BIOC 460, Spring 2008LEC 18, Membranes 1 - Lipids and LipidBilayers 1Lecture 18Membranes 1: Lipids andLipid BilayersSubsequent 3 lectures:– Membrane Proteins– 2 lectures on Membrane TransportReading: Berg, Tymoczko & Stryer, 6th ed., Chapter 12,pp. 326-335Problems: Chapter 12, p. 150, #9Key Concepts• Major functions of lipids: energy storage, major membrane components– Other functions: signals, electron carriers, emulsifying agents....• Membrane lipids (amphipathic) -- responsible for spontaneousformation of lipid bilayers– Glycerophospholipids: glycerol backbone + 2 fatty acyl "tails" inester linkage + a polar "head group” (a phosphate ester of anotheralcohol like choline, ethanolamine, serine, inositol, etc.)– Sphingolipids: sphingosine backbone (1 "tail") + fatty acid chain inamide linkage (another "tail") + either carbohydrate (glycosidic bond tosphingosine) or phosphate ester of another alcohol like choline orethanolamine (ester bond to sphingosine)• glycosphingolipids (cerebrosides, gangliosides)• phosphosphingolipids (sphingomyelins)– Cholesterol• Membrane fluidity (vital to membrane function) depends on lipidcomposition of bilayer.– fatty acid chainlength (more C atoms → more packing of tails, lessfluidity)– fatty acid numbers of double bonds (fewer double bonds → morepacking of tails, less fluidity)– cholesterol content ("buffers" fluidity)BIOC 460, Spring 2008LEC 18, Membranes 1 - Lipids and LipidBilayers 2Learning Objectives• Terminology: micelle, lipid bilayer, amphipathic• List the biological roles and the molecular components of membranes.• With the structure of a lipid as an example, point out the features thatmake a molecule amphipathic.• Explain why amphipathic membrane lipids form self-sealing bilayers inaqueous environments, including the types of interactions stabilizingthe bilayer structure.• Write out the structure of a 16-carbon saturated fatty acid (i.e., nodouble bonds), and describe the general properties of the fatty acylcomponents of membrane lipids.• Be able to recognize the structures of phosphoglycerides,phosphosphingolipids, glycosphingolipids, and cholesterol. What typeof lipids are cerebrosides and gangliosides?• Briefly explain the consequences if an individual has a geneticdeficiency in any one specific enzyme involved in glycosphingolipiddegradation.• What bond in a glycerophospholipid is cleaved (hydrolyzed) byphospholipase A1? A2? C? D?Learning Objectives, continued• Discuss how living organisms regulate the fluidity of their membranes,including in your discussion the effects on fluidity of temperature, fattyacyl chainlength, and number of double bonds.• Discuss the concepts of lateral and transverse (“flip-flop”) diffusion ofmembrane lipids and proteins, and the asymmetric distribution ofmembrane components (especially carbohydrate portions) on theextracellular and intracellular sides of the bilayer.• Describe the permeability properties of lipid bilayers.BIOC 460, Spring 2008LEC 18, Membranes 1 - Lipids and LipidBilayers 3Biological Membranes• sheet-like structures, a few molecules thick, forming closed boundaries(self-sealing)– amphipathic lipids: polar "head" groups and nonpolar "tails”• With 2 hydrophobic "tails", amphipathic lipids form bilayersinstead of micelles.– Proteins carry out most of the specific functions.– carbohydrates (covalently attached to lipids = glycolipids, or toproteins = glycoproteins) - important in communication/recognition• noncovalent assembly (interactions between components) into a fluid2-dimensional solution– Proteins and lipids can diffuse rapidly in plane of membrane, but– Proteins and lipids do not rotate across the membrane (no "flip-flop" in orientation across membrane).– asymmetric arrangement• 2 sides (faces) different• biosynthesized that way• Components don’t "flip-flop" their orientation.• Membranes always synthesized by growth of preexisting membranesAmphipathic nature of membrane lipids• hydrophilic portion and hydrophobic portion– hydrophilic portion = "head"; hydrophobic chain(s) = "tails"• Consequence: Amphipathic lipids form micelles or bilayers, to bury theirhydrophobic tails so they're NOT exposed to H2O, but keep the hydrophilichead groups in contact with H2O.• Lipids with single hydrophobic tails can form micelles, but• Membrane lipids almost all have 2 tails, and thus form bilayers.– Bilayers curve around and seal edges → closed vesicles (liposomes).• The hydrophobic effect provides the major driving force for the formationof lipid bilayers.“slice” through a micelleBerg et al., Fig. 12-9“slice” through a bilayerBerg et al., Fig. 12-10BIOC 460, Spring 2008LEC 18, Membranes 1 - Lipids and LipidBilayers 4Liposomes• lipid vesicles, aqueous compartments enclosed by a lipid bilayer• experimental tools for studying membrane permeability• vehicles for delivery to cells of chemicals/drugs/DNA for gene therapy“slice” through a liposomeBerg et al., Fig. 12-12Membrane Functions1) HIGHLY SELECTIVE PERMEABILITY BARRIERS regulate molecular & ionic compositions of cells and intracellular organellesa) channels and pumps (proteins that act as selective transport systems)b) electrical polarization of membrane (inside of plasma membranenegative, typically - 60 millivolts)(maintain different ionic concentrations on opposite sides of membrane)2) INFORMATION PROCESSING - biological communicationa) signal reception by specific protein receptors (BINDING)b) transmission/transduction of signals (via protein conformational changes)sometimes generation of signals, chemical or electrical, e.g.,nerve impulses3) ENERGY CONVERSION - ordered arrays of enzymes and other proteins,organization of reaction sequencesa) photosynthesis (light energy → chemical bond energy): innermembranes of chloroplasts, and plasma membranes of someprokaryotesb) oxidative phosphorylation (oxidation of fuel molecules → chemical bondenergy "stored" in ATP): inner membranes of mitochondria, and plasmamembranes of prokaryotesBIOC 460, Spring 2008LEC 18, Membranes 1 - Lipids and LipidBilayers 5Lipid Components of Animal Cell Membranes• LIPIDS (definition): water-insoluble biomolecules that are highly solublein organic solvents– Biological functions:• fuels (highly concentrated energy stores)• signaling molecules• membrane components• Membrane lipid functions:– bilayer structure →
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