

Biomembranes fascinate physicists for several reasons:

(i) They are examples of two-dimensional colloidal systems exhibiting various novel physical properties (e.g., non-classical elastic properties) which are simultaneously essential for their biological function.
(ii) Their composition involves about a hundred components and thus poses a real challenge for the development of new concepts of the physical basis of self-organization of multi-component systems.
(iii) Despite their complexity they allow us to explore the interplay between biochemical modulations of the physical properties of biomaterials and the control of biological functions (e.g., in the course of signal transduction processes).
(iv) By reconstitution of model membranes from a few lipids and membrane proteins, specific membrane function can be studied on a molecular level.
(v) Studies of biomembranes yield direct insight into the possible role of universal physical properties for the behaviour and function of biological materials (such as scaling laws or logarithmic laws typical for two-dimensional systems).

For the above reasons, artificial and biological membranes have become a basic topic within the new field of complex fluids.

There is a second motivating aspect. It is hoped that we learn to exploit the tricks of nature for biotechnical applications. Examples are the use of vesicles for drug delivery systems or the combination of membranes with electronic or optoelectronic devices in order to build biosensors.

In the present introductory section the basic principles of the molecular design of biomembranes and some of their fundamental functions are introduced. It is also an attempt to point out that universal physical properties can play a role in biological functions. Examples are the entropy driven repulsion forces and their role for bioadhesion or the role of membrane bending energy for the stabilization of cell shapes or for shape transitions.

This chapter is considered as an introduction in the field and for that reason only a few references are given for further reading where further references can be found.

1.2. Biomembranes enable the modular design of cells

Cells exhibit a modular design. They are made up of compartments which are specialized for one or several well defined functions. The most important functional compartments of eucaryotic cells are exhibited in fig. 1.

Clearly, the modular design was an essential evolutionary step in order to create some order within the cell and to facilitate the control of such a complex machine. Some important consequences and advantages of this design principle are:

The intracellular space is divided into two sub-spaces: the lumina of the various organelles and the cytosol.
An enormous number of different classes of molecules are distributed among these different subspaces, thus reducing the number of directly interacting molecular species.
It allows the formation of a remarkable gradient of composition between the nucleus and the plasma membrane which is essential for the directed flow of newly synthesized material from the endoplasmatic reticulum to the plasma membrane or the extracellular space as well as for the trafficking of nutrition molecules in the opposite direction.
Different ionic compositions or pH's can be established in the lumina of the various organelles which is essential (i) for the establishment of electrochemical gradients across the membranes, (ii) the control of the activity of specialized proteins (such as the digestive enzymes of lysosomes) and (iii) the accumulation of specific proteins within the various subspaces.

Fig. 1. Modular design of the cell. Schematic view of eucaryotic cell composed of modules with well defined functions. These include: The nucleus, N, (the site of information storage); the endoplasmatic reticulum. ER, with associated ribosomes (the location of protein and lipid synthesis); the Golgi apparatus, G, (serving the modification and sorting of newly synthesized proteins and lipids and their directed distribution to other compartments or membranes); the mitochondria, M, (organelles where ATP is produced); the lysosomes, L, (specialized for intracellular digestion). V denotes a whole palette of vesicles (e.g., endosomes) which are required for the molecular transport within the cell and between the cell and its environment. The plasma membrane (PM) forms a selective filter and regulates communication between cells. The intracellular compartments are embedded in the cytoskeleton: a soft network of protein filaments (not shown). The cytoskeleton helps to establish some order within the cytoplasm and, together with the plasma membrane, determines the mechanical stability of the cell. The membranes create three subspaces: the lumina inside of the compartments, the cytosol which is the zone between the compartments, and the extracellular space.

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