What Are The Major Components of Biological Membranes And How Do They Contribute
To Membrane Function?.


Summary.

The role of the biological membrane has proved to be vital in countless
mechanisms necessary to a cells survival. The phospholipid bilayer performs the
simpler functions such as compartmentation, protection and osmoregulation. The
proteins perform a wider range of functions such as extracellular interactions
and metabolic processes. The carbohydrates are found in conjunction with both
the lipids and proteins, and therefore enhance the properties of both. This may
vary from recognition to protection.
Overall the biological membrane is an extensive, self-sealing, fluid,
asymmetric, selectively permeable, compartmental barrier essential for a cell or
organelles correct functioning, and thus its survival.

Introduction.

Biological membranes surround all living cells, and may also be found
surrounding many of an eukaryotes organelles. The membrane is essential to the
survival of a cell due to its diverse range of functions. There are general
functions common to all membranes such as control of permeability, and then
there are specialised functions that depend upon the cell type, such as
conveyance of an action potential in neurones. However, despite the diversity of
function, the structure of membranes is remarkably similar.
All membranes are composed of lipid, protein and carbohydrate, but it is
the ratio of these components that varies. For example the protein component may
be as high as 80% in Erythrocytes, and as low as 18% in myelinated neurones.
Alternately, the lipid component may be as high as 80% in myelinated neurones,
and as low as 15% in skeletal muscle fibres.
The initial model for membrane structure was proposed by Danielli and
Davson in the late 1930s. They suggested that the plasma membrane consisted of a
lipid bilayer coated on both sides by protein. In 1960, Michael Robertson
proposed the Unit Membrane Hypothesis which suggests that all biological
membranes -regardless of location- have a similar basic structure. This has been
confirmed by research techniques. In the 1970s, Singer and Nicholson announced a
modified version of Danielli and Davsons membrane model, which they called the
Fluid Mosaic Model. This suggested that the lipid bilayer supplies the backbone
of the membrane, and proteins associated with the membrane are not fixed in
regular positions. This model has yet to be disproved and will therefore be the
basis of this essay.

The lipid component.

Lipid and protein are the two predominant components of the biological
membrane. There are a variety of lipids found in membranes, the majority of
which are phospholipids. The phosphate head of a lipid molecule is hydrophilic,
while the long fatty acid tails are hydrophobic. This gives the overall molecule
an amphipathic nature. The fatty acid tails of lipid molecules are attracted
together by hydrophobic forces and this causes the formation of a bilayer that
is exclusive of water. This bilayer is the basis of all membrane structure. The
significance of the hydrophobic forces between fatty acids is that the membrane
is capable of spontaneous reforming should it become damaged.
The major lipid of animal cells is phospatidylcholine. It is a typical
phospholipid with two fatty acid chains. One of these chains is saturated, the
other unsaturated. The unsaturated chain is especially important because the
kink due to the double bond increases the distance between neighbouring
molecules, and this in turn increases the fluidity of the membrane. Other
important phospholipids include phospatidylserine and phosphatidylethanolamine,
the latter of which is found in bacteria.
The phosphate group of phospholipids acts as a polar head, but it is not
always the only polar group that can be present. Some plants contain
sulphonolipids in their membranes, and more commonly a carbohydrate may be
present to give a glycolipid. The main carbohydrate found in glycolipids is
galactose. Glycolipids tend to only be found on the outer face of the plasma
membrane where in animals they constitute about 5% of all lipid present. The
precise functions of glycolipids is still unclear, but suggestions include
protecting the membrane in harsh conditions, electrical insulation in neurones,
and maintenance of ionic concentration gradients through the charges on the
sugar units. However the most important role seems to be the behaviour of
glycolipids in cellular recognition, where the charged sugar units interact with
extracellular molecules. An example of this is the interaction between a
ganglioside called GM1 and the Cholera toxin. The ganglioside triggers a chain
of events that leads to the characteristic diarrhoea of Cholera sufferers. Cells
lacking GM1 are not affected by the Cholera toxin.
Eukaryotes also contain sterols in their membranes, associated with
lipids. In plants the main sterol present is ergosterol, and in animals the main
sterol is cholesterol. There may be as