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Components of Biological Membranes
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 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 cryoprotectant, 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 to 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 important 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 fig.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%. The protein component. All biological membranes contain a certain amount of protein. The mass ratio of protein to lipid may vary from 0.25:1 to 3.6:1, although the average is usually 1:1. The proteins of a biological membrane can be classified into five groups depending upon their location, as follows; Class 1. Peripheral.------------These proteins lack anchor chains. They are usually found on the external face of membranes associated by polar interactions. Class 2. Partially Anchored-----These proteins have a short hydrophobic anchor chain that cannot completely span the membrane. Class 3. Integral (1)-----------These proteins have one anchor chain that spans the membrane. Class 4. Integral (5)-----------These proteins have five anchor chains that span the membrane. Class 5. Lipid Anchored---------These proteins undergo substitution with the carbohydrate groups of glycolipids, therefore binding covalently with the lipid. This classification is not definitive in including all proteins, since there may well be other examples that span the membrane with different numbers of anchor chains. The structure of proteins varies greatly. The first factor affecting structure is the proteins function, but equally important is the proteins location, as shown above. Those proteins that span the membrane have regions of hydrophobic amino acids arranged in alpha-helices that act as anchors. The alpha-helix allows maximum Hydrogen bonding, and therefore water exclusion. Proteins that pass completely through the membrane are never symmetrical in their structure. The outer face of the plasma membrane at least always has the bulk of the proteins structure. It is usually rich in disulphide bonds, oligasaccharides, and when relevant, prosthetic groups. The proteins found in biological membranes all have distinctive functions, such that the overall function of a cell or organelle may depend on the proteins present. Also, different membranes within a cell, (i.e. those membranes surrounding organelles) can be recognised solely on the presence of membranous marker proteins. In the majority of cases membranous proteins perform regulatory functions. The first group of such proteins are the ionophores, as mentioned before. The proteinous ionophores are found in the greatest concentration in neurones. Here, the diffusion of inorganic ions is essential to maintaining the required membrane potential. The main ions responsible for this are Sodium, Potassium and Chloride - each of which has its own channel forming ionophore. The observed rate of diffusion of many other solutes is much greater than can be explained by physical processes. It is widely accepted that membranous proteins carry certain solutes across the membrane by the process of facilitated diffusion. This is done by the forming of pores of a complimentary size and charge, to accept specific ions or organic molecules. The pores are opened and closed by conformational changes in the proteins structure. There are three main types of facilitated diffusion. None of these processes require an energy input. Active transport is the movement of solutes across a membrane, against the concentration gradient, and it therefore utilises energy from ATP. An example of this is the Sodium-Potassium-ATPase pump, which is an active antiport carrier protein common to nearly all living cells. It maintains a high [Potassium ion] within the cell while simultaneously maintaining a high [Sodium ion] outside the cell. The reason for this is that by pumping Sodium out of the cell, it can diffuse in again at a different site where it couples to a nutrient. As well as transporting solutes across a membrane, there are many proteins that transport solutes along the membrane. An example of this are the respiratory enzyme complexes of the inner mitochondrial membrane. These complexes are located in a close proximity to each other, and pass electrons through what is known as the respiratory chain. The orientation of the complexes is vital for their correct functioning. Another key role of membranous proteins is to oversee interactions with the extracellular matrix. Many hormones interact with cells through the membranous enzyme - adenylcyclase. The binding of specific hormones activates adenylcyclase, to produce cyclic adenosine monophosphate (c.AMP) from adenosine triphosphate (ATP). c.AMP acts as a secondary messenger within the cell. A wide variety of extracellular signalling molecules work by controlling intracellular c.AMP levels. Insulin is an exception to this generalisation, because its receptor is enzyme linked rather than ligand linked. This means that the cystolic face of the receptor has enzymatic activity rather than ligand forming activity. The enzymatic activity of the Insulin receptor is in the reversible phosphorylation of phospoinosite. Vision and smell rely on a family of receptors called the G-protein receptors. The cystolic faces of these receptors bind with guanosine triphosphate (GTP). This action is coupled to ion channels, so that the activation of a receptor changes the intracellular levels of c.GMP, which in turn activates the ion channels, and thus allows a membrane potential to be developed. The composition of proteins in the biological membrane is far from static. Receptors are constantly being regenerated and replaced, and this is important in the ever changing environment of the cell. For example, the transferrin receptor is responsible for the uptake of Iron. In the cytosol, an enzyme called aconitase is present which inhibits the synthesis of transferrin by binding to transferrins mRNA. In a low Iron concentration, aconitase releases the mRNA allowing transferrin to be synthesised. A similar process occurs with the Low Density Lipoprotein (LDL) receptor. This receptor traps LDL particles which are rich in cholesterol. The LDL receptor is only produced by the cell, when the cell requires cholesterol for membrane synthesis. The number of receptors in a biological membrane varies greatly between different type of receptor. The immune responses of cells are controlled by a superfamily of membranous proteins called the Ig superfamily. This superfamily contains all the molecules involved in intercellular and antigenic recognition. This includes major histocompatability complexes, Thymus T-cells, Bursa B-cells, antibodies and so on. Although this family is vast, the important point is that all antigenic responses are mediated by membranous proteins. As there are glycolipids in the biological membrane, there are also glycoproteins. One of the key roles of glycoproteins is in intercellular adhesion. The Cadherins are a family of Calcium dependant adhesives. They are firmly anchored through the membrane, and have glycolated heads that covalently bind to neighbouring molecules. They seem to be important in embryonic morphogenesis during the differentiation of tissue types. The Lectins and Selectins are similar families of molecules responsible for adhesion in the bloodstream. However the most abundant adhesives are the Integrins, which are responsible for binding the cellular cytoskeleton to the extracellular matrix. The range of membranous proteins has proved to be vast, due to the wide variety of functions that must be performed. It would be possible to continue describing proteins for many more pages, but one final example will be used in conclusion, and that is the photochemical reaction centre of photosynthesis. This very large protein complex is found in the Thylakoid membrane of chloroplasts. Each reaction centre has an antenna complex comprising hundreds of chlorophyll molecules that trap light and funnel the energy through to a trap where an excited electron is passed down a chain of several membranous electron acceptors. Conclusion. 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. Bibliography: Alberts,B; Bray,D; Lewis,J; Raff,M; Roberts,K; Watson,J.D. Molecular Biology of the Cell, Third Edition. p.195-212, p.478-504. Garland Publishing, 1994. Beach; Cerejidol; Gordon; Rotunno. Introduction to the study of Biological Membranes. p.12. 1970. Fleischer; Haleti; Maclennan; Tzagoloff. The Molecular Biology of Membranes. p.138-182. Plenum Press, 1978. Perkins,H.R; Rogers,H.J. Cell Walls and Membranes. p.334-338. E & F.N. Spon Ltd, 1968. Quinn,P. The Molecular Biology of Cell Membranes. p.30-34, p.173-207. Macmillan Press, 1982. Stryer,L. Biochemistry, Third Edition. p.283-309. W.H. Freeman & Co, 1994. Yeagle,P. The Membranes of Cells. p.4-16, p.23-39. Academic Press Inc, 1987.

 



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