Components of Biological Membranes - Coursework Example

 Introduction Biological membranes are an active component of all cells. They are selectively permeable membranes, which in addition to controlling the movement of substances through the cell, regulates fluid composition within the cells, recognize and send signals to other cells, and are involved in photosynthesis and oxidative phosphorylation. Components of Biological Membranes The three major components are lipids, proteins and sugars, with a small amount of other materials such as fatty acids. The lipid molecules are two-layered (bilipid layer) with embedded proteins, and both are held together mainly by covalent interactions. Covalent bonds help in the attachment of sugars to the lipid and protein molecules, and are found on one side of the membrane. Lipid Lipids in biological membranes are of three major types: phospholipids, glycolipids and cholesterol. Lipids are soluble in organic solvents like ethanol, propanonone etc but are insoluble in water. a. Phospholipids- glycerol is the most common type of phospholipid (glycerophospholipids), which is linked to two fatty acid chains, phosphate and choline. The glycerophospholipids are of three major types, and contain choline, serine or ethanolamine attached to the phosphate. Sphingomyelin is a phospholipid where choline is attached to phosphate (Brown BS.) The most biologically prevalent lipid head group is phosphatidylcholine (PC). The common negatively charged lipid head groups include phosphaditic acid (PA), phosphatidylglycerol (PG) and phosphatidylserine (PS). The hydrocarbon chains of lipids, which are usually 14-24 carbons long, have the chains aligned parallel to each other, and are stabilized by extended dispersion forces. Either a trans or gauche configuration can occur by rotation around the C-C bonds along the chains. (Scarlata S) b. Glycolipids- glycolipids contain a sugar (glucose or galactose) instead of the phosphate, thereby differing from phospholipids. However, like phospholipids, glycolipids contain either glycerol or spingosine, which are linked to fatty acid chains. Biological membranes in animals contain sphingosine, in contrast to bacterial and plant membranes, which contain glycerol. However, in general, glycolipids are present on the outer surface of the plasma membrane. c. Cholesterol-unlike phospholipids or glycolipids, cholesterol has a four-ring steroid structure along with a hydrocarbon side-chain and a hydroxy group (Brown BS). The membrane fluidity can be affected by cholesterol to varying degrees. Cholesterol tends to decrease the rotational freedom of the neighboring hydrocarbon chains in the fluid phase, which decreases the fluidity and stiffens the membrane. By acting as a contaminant in the gel phases, cholesterol decreases the order of the well-packed lipid chains (Scarlata S.) One part of the phospholipid and the cholesterol molecules is soluble in water (hydrophilic or water loving) whereas the other part is soluble only in fats (hydrophobic or water fearing). The fatty portions occupy the centre of the membrane and the hydrophilic portions project to the two surfaces in contact with the water (Guyton AC.) The Fluid Mosaic Model According to this model, proposed by Singer and Nicolson, the lipid bilayers are fluid, and individual proteins, cholesterol, and phospholipids diffuse rapidly throughout the two dimensional surface of the membrane (mosaic). This model has been further refined to include non-homogeneous distribution of components and the effect of cytoskeletal elements. The bilipid layer is a selectively permeable membrane because it is impermeable to water-soluble substances like ions, glucose, urea etc, whereas it is permeable to fat-soluble substances like oxygen and alcohols. Proteins The membrane proteins, most of which are glycoproteins, are large, globular masses floating in the lipid bilayer. There is a specific upside-down or right-side-up orientation of all membrane proteins in the bilayer. Whereas some proteins are anchored to the membrane by ionic interactions between residues with side chains that are positively charged and lipid head groups that are negatively charged, other proteins are anchored by post-synthetic attachment of a hydrocarbon chain such as myristoyl, palmitoyl, etc or a lipid such as glycosyl-phosphatidyl-inositol (GPI), or are anchored to the surface by ionic contacts (Scarlata S.) There are two types of proteins, integral proteins, which protrude all the way through the membranes, and the peripheral proteins, which are attached to the surface the membrane (Guyton AC). Ions, sugars and amino acids, which cannot diffuse across phospholipid bilayer, are primarily transported by integral membrane proteins called transport proteins (mediated transport). Two types of transport protein can be considered: channel proteins and carrier proteins (Brown BS.) Channel proteins provide structural channels or pores across the lipid bilayer through which water-soluble substances, especially the inorganic ions can move down their electrochemical gradient. Then selectivity of the pores is determined by the amino acid side-chains, which line the pore. Those pores, which are lined by negative side-chains, admit positive ions, whereas those lined by positive side-chains, admit negative ions. In addition, the diameter of the narrowest part of the pore, determines the size of the ion, which are admitted through the pores. The binding of signal molecules to the protein subunits or the change in the membrane potential determines whether the channel is closed or open. Channels which are opened by the binding of a signal molecule is said to be ligand-gated, and channels which are opened by a change in membrane potential is said to be voltage-gated. The transport rate for channel proteins is about 10 8 ions/second. Carrier proteins transport specific molecules or ions by binding to them and subsequently transferring them across the membrane. Each carrier protein transports one type of molecule and one particular molecule of its class. In order to transport materials across the membrane, the carrier has to change its shape or conformation. The maximal transport rate is 10 4 molecules/second. Some carrier proteins transport only one solute across the membrane, and are called uniports (eg. Glucose transporters in RBC membranes), while some transport two solutes at a time, and are called co-transporters. If they are transported in the same direction, they are called symport, while transport in the opposite directions is called antiport. Passive transport or facilitated transport is the transfer of molecules or ions without the expenditure of energy. The transport of molecules or ions with the use of energy and against their electrochemical gradient is called active transport. The most common source of this energy is from the hydrolysis of ATP. Whereas membranes can transport small molecules like sugars, amino acids and inorganic ions by both active and passive transport, they can also transport macromolecules like proteins or bacterial cells by the process of exocytosis and endocytosis. (Brown BS.) In endocytosis, there is formation of small vesicles at the cell membrane, which contain particulate matter or extracellular fluid. The vesicles then pinch off to the interior of the cell and release its content inside the cell. In exocytosis, the vesicles fuse with the plasma membrane to release the contents outside the cell (Guyton AC.) Fatty acids- fatty acids when compared to their corresponding two-chained lipid can freely partition into membranes, and basically consist of a carboxylic group attached to a single CH2 chain. They act as carriers to membrane proteins (Scarlata S.) Critical discussion The interactions between lipid-lipid and lipid-protein in membranes are extremely important for membrane function. The fluid-mosaic model seems to imply that the bilayer is disorderly and is a random structure, whereas actually, it is a highly dynamic structure and complex. The embedded proteins in the lipid bilayer make a hydrophobic match by deformation. Interactions occur between amino acids and specific acyl-chains in the hydrophobic core, and hydrophilic amino acids with the charged lipid head-groups at the water membrane interface. Lipids also affect the function of membrane proteins by organizing in non-lamellar structures, and in addition, can also segregate in local domains in the membrane. (Eg. formation of sphingolipid-cholesterol domains) (Children’s Hospital Oakland Research Institute.) The ability of lipids to self-organize into membrane microdomains and the specific interactions between lipids and proteins in membrane traffic and signalling are subjects of current research (Thiele C.) Conclusion The three major components of biological membranes are lipids, proteins and sugars, and it is a selectively permeable membrane. The membrane proteins, most of which are glycoproteins, are embedded in the lipid bilayer. There are two types of transport protein, channel proteins and carrier proteins. Transport can also occur by means active and passive methods, in addition to processes like endocytosis and exocytosis. The traditional view of membrane lipids as a passive two-dimensional solvent for membrane proteins is however not true, and they are of complex organization and function. ***************************************************************************** References Brown BS. Biological Membranes. [Online] . Children's Hospital Oakland Research Institute. Lipid Biology in Health & Disease: Areas of Research. [Online] Guyton AC (1986). Textbook of Medical Physiology. 7th ed. pp. 3-5. Scarlata S. Membrane protein structure. [Online]. Thiele C. The cell biology of lipid-protein interactions. [Online] Read More