Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

Britta Lindholm-Sethson
membrane proteins can be incorporated without loss of activity with the aim of
studying their specific functions or in the development of biosensing devices,
2. Objective
2.1. THE PLASMA MEMBRANE
The lipid bilayer in the plasma membrane is composed mainly of three types of lipids
where the phospholipids are the most abundant, but there are also significant amounts
of cholesterol and glycolipids. The mixture of lipids in the inner and outer monolayers
of the plasma membrane are different and a large variation is also prevalent in the lipid
composition in membranes of different types of cells. The reason for the large variation
in lipid composition is in most cases not well understood. The lipid bilayer is fluid and
it is known that the presence of cholesterol increases the fluidity, elasticity and
mechanical stability of the lipid bilayer and moreover it is believed to decrease the
permeability of small water-soluble molecules.
One family of the glycolipids is the gangliosides, which contributes with up to 10%
of the total lipid mass in the nerve cell membrane. The ganglioside, G M1 , for instance,
binds bacterial toxins and has been used as a model receptor in supported lipid
membranes to signal for cholera toxins [2-7]. The main function for the gangliosides is
obviously not to signal for cholera toxin, but probably to serve as receptors in the
signalling between cells. The surface of a biological cell membrane is covered with a
layer of charges, that might be up to 20 nm thick. The contribution to this surface
charge layer comes from a non-uniform ion distribution near the membrane surface that
is governed by ordinary coulombic surface interactions, but also other ion-surface
affinities that are not electrostatic to their nature. The dipole potential of zwitterionic
amphiphiles must also be taken into account when considering the electrostatic
potentials of the surface region [8]. The net charge of the biological membrane is most
often negative and the charge density is rather low, typically -(0.02-0.2)C*m -2 . The
electrostatic membrane surface potential plays an important role in the processes of
membrane interaction, recognition and solute binding.
2.2. THE ARTIFICIAL CELL MEMBRANE
The discussion above underlines the complexity of the biological membrane and the
problems the architect of an artificial lipid membrane faces. The quest to build a
structure that totally mimics the plasma membrane in all its details is overwhelming.
Instead one has to focus on the creation of an overall structure, which possesses the
most important characteristics of the living cell membrane. The artificial membrane
should therefore consist of a continuous bilayer of phospholipids where the inner core
is composed of the hydrocarbon chains and the hydrophilic head groups in both leaflets
are facing an aqueous environment. Furthermore, the phospholipids should be able to
diffuse freely within the two monolayers. This is of vital importance for the long term
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