Biophysical studies of membrane proteins/peptides. Interaction with ...

INTRODUCTION: BIOMEMBRANES
I
INTRODUCTION
1.BIOMEMBRANES
1.1. Function and architecture of biomembranes
Biomembranes delineate the boundaries of cells as plasma membranes, and in the
eukaryotic cell, enclose organelles in the form of intracellular membranes, allowing the
subdivision of cellular activities and the more diverse and specialized functions found in
eukaryotes. The biomembrane is the passive permeability barrier that allows the
maintenance of different molecular environments in the inside and outside of the cell or
organelle. The importance of this task is emphasized by the fact that a significant
fraction of the energy required for life is expended in the preservation of the differences
in molecular environments across biomembranes.
The foremost structural framework of the biomembrane is the lipid bilayer. The
lipid bilayer is formed by spontaneous self-assembly of lipid molecules. During this
process, the decrease in entropy resulting from hydrocarbon-water interaction, which is
also called the hydrophobic effect (Tanford, 1980), acts to organize lipid molecules so
that acyl-chains are screened from the water environment. This can also lead to the
formation of micelles or other types of organization depending on the structure of the
lipid, as will be discussed in chapter 1.4.
Due to the amphipatic nature of most lipids, the lipid bilayer presents a hydrocarbon
environment in the interior core screened from water molecules by the polar groups of
lipids (see Fig. I.1). The hydrophobic acyl-chains are in a fluid state whereas the polar
groups of the lipids are assembled in an orderly array as in a liquid crystal. This
hydrophobic core of the lipid bilayer is the most important structural feature in the role
of biomembranes as barriers to passive molecular diffusion. It allows the passage of
water and other small uncharged molecules but presents great impediment to passive
1