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, symmetric, 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 unction, 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 asis 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 ignificance 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, he 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 rotecting 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 many cholesterol molecules in a membrane as there are phospholipid molecules. Cholesterol orientates in such a way that it significantly affects the fluidity of the membrane. In regions of high cholesterol content, permeability is greatly restricted so that even the smallest molecules can no longer cross the membrane. This is advantageous in localised regions of membrane.
Cholesterol also acts as a very efficient ryoprotectant, preventing the lipid bilayer from crystallising in cold conditions. The biological membrane is responsible for defining cell and organelle boundaries. This is important in separating matrices that may have very different compositions. Since there are no covalent forces between lipids in a bilayer, the individual molecules are able to diffuse laterally, and occasionally across the membrane. This freedom of movement aids the process of simple diffusion, which is the only way that small molecules can cross the membrane without the aid of proteins.
The limit of permeability of the membrane o the diffusion of small solutes is selectively controlled by the distribution of cholesterol. Another role of lipids is their to dissolve proteins and enzymes that would otherwise be insoluble. When an enzyme becomes partially embedded in the lipid bilayer it can more readily undergo conformational changes, that increase its activity, or specificity to its substrate. For example, mitochondrial ATPase is a membranous enzyme that has a greatly decreased Km and Vmax following delipidation. The same applies to glucose-6-phospatase, and many other enzymes.
The ability of the lipid bilayer to act as an organic solvent is very mportant in the reception of the Intracellular Receptor Superfamily. These are hormones such as the steroids, thyroids and retinoids which are all small enough to pass directly through the membrane. Ionophores are another family of compounds often found embedded in the plasma membrane. Although some are proteinous, the majority are polyaromatic hydrocarbons, or hydrocarbons with a net ring structure. Their presence in the membrane produces channels that increases permeability to specific inorganic ions. Ionophores may be either mobile ion-carriers or channel formers. see ig. 4)
The two layers of lipid tend to have different functions or at least uneven distribution of the work involved in a function, and to this end the distribution of types of lipid molecules is asymmetrical, usually in favour of the outer face. In general internal membranes are also a lot simpler in composition than the plasma membrane. Mitochondria, the endoplasmic reticulum, and the nucleus do not contain any glycolipids. The nuclear membrane is distinct in the fact that over 60% of its lipid is phospatidylcholine, whereas in the plasma membrane the figure is nearer 35%.
