Why do we have Biomembrane. Protect internal environment from outer environment. Helps in compartmentation within the cell.
Membrane properties Membranes are highly selective permeability barriers, rather than impervious walls because they contain specific molecular pumps and gates. Membranes also control the flow of information between cells and their environment, they contain specific receptors for external stimuli.(e,g bacteria,hormone receptor complex) Some membranes generate signals which can be chemical or electrical. The two most Important energy conversion process in biological systems are carried out by membrane system that contain highly ordered array of enzymes and other protein, Photosynthesis,in which light is converted into chemical- bond energy occurs in the inner membranes of chloroplasts. In oxidative phosphorylation,inwhich ATP is formed by the oxidation of fuel molecules, takes place in the inner membranes of mitochondria.
Common features of biological membrane. Membranes are sheet like structure, only a few molecules thick that form closed boundaries between compartments of different composition. The thickness of most membranes is between 60 and 100A Membranes consist mainly of lipids and proteins. The weight ratio of protein to lipid in most biological membranes ranges from 1:4 to 4:1.They also contain carbohydrates that are linked to lipids and proteins Membrane lipids are relatively small molecules that have both hydrophilic and hydrophobic moiety. These lipids spontaneously form closed bimolecular sheets in aqueous media. These lipid bilayers are barrier to the flow of polar molecules.
Specific proteins mediate distinctive function of membranes. Proteins serve as pumps, gates, receptors, energy transducers and enzymes. Membrane protein are intercalated into lipid bi layers, which create a suitable environment for the action of these proteins. Membranes are noncovalent assemblies. The constituent protein and lipid molecules are held together by many non covalent interactions, which are cooperative in character. Membranes are asymmetric. The inside and outside face of membranes are different. Membranes are fluid structures. Lipidmolecules diffuse rapidly in the plane of the membrane, as do proteins, unless anchored by specific interactions.Membranes can be regarded as two-dimensional solutions of oriented protein and lipids.
Most cell membranes are electrically polarized, such that the inside is negative [typically -60 millivolts (mV)]. Membrane potential plays a key role in transport, energy conversion, and excitability
Membrane composition depends on their function Neuron Myelin sheath only 18% protein 82% lipid to provide good insulation Oxidative phosphorylation Inner mitochondrial membrane contain 76% protein
What are membrane made up of? The three major classes of membrane lipids are. 1. Phosholipids a. Phosphoglycerides: derived from glycerol(3 C alcohol) b. Sphingosine ( a more complex alcohol) 2. Glycolipids 3. Cholesterol
Phospholipids are the major Class of membrane Lipids. Phospholipids are derived from either glycerol(3c alcohol),or sphingosine(complex Alcohol). Phospholipids derived from glycerol are called phosphoglyceroids.it consists of a glycerol backbone,two fatty acid chains and a phosphorylated alcohol. The fatty acid chain usually contain even number of C atom, between 14-24. the 16 and 18 are most common.Fatty acids may be saturated or unsaturated.the configuration of double bonds in unsaturated fatty acids is nearly always cis. The length and the degree of of unsaturation of fatty acids chains in membrane lipids have a profound effect on membrane fluidity.
Structure of Phosphoglycerides In phosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the two fatty acid chains. The C-3 hydroxyl group of the glycerol backbone is esterified to phosphoric acid. When no further additions are made, the resulting compound is phosphati-date (diacylglycerol 3- phosphate), the simplest phosphoglyceride. Only small amounts of phosphatidate are present in membranes. However, the molecule is a key intermediate in the biosynthesis of the other phosphoglycerides
Some Common Phosphoglycerides Found in Membranes
Structures of Sphingosine and Sphingomyelin Sphingomyelin is a phospholipid found in membranes that is not derived from glycerol. Instead, the backbone in sphingomyelin is sphingosine, an amino alcohol that contains a long, unsaturated hydrocarbon chain.
Glycolipid Glycolipids, as their name implies, are sugar- containing lipids. Like sphingomyelin, the glycolipids in animal cells are derived from sphingosine. The amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin. Glycolipids differ from sphingomyelin in the identity of the unit that is linked to the primary hydroxyl group of the sphingosine backbone. In glycolipids, one or more sugars (rather than phosphoryl choline) are attached to this group. The simplest glycolipid, called a cerebroside, contains a single sugar residue, either glucose or galactose. Gangliosides contain a branched chain as many as 7 sugar residues.
Membrane Lipids Can Also Include Carbohydrate Moieties
Cholesterol Present in eukaryotes and not in most procaryotes. It constitutes almost 25% of the membrane lipids in certain nerve cells but is essentially absent from some intracellular membranes.
Liposomes When a suspension of phospholipids is mechanically dispersed in aqueous solution, the phospholipids aggregate into one of three forms: spherical micelles and liposomes and sheetlike, two-molecule- thick phospholipid bilayer.