BIOC 460, Summer 2011 Membranes Lipids 1 Membranes 1: Lipids and Lipid Bilayers Reading: Berg, Tymoczko & Stryer, 6th ed., Chapter 12, pp. 326-335 Problems: Chapter 12, p. 150, #9 This animation is excellent for its representation of dynamics of membrane lipids as well as the proteins embedded with the lipid bilayer and is highly recommended: http://multimedia.mcb.harvard.edu/anim_innerlife.html Excellent animations on lipids, membranes, and transport processes http://telstar.ote.cmu.edu/biology/animation/index.html Key Concepts • Major functions of lipids: energy storage, major membrane components – Other functions: signals, electron carriers, emulsifying agents.... • Membrane lipids (amphipathic) -- responsible for spontaneous formation of lipid bilayers – Glycerophospholipids: glycerol backbone + 2 fatty acyl "tails" in ester linkage + a polar "head group” (a phosphate ester of another alcohol like choline, ethanolamine, serine, inositol, etc.) – Sphingolipids: sphingosine backbone (1 "tail") + fatty acid chain in amide linkage (another "tail") + either carbohydrate (glycosidic bond to sphingosine) or phosphate ester of another alcohol like choline or ethanolamine (ester bond to sphingosine) • glycosphingolipids (cerebrosides, gangliosides) • phosphosphingolipids (sphingomyelins) – Cholesterol • Membrane fluidity (vital to membrane function) depends on lipid composition of bilayer. – fatty acid chainlength (more C atoms → more packing of tails, less fluidity) – fatty acid numbers of double bonds (fewer double bonds → more packing of tails, less fluidity) – cholesterol content ("buffers" fluidity)BIOC 460, Summer 2011 Membranes Lipids 2 Biological 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 bilayers instead of micelles. – Proteins carry out most of the specific functions. – carbohydrates (covalently attached to lipids = glycolipids, or to proteins = glycoproteins) - important in communication/recognition • noncovalent assembly (interactions between components) into a fluid 2-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 membranes Amphipathic nature of polar lipids • Polar hydrophilic “head” group and non-polar hydrophobic “tail”portion • Amphipathic lipids self-assemble forming micelles (detergents) or bilayers (phospholipids). • hydrophobic tails are buried, NOT exposed to H2O. • hydrophilic head groups in contact with H2O. • Detergents: single hydrophobic tails can form micelles • Membrane lipids: have 2 tails, spontaneously form bilayers. • The hydrophobic effect provides the major driving force for the formation of lipid bilayers.BIOC 460, Summer 2011 Membranes Lipids 3 Liposomes or Vesicles • Membranes curve and spontaneously form vesicles or liposomes • 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 • possible to incorporate proteins in membrane or inside vesicle interior Berg et al., Fig. 12-12 Membrane Functions 1) HIGHLY SELECTIVE PERMEABILITY BARRIERS regulate molecular & ionic compositions of cells and intracellular organelles a) channels and pumps (proteins that act as selective transport systems) b) electrical polarization of membrane (inside of plasma membrane negative, typically - 60 millivolts) (maintain different ionic concentrations on opposite sides of membrane) 2) INFORMATION PROCESSING - biological communication a) 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 impulses 3) ENERGY CONVERSION - ordered arrays of enzymes and other proteins, organization of reaction sequences a) photosynthesis (light energy → chemical bond energy): inner membranes of chloroplasts, and plasma membranes of some prokaryotes b) oxidative phosphorylation (oxidation of fuel molecules → chemical bond energy "stored" in ATP): inner membranes of mitochondria, and plasma membranes of prokaryotesBIOC 460, Summer 2011 Membranes Lipids 4 Lipid Components of Animal Cell Membranes • LIPIDS (definition): water-insoluble biomolecules that are highly soluble in organic solvents – Biological functions: • fuels (highly concentrated energy stores) • signaling molecules • membrane components • Membrane lipid functions: – bilayer structure → compartments/permeability barriers – provide environment for proteins to work – electrical insulation (e.g., myelin sheath on myelinated nerve fibers, but also maintenance of electrical potential in other cells) • Membrane lipid distribution: functional significance of all the differences not really understood – proportions of different lipids vary by • type of membrane (plasma membrane vs. mitochondrial membrane vs. nuclear membrane, etc.) • type of cell Membranes are Asymmetric! – inner vs. outer "leaflets" [layers of bilayer] -- different lipid compositions, different proteins or protein orientation – asymmetry maintained by extremely slow rate of rotation of components across membrane – "flip-flop" essentially doesn’t occur except when catalyzed by "flippases" (proteins involved in creating/maintaining lipid asymmetries across membrane) • Carbohydrate components: almost always on outer surface of membrane – Glycolipids: lipid with attached carbohydrate found only in outer leaflet of plasma membranes. – Glycoproteins: Carbohydrate component found only on outsides of cells, even when protein itself spans membrane. Berg et al., Fig. 12-30BIOC 460, Summer 2011 Membranes Lipids 5 Fatty Acids • Fatty acyl groups are components of membrane lipids, in ester or amide linkages. • longchain carboxylic acids, typically 14-24 C atoms • C16 and C18 most common (amphipathic) RCOO– with 0 - 4
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