Biological Membranes Lecture Notes PDF

BIOLOGICAL MEMBRANES DR. REHAM ABD ELMONSIF LECTURER OF CELL BIOLOGY I.OVERVIEW Membranes found around: The cell The nucleus ER Golgi bodies Vacuoles Mitochondria Selectively permeable Phospholipid bilayer 1- Inner leaflet 2- Outer leaflet II. COMPONENTS The memb...

BIOLOGICAL MEMBRANES DR. REHAM ABD ELMONSIF LECTURER OF CELL BIOLOGY I.OVERVIEW Membranes found around: The cell The nucleus ER Golgi bodies Vacuoles Mitochondria Selectively permeable Phospholipid bilayer 1- Inner leaflet 2- Outer leaflet II. COMPONENTS The membrane is a fluid mosaic of: A- Lipids B- Proteins C- Carbohydrates.  A. Lipids The most abundant macromolecules 40-80% Provide basic structure and framework of membrane Three types 1- Phospholipids 2- Cholesterol 3- Glycolipids 1. Phospholipids The most abundant lipids in membrane Polar Amphibathic (having both polar and non-polar regions of the molecule) 1. Phospholipids Arranged as a bilayer Compose of: 1- Phosphate group head (hydrophilic) 2- Fatty acid tails (hydrophobic)  1. Phospholipids Serine Ethanolamine Inositol Choline - Phosphatidylserine - Phosphatidyethanolamine - Phosphatidylinositol - Phosphatidylcholine - Sphengomyelin Lipid bilayer, liposmoes, micelles 1. Phospholipids Hydrophobic portion Long hydrocarbon chain (fatty acid tail) Saturated or (C-C) Unsaturated (C=C) Lipids asymmetrically distributed in membrane (Different lipids in each side of bilayer) Motion Flexion Rotation Lateral movement (fip-fop) Fluidity depends on lipid composition Saturated fatty acids All C-C bonds are single bonds Straight chain allows maximum interaction of fatty acid tails Make membrane less fluid Solid at room temperature "Bad fats" that clog arteries (animal fats) Unsaturated fatty acids Some C=C bond (double bonds) Bent chain keeping tails apart Make membrane more fluid Polyunsaturated fats have multiple double bonds and bends Liquid at room temperature "Good fats" which do not clog arteries (vegetable fats) Cholesterol Reduces membrane fluidity by reducing phospholipid movement Hinders solidification at low (room) temperatures  2.Cholesterol Can comprise up to 50% of Plasma membrane Hydrophilic OH groups toward surface Smaller than a phospholipid and less amphipathic 2.Cholesterol Reduces membrane fluidity by reducing phospholipid movement. Hinders solidification at low (room) temperatures. 3. Glycolipids Lipids attached to carbohydrates Always oriented toward outside Involved in cell-cell interactions Determine blood group antigen Receptor for toxins (cholera & tetanus) B. Proteins I- Membrane associated proteins: 1- Integral membrane proteins a- Transmembrane proteins The polypeptide chain traverses the lipid bilayer b- Lipid anchored proteins Attached to lipids in the bilayer 2- Peripheral membrane proteins More loosely associated with the membrane 2. Membrane protein function A- integral proteins 1- Cell adhesion molecules Cell-cell contact 2- Ion channel proteins 3- Transport proteins 4- Ligand receptors Enable cells to respond to hormones & signaling molecule 5- Lipid anchored proteins G proteins: Proteins involved in transmitting signals from outside a cell into the inside of the cell ( respond to hormones).  2. Membrane protein function B. Peripheral proteins 1- Regulate membrane shape 2- Stabilize membrane structure Many functions of membrane proteins Outside Plasma membrane Inside Transporter Enzyme Cell surface activity receptor Cell surface Cell adhesion Attachment to the identity marker cytoskeleton C- Carbohydrates 1- Play a key role in cell-cell recognition Ability of a cell to distinguish one cell from another (Antigens) 2- Important in organ & tissue development 3- Basis for rejection of foreign cells by immune system Characteristics of biological membranes Bilayer Selectively permeable Fluid mosaic Self sealing Asymmetric III. Structure A- Bilayer arrangement B- Asymmetry C- Fluid mosaic model D- Lipid rafts  A- Bilayer arrangement polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads  B- Asymmetry Outside: more phosphatidylcholine (red) Inside: more phosphatidylserine (green) Glycolipids: only on outside face (blue) B- Asymmetry C- Fluid mosaic model A mixed composition of phospholipids, glycolipids, sterols and proteins. Fluid because it can change shape but also because the phospholipids can change position in the same plane. Mosaic as the membrane has protein molecules embedded and attached to its surface. C- Fluid mosaic model Fluidity : lipid movements Mosaic : proteins are dispersed throughout the membrane – Icebergs (proteins) floating with the sea of lipids D- Lipid rafts Glycolipoprotein (microdomain) Membrane regions enriched in specialized lipids that function in cholesterol transport, endocytosis and signal transduction D- Lipid rafts Glycolipoprotein (microdomain) Membrane regions enriched in specialized lipids: Glycosphengolipids Polyphosphoinositol Caveolae(caveolin) Function in: Cholesterol transport, Endocytosis, Signal transduction