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Text Structure and Functions of the Cell Membrane The cell membrane (also called the plasma membrane or plasmalemma) is the biological membrane which separates the interior of a cell from the outside environment. Plasma membranes bounds all plant and animal cells and all single-celled eukaryotes and prokaryotes. They are fluid mosaics of lipids, proteins, and carbohydrates. Structurally, they show resemblance to other cellular membranes, but differ in their lipid composition and more drastically in their protein content from one cell type to another and from intracellular membranes. In this topic we will describe these three structures and see how they organise and function in the cell membrane. Lipids Lipids form an important group of organic molecules which includes fatty acids and their naturally-occurring derivatives like carotenoids, waxes, sterols and fat-soluble vitamins (such as vitamins A, D, E and K). Lipid molecules are generally insoluble in water. They may be also be defined as hydrophobic small molecules consisting of long, 18-22 carbon, hydrocarbon backbones with only a small amount of oxygen-containing groups. Lipids are the biomolecules which serve innumerable functions in organisms. Phospholipids, glycolipids and steroids are very important for the organisation membranes in cells. Basic Lipid Structure and functioning of Fats (triacylglycerols) Fats can be defined as a diverse group of compounds that are generally insoluble in water but soluble in organic solvents. Chemically, fats are triesters of glycerol and fatty acids. The fatty acids are long, unbranched hydrocarbons that terminate with a monocarboxylic acid. Depending upon the double bonds, fatty acids can be of two types: saturated and unsaturated. In a typical fatty acid, each carbon atom can be bonded to two hydrogen atoms. Fatty acids which have no double bonds are called as saturated fatty acids, because the carbon atoms are saturated with hydrogen which means that they are bonded to the maximum number of hydrogen atoms possible. On the other hand, fatty acids which have one or more double bonds are called as unsaturated fatty acids. Fatty acids which have only one double bond are known as "monounsaturated" fatty acids, whereas those which have more than one double bond are called as "polyunsaturated" fatty acids. These double bonds introduce "kinks" in the carbon chain which has a direct bearing on the fluidity of lipid membranes. Therefore, a fat molecule can be made of one, two, or three different types of fatty acids. Depending upon the type of fatty acid, fats can be saturated or unsaturated. A saturated fat does not contain any unsaturated fatty acid while as an unsaturated fat contains a minimum of one unsaturated fatty acid esterified with the glycerol. The former are typically solid at room temperature and the latter are fluid at low temperatures because of the kinks introduced by the double bonds in the fatty acid chains, which do not allow their close packing. Phospholipids Phospholipids are a diverse and important class of lipids known for their role in cell membrane organisation and structure. They serve as the major constituent of all cell membranes as they nicely assemble to form the lipid bilayer of membranes. A phospholipid molecule consists of two parts: a hydrophilic polar head and a non polar hydrophobic tail. The phosphate group along with the glycerol constitute the hydrophilic polar head, whereas the fatty acid molecules form the hydrophobic tail. Thus phospholipids are amphipathic, with polar head and non polar tail. Most of the phospholipids contain a glycerol, two fatty acid chains, one or more phosphate groups, or also a simple organic molecule such as choline as shown in the fig. Phospholipids, being amphipathic have both water loving and water hating areas, this property of phospholipids is basic to the formation of micelles and lipid bilayers. Phospholipid Structure When phospholipids are exposed to aqueous environments, they self assemble into structures called micelles and bilayers, in which the hydrophobic tails (water-repelling parts) form the core and remain hidden from the water while as the hydrophilic polar heads (water-loving regions) remain in contact with water as shown below in the diagram. These specific properties prove selective in allowing phospholipids to play an important role in the phospholipid bilayer synthesis. Lipid bilayers occur when hydrophobic or non polar tails of phospholipid molecules line up against one another, forming a membrane with hydrophilic heads on both sides facing the water and an inner hydrophobic core. In living systems, the phospholipids often occur in association with other molecules (e.g., proteins, glycolipids, cholesterol) in a bilayer, such as a cell membrane that surrounds the cell and intracellular structures like chloroplast, mitochondria and other membrane-bound organelles. There are a few important features of phospholipid bilayer which are critical to membrane function. To begin with, the structure of phospholipids is responsible for the basic function of membranes as barriers between two aqueous compartments, interior cell compartment and outer environment. Since, the core of the phospholipid bilayer is occupied by hydrophobic fatty acid tails, this hydrophobic core lends membrane the property of impermeability biological to water-soluble molecules. Second molecules, important ions and feature of other the phospholipid bilayer is its viscous fluid nature. The phospholipids have a good degree of unsaturation in their fatty acids, having one or more double bonds, which cause kinks in the hydrocarbon chains making them difficult to pack together. This causes the long hydrocarbon chains of the fatty acids to move freely in the interior of the membrane, so the membrane itself is soft and fluid. Because of this fluidity, both phospholipids and proteins are free to diffuse laterally within the membrane, property which forms the basis of many membrane functions. Steroids In addition to phospholipids, the cell membranes of animal cells also contain glycolipids and steroids. The steroids are a class of lipids which do have a molecular structure which comprises of four fused rings and are fat-soluble organic compounds. This class of lipids includes many hormones such as androgens of animals and cholesterol. Cholesterol is an important steroid which is an integral constituent of animal cell membranes. Cholesterol functions to enhance the fluidity of membranes by preventing close packing of fatty acid chains and also enhances the membrane rigidity. Cholesterol also influences the cell membrane permeability. Proteins Proteins are also the vital components which enter into the constitution of cell membranes. The number of protein molecules in the membrane weight/weight basis. is less than lipids, but are equal on Two classes of membrane-associated proteins distinguished are extrinsic and intrinsic. 1. Extrinsic or Peripheral proteins: They are superficially located and can be easily extracted from the membrane following treatments with polar reagents, such as solutions of extreme pH or high salt concentration that do not disrupt the phospholipid bilayer. They are soluble in aqueous solution and constitute about 30% of the protein content of plasma membrane e.g. Cytochrome c on the mitochondrial surface. 2. Intrinsic or Integral protein: They constitute about 70% of the protein content of plasma membrane. They are tightly held and difficult to remove from the membrane. They are insoluble in aqueous solutions. To obtain the integral proteins, the membrane has to be disrupted. Some of the integral or intrinsic proteins partially traverse the lipid bilayer and are found inserted on one side of the membrane only, but many integral proteins called tunnel proteins or transmembrane proteins completely traverse the lipid bilayer on both sides. When a protein crosses the lipid bilayer it adopts an alpha-helical configuration. The integral transmembrane proteins may be called as single-pass proteins, bi-pass protein or multi-pass proteins depending on whether the protein makes one, two or multiple turns across the lipid bilayer. Because of their property of spanning the membrane completely, transmembrane proteins are able to perform functions both inside and outside of the cell. A good number of transmembrane proteins are also believed to have channels through which ions and other water-soluble materials are diffused into the cells. Membrane Proteins The transmembrane proteins like the phospholipid molecules have hydrophobic and hydrophilic groups and regions. The non polar or hydrophobic regions of the integral protein are embedded in the hydrophobic interior of the lipid bilayer while as the hydrophilic regions protrude from the bilayer surface. Owing to the semi-permeability of the cell membrane, the cell depends on special mechanisms for communication with other cells and exchange of nutrients with the extracellular space. These special roles are primarily performed by proteins. The intrinsic proteins embedded in the lipid bilayer of the membrane serve many of the membrane functions, besides acting as structural components. Their specific and unique shapes also allow them to function as receptors and receptor sites, signal transducers and transporters. The transport proteins or carrier proteins help in transporting substances across the membrane. The extrinsic proteins serve as anchoring sites for the cytoskeleton or extracellular fibres. Carbohydrates In addition to the lipids and proteins, carbohydrates also form an important constituent of the cell membrane. The outer surface of the protein-lipid membrane bilayer is coated with a layer of carbohydrate chains. This carbohydrate layer is called as glyocalyx. The lipids and intrinsic proteins of the membrane are bound to carbohydrates, forming glycolipids and glycoproteins, respectively. These carbohydrates which are normally short chain oligosaccharides give cells their identity and also play the roles of cell-cell interaction and cell recognition. MEMBRANE ORGANIZATION Many models of plasma membrane structure have been proposed by scientists from time to time. In 1935, Danielli and Davson proposed a model according to which the plasma membrane is a trilaminar structure with a phospholipid layer sandwiched by two protein layers. In 1959, David Robertson proposed the Unit membrane model which means that the trilaminar structure is of common occurrence in all biological membranes. The three layers have a total thickness of 75 A0 to 100 A0. It advocates that all membranes do have a common structure. This model, however, could not explain the dynamic nature and functional specificity of the membrane. Fluid Mosaic Model The fluid mosaic model of cell membrane was proposed by two scientists, Jonathan Singer and Garth Nicolson in 1972. This model of membrane structure is now the most widely accepted, since it explains the basic organization of all biological membranes. According to this model, cell membrane consists of a highly viscous fluid matrix made up of phospholipid bilayer to which proteins are associated. The lipid forms the ocean in which proteins are immersed as ice bergs. This model came to be known as fluid mosaic model because it views the membranes as two-dimensional fluids, in which the phospholipids and membrane proteins are free to diffuse laterally. The mosaic pattern in the fluid membrane is attributed to the scattered arrangement of protein molecules in the fluid of phospholipid matrix; hence the name mosaic to the model. According to the fluid mosaic model, lipids are the basic structural components of membranes while as the proteins within the phospholipid bilayer carry out specific membrane functions. Generally, the plasma membranes are constituted of approximately 50% lipid and 50% protein by weight, while as the carbohydrates are relatively minor and make up only 5 to 10% of the membrane weight. Membrane asymmetry The plasma membrane shows asymmetry in its structure. The two layers of the lipid bilayer differ in their lipid and protein composition. The outer surface of the membrane bilayer, which faces the extracellular matrix, contains oligosaccharides (glycolipids and glycoproteins) which give identity to the cell. It also possesses the end of the integral proteins which receive signals from outside the cell. The inner side of the membrane lies in contact with the cell cytoplasm. It remains attached to the cytoskeleton and also contains the end of the integral proteins which transmit the signals that are received on the extracellular side. Membrane Fluidity The fluidity of cell membrane is because of the phospholipids and is primary to all the functions of the membrane and thus of the cell also. Fluidity of membrane allows the constituent lipids and proteins to mobilise within the bilayer. This mobility is having a tremendous biological significance since it governs the transport or exchange of life-driving commodities in and out of the cell. The fluidity of membranes depends upon the the structure of fatty acid chain and temperature. Higher degree of unsaturated fatty acids increases membrane fluidity while as their lower content decreases it. So membrane fluidity at any temperature is maintained by the right ratio of saturated to unsaturated fatty acids. During cold periods, some animals and plants respond to decreasing unsaturated temperatures fatty acids by in increasing cell the amount of membranes. Presence of cholesterol in animal cell membranes prevents the close packing of fatty acid tails, thereby lowering the need of unsaturated fatty acids. This prevents the cell membrane from becoming too liquid at body temperature and therefore, maintaining the desired fluidity. The lipids present in the cell membrane are randomly moving at the rate of 22 µm per second. Usually the phospholipids present in the same lipid layer of the membrane move freely but very rarely they flip to the other layer. Flipping is an energy-dependent process and it occurs rarely because it requires the hydrophillic head of the phospholipid to traverse the hydrophobic region of the bilayer. Membrane Functions or Membrane Transport Cell membranes are called selectively or differentially permeable i.e. they permit the passage of certain ions and molecules while excluding others. Membranes are relatively permeable to water, some simple sugars, amino acids and lipidsoluble materials but are relatively impermeable to very large molecules such as proteins, polysaccharides, etc. It is observed that the negatively charged ions pass rapidly than the positively charged ions, though non-electrolytes pass most rapidly. This means that the membrane is positively charged. There are different processes used by the molecules and ions to move into and out of the cells through cell membrane. A. Passive transport: 1. Simple diffusion: In this process of diffusion, the molecules and ions pass through a membrane along their concentration gradient, without involving expenditure of energy. Lipid-soluble substances and water-soluble substances can pass by simple diffusion. The smaller molecules pass more rapidly than the larger ones. Passage of charged ions leads to change in the potential across the membrane which is unfavourable for further passage of the same ion. In order to avoid the effect, a positively charged ion should accompany a negatively charged ion. Alternatively, movement of a cation in one direction should be accompanied by movement of a cation in the other direction. 2. Facilitated diffusion: Some substances are transferred across membranes more readily than is expected from a process of passive diffusion, although no expenditure of energy is involved in their transport. The process of such transfers is called the Facilitated diffusion and occurs according to the concentration gradient. The process requires participation of a transmembrane protein called the carrier, transporter or permease. Besides, the carrier also binds the transported compounds specifically. Specificity, however, may not be absolute. Thus, the glucose carrier for the erythrocyte membrane has maximum affinity for glucose, mannose and fructose. Fructose in small intestine is absorbed by Facilitated diffusion. Mechanism of Facilitated diffusion: After binding of the substance for transport, to the carrier, the molecular events for transport are clear. In the “ping pong model” the carrier protein is believed to have two conformations. In one, the binding site of the carrier molecule faces one side of the membrane and in the other conformation it faces the other side. Exchange between the two conformations is obligatory but exchange is slow if the binding sites are empty but becomes fast if these sites are occupied. Osmosis: This is a special diffusion of solvent (water). The solvent permeable molecules membrane pass (cell through a membrane) selectively from dilute solutions to water by the process of osmosis. Ion channels: These are membrane proteins that allow the passage of ions that would ordinarily be stopped by the lipid bilayer of the membrane. These small passageways are specific for one type of ion, such that a calcium ion could not pass through an iron ion channel. The ion channels also serve as gates because they regulate ion flow in response to two environmental factors: chemical or electrical signals from the cells and membrane movement. This happens in your body when a nervous impulse encounters a gap or synaptic cleft between nerve cells. The electrical stimulation is continued because ion channels are opened to allow specific ions to pass through the receiving membrane, which continues the electrical stimulation to the next nerve cell. B. Active transport: There is evidence that dissolved substances, especially mineral ions, continue to move into the cells even though there is a greater concentration of them within the cell than outside. Such a movement of material against the concentration gradient is called active transport, which involves the expenditure of metabolic energy. Active and Passive Transport Proteins For example Sodium-potassium pump or Na+-K+ ATPase: It is present in the plasma membrane of many cells. Na+-K+ ATPase exchanges 3 Na+ (from cell to outside) with 2 K+ (into the cell from outside). Steps involved 1. Binding side towards the cell--Na+ binds. 2. Phosphorylation of transporter-binding site gets everted 3. Na+ released, conformation for K+ acquired. Thus, K+ binds. 4. Dephosphorylation of transporter. 5. Original position and process repeats. Uniport and co-transport Active transport system may act as Uniport or Cotransport processes. In a uniport process, only a single molecule is transported. In Co-transported processes, transport of one molecule is linked with transport of another molecule in the same direction (symport) or in the opposite direction (antiport). C. Membrane Transport of Macromolecules (Bulk Flow) Many cells usually use the processes of exocytosis and endocytosis for secretion and ingestion of macromolecules, respectively. In exocytosis, the vesicle fuses with the cell membrane and releases the contents to the exterior of the cell while as in endocytosis, the membrane invaginates and pinches off, engulfing the molecule. The endocytotic vesicles are of two different sizespinocytotic (which are small and contain the dissolved solutes) and phagocytic (which are large and contain the solid particles). During receptor-mediated endocytosis, coated vesicles bind to specific receptors on the cell surface, guiding the cell to select what molecules to take and what to reject. D. Membrane Receptors Receptor proteins are the specialized transmembrane proteins which act as the communication office of the cell, thereby allowing the cell to interact with the outside environment. The exterior end of the receptor protein binds to a specific messenger molecule. This signal is transmitted, causing the interior end of the protein to change shape, which triggers a reaction inside the cell. These receptor proteins are highly specific.