Volume
1. Structure and Dynamics of Membranes
Chapter 5. Physical basis of self-organization and
function of membranes: physics of vesicles
E. Sackmann
Technische Universität München,
James-Franck-Strasse, D-85748 Garching, Germany
1. Introduction
A continuous discussion between cell biologists and membrane biophysicists
is whether lipids play an active role in biochemical membrane processes
such as the formation of functional complexes (e.g., activation of adenylate
cyclase by G-protein after hormone binding, cf. chapter 1) or whether they
provide just an inert matrix of the right fluidity for the real functional
entities: the proteins. Arguments for the former view are for instance:
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that the average lipid composition of cells but also the distribution of
the various lipids among the cellular organelles are well preserved despite
a rapid material exchange within the cell (cf. chapter 1);
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that deviation from the natural lipid composition may lead to severe health
problems (such as the Tay-Sachs disease, cf. chapter 1).
Arguments for the latter view are that the activity of ion channels (such
as band III) does not depend very critically on the lipid chainlength provided
the bilayer is in a fluid state. The growth of cells (e.g., mycoplasts)
requires that the lipid bilayer moiety is in a fluid state whereas the
growth rate does not depend critically on the lipid chain structure [1].
This unspecific effect is also the basis of the adaption of the lipid composition
of plant cells to the environment.
Whatever the answer, there is little doubt that nature was extremely
clever by choosing lipids as basic building unit of membranes. Thus,
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Two-chain lipids associate in water at extremely low concentrations (~10-11
M) to form bilayer vesicles.
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The vesicles are extremely stable for instance if the bilayer undergoes
fluid-to-solid phase transition as is shown in the examples of fig. 1a.
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Lipid bilayers are quite permeable for water but simultaneously rather
imper-meable for ions which allows the rapid establishment of the osmotic
equilibrium of cells (as discussed in chapter 1) and renders bilayers simultaneously
excellent insulating properties.
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The two-dimensional-fluid character of bilayers entails a large increase
in efficiency of diffusion controlled processes and the lateral organization
of membranes may be controlled externally (e.g., by the adsorption of proteins
[2, 3]).
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Bilayers are extremely soft with respect to bending but essentially incompressible
under lateral tension. It is this unique combination of elastic properties
of the lipid bilayer which allows migrating cells (such as erythrocytes)
to travel for several hundred km through narrow body channels without loss
of ions [2].
The intention of this chapter is to review phenomenological physical properties
of isolated lipid bilayers: namely vesicles. The first part deals with
the structural dynamic and elastic properties of fluid and solid bilayer
vesicles together with the thermodynamics of the thermotropic phase transitions.
In the second part the advantage of lipid bilayers as two dimensional solvent
are described and the physics of lipid mixtures is discussed on a thermodynamic
basis. In all parts we endeavour to point out how nature could exploit
the exceptional physical properties of lipid bilayers to control the self-organization
and function of cell membranes. Even more important is to show that lipid
bilayers are exciting as model systems to explore the
novel and rich physical properties of soft two-dimensional materials.
Main emphasis is laid on essential phenomenological physical properties
of isolated vesicles. Concerning the molecular structure of membranes,
recent reviews are now available [4-6]. Aspects treated in other chapters
of this volume are only indicated.
Fig. 1a. Demonstration of stability of single bilayer
vesicles: Shape change of giant DMPC-vesicle in 500 mM NaCl buffer caused
by transition from fluid (smectic A) to solid (smectic B) state demonstrating
the astonishing stability of lipid bilayers. |
Fig. 1b. Demonstration of stability of single bilayer
vesicles: Spontaneous budding leading to formation of tube-like (and branched)
protrusions. The budding is induced by a small increase of area-to-volume
ratio. (Bar: 10
mm). |
Fig. 1c. Demonstration of stability of single bilayer
vesicles: Bilayer vesicle of partially cross-linked lipid (from diacetylene
PC and DMPC) undergoing transition from swollen (I) to strongly wrinkled
state (II) due to phase separation, [12]. |
Fig. 1d. Demonstration of stability of single bilayer
vesicles: Torus-shaped vesicle of diacetylene PC according to [45]. Bar:
10 mm. |
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