Lec 1.1 The Membrane Bilayer PDF

Membrane and receptors module Session 1 The Lecture 1.1Membrane Bilayer Dr. Asmaa hamza Objectives: To know the basic structure of biological membranes To introduce the characteristics of bilayer mechanisms for the correct insertion of References 1. Introduction to Genera...

Membrane and receptors module Session 1 The Lecture 1.1Membrane Bilayer Dr. Asmaa hamza Objectives: To know the basic structure of biological membranes To introduce the characteristics of bilayer mechanisms for the correct insertion of References 1. Introduction to General, Organic and Biochemistry Frederick A. Bettelheim 2. Lehninger principles of biochemistry. Albert L. Lehninger Objective number (1): To know the basic structure of biological membranes  General functions of biological membranes (cells and organelles) 1- Continuous. highly selective permeability barrier. 2- Allows control of the enclosed chemical environment 3-Communication - control the flow of information between cells and their environment. 4. Recognition - signalling molecules, adhesion proteins, immune surveillance 5. Signal generation in response to stimuli - electrical, chemical  Different Biological Membrane have specialized functions Plasma membrane Plasma membrane - all of the above functions. Mitochondrial membrane energy conservation by oxidative phosphorylation. * oxidative phosphorylation:- deponds on the reduction of oxygen and using of NADH and FADH as donars to generate ATP Membrane Composition Varies with source but generally membranes contain approximately: 1- 40% lipid. 2- 60% protein. 3- 1-10% carbohydrate (dry weight). N.B. the membrane bilayer is a hydrated structure and hence 20% of total membrane weight is water.  Properties of Membrane lipids amphipathic molecules :- -They contain both hydrophilic and hydrophobic moieties. -Distribution varies depending on cell type Phospholipid Sphingomyelin Cholesterol Glycolipids  1- Phospholipids It is Predominant lipids of membrane e.g. phosphatidylcholine. It is consists of :- one head and two tails. The head is phosphate group and it is polar ( has negative charge) so it is hydrophilic. The two tails are two fatty acids ,they are non polar ( they can not interact with water) so it is hydrophobic. phospholipids are derived from Glycerol -3-P What are the properties of head group? - A range of polar head groups are employed -choline, amines, amino acids, sugars Fatty acid chains. - What are the properties of tails group? - Enormous variety, C16 and C18 most prevalent - May be saturated or unsaturated. *Unsaturated fatty acid side chains (double bonds) in the cis conformation introduce a kink in the chain which reduces phospholipid packing 2- Sphingomyelin 3- Cholesterol - It is one of plasma membrane lipid, consists 45% of the total membrane lipid. Distribution of different lipids is tissue specific and related to function  What is the importance of unsaturated fatty acids and cholesterol in the cell membrane? Unsaturated double bonds in the fatty acid side chains disrupt the hexagonal packing of phospholipids and so increase membrane fluidity. Cholesterol plays an important role in stabilizing the fluidity of plasma membrane, OH group of Cholesterol interacts with the polar groups of the phospholipids as well as the remaining of molecule with the fatty acids by hydrogen bonding to plasma  4- Glycolipids : sugar containing lipids May contain glycerol or sphingosine and fatty acids. They are classified as: Cerebrosides - head group sugar monomers Gangliosides - head group oligosaccharides (sugar multimers) Distribution of different lipids in tissues depends on the function Lipid % Phostcho 45-55 15-25 PhostEth 10-15 PhostIno 5-10 PhostSer Cholesterol 10-20 Others ?  Lipid Bilayer structures Amphipathic molecules form one of two structures in water, micelles and bilayers. Bilayers are the favoured structure for phospholipids and glycolipids in aqueous media. Different structures may be formed liposome is a tiny bubble (vesicle), made out of the same material as a cell membrane. Liposomes can be filled with drugs, and used to deliver drugs for cancer and other diseases. Membranes are usually made of phospholipids, which are molecules that have a head group and a tail group. Objectives (2) : To introduce the characteristics of bilayer  Characteristics of the bilayer Bilayer formation is spontaneous in water driven by the van der Walls attractive forces between the hydrophobic tails. The co-operative structure is stabilized by non-covalent forces; electrostatic and hydrogen bonding between hydrophilic moieties and interactions between hydrophilic groups and water. *Pure lipid bilayers have a very low permeability to ions and most polar molecules.  Dynamics in lipid bilayers Membranes are fluid structures. Lipid molecules possess four permitted modes of mobility in a lipid bilayer. 1 -Fast lateral diffusion within the plane of the bilayer.. 2-Fast axial rotation. 3- Intra-chain motion - kink formation in the fatty acyl chains. 4- Flip-flop - movement of lipid molecules from one half of the bilayer to the other on one for one an exchange basis Objective 3 - mechanisms for the correct insertion of membrane proteins into the lipid bilayer Membrane proteins Membrane proteins carry out the distinctive functions of membranes which include enzymes, transporters, pumps, ion channels, receptors, and energy transducers. Protein content can vary from approximately 18% in myelin (nerve cell *insulator*) to 75% in the mitochondria. Normally membranes contain approximately 60% dry weight of protein. Mobility of proteins in bilayers:- Three modes of motion permitted, 1- conformational change 2- rotational 3- lateral 4-NO FLIP-FLOP Restraints on mobility -lipid mediated effects proteins tend to separate out into the fluid phase or cholesterol poor regions. - membrane protein associations - association with extra-membranous proteins (peripheral proteins) e.g. cytoskeleton membrane Thank you