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.
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...