Membranes and Receptors Module - Session 1-1 - PDF

MEMBRANES AND RECEPTORS MODULE SESSION 1-1 THE MEMBRANE BILAYER Dr. Aveen Hassan Mustafa FKBMS-Chemical Pathologist 4th Sept. 2023 LEARNING OUTCOMES 1. List the main kinds of lipids and general properties of fatty acids. 2. Describe the properties of amphipathic molecules and explain the process...

MEMBRANES AND RECEPTORS MODULE SESSION 1-1 THE MEMBRANE BILAYER Dr. Aveen Hassan Mustafa FKBMS-Chemical Pathologist 4th Sept. 2023 LEARNING OUTCOMES 1. List the main kinds of lipids and general properties of fatty acids. 2. Describe the properties of amphipathic molecules and explain the process of formation of lipid bilayers 3. Discuss the influence of unsaturated fatty acids and cholesterol on membrane fluidity. 4. Explain the restrictions on protein movement in the membrane, including potential interactions with cytoskeletal elements General functions of biological LO -1 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. 3 Membrane Composition LO -1 • Generally membranes contain approximately: • 40% lipid, • 60% protein and Dry Weight • 1-10% carbohydrate • 20% Water (membrane is hydrated) 4 Membrane lipids LO -2 Membrane lipids are a group of compounds which form the double-layered surface of all cells Amphipathic molecules: i.e. they contain both hydrophilic and hydrophobic moieties. The three major classes of membrane lipids are • Phospholipid • Glycolipids • Cholesterol 5 6 Phospholipids LO -1 Phospholipids are the predominant lipid. E.g. Phosphatidylcholine Head groups - a range of polar head groups are employed choline, amines, amino acids, sugars Fatty acid chains o enormous variety, C16 and C18 most prevalent o unsaturated fatty acid side chains (double bonds) in the cis conformation introduce a kink in the chain which reduces phospholipid packing. 7 8 Plasmalogens (Non-classical phospholipids) LO -1 • Plasmalogens are a subclass of phospholipids, found in cell membranes in the nervous, immune & cardiovascular systems. • Sphingomyelin is a type of sphingolipid found in cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons, is the only phospholipid not based on glycerol. 9 Glycolipids LO -1 • Glycolipids are carbohydrate-containing molecules, possessing a hydrophobic, fatty acid tail that embeds them into membrane bilayers. • Cerebrosides - head group sugar monomers • Gangliosides - head group oligosaccharides (sugar multimers) 10 Cholesterol LO -1 • Cholesterol is a plasma membrane lipid. • It makes up 45% of total membrane lipid. • Distribution of different lipids is tissue specific and related to function. • Cholesterol stabilizes the plasma membrane by hydrogen bonding to the fatty acid chains. • This abolishes the endothermic phase transition of phospholipid bilayers. • Cholesterol reduces phospholipid packing, increasing membrane fluidity. • However, it also reduces phospholipid chain motion, 11 decreasing membrane fluidity Cholesterol 12 Lipid Bilayer LO -2 • Amphipathic molecules form one of two structures in water, micelles and bilayers. • Bilayer formation is spontaneous in water and is driven by the van der Waals forces between the hydrophobic tails. • Co-operative structure is stabilized by non-covalent forces; electrostatic and hydrogen bonding between hydrophilic moieties and interactions between hydrophilic groups and water. • Bilayers are the favored structure for phospholipids and glycolipids in aqueous media. • Pure lipid bilayers have a very low permeability to ions and most polar molecules. • • Watch this link https://www.youtube.com/watch?v=MTwUy3c5Etc 13 Different structures may be formed 14 Dynamics in lipid bilayers LO -2 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. Flip-flop - movement of lipid molecules from one half of the bilayer to the other on a one for one exchange basis. 3. Intra-chain motion - kink formation in the fatty acyl chains 4. Fast axial rotation. . 15 16 1- Fast lateral diffusion within the plane of the bilayer. 17 18 LO -3 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. 19 20 Membrane proteins LO -4 • 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. 21 Mobility of proteins in bilayers LO -4 • Three modes of motion permitted – conformational change, rotational and lateral NO FLIP-FLOP, as they have large hydrophilic moieties, and large amounts of energy would be required for them to pass through the hydrophobic region of the bilayer. 22 Restrictions on mobility: 1. lipid mediated effects - proteins tend to separate out into the fluid phase or cholesterol poor regions. 2. membrane protein associations 3. association with extra-membranous proteins (peripheral proteins) e.g. cytoskeleton. 23