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UNC-Chapel Hill BIOL 205 - Cellular Building Blocks- Lipids and Membranes

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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 2201.21.11Lecture 4 Cellular Building Blocks: Lipids and MembranesMembranes form many compartmentsin the cellBiological membranes are composed of a lipid bilayer and proteinsMembrane lipids are amphipathic molecules•Membrane lipids are amphipathic•Hydrophilic heads (polar) form hydrogen bonds with water•Hydrophobic tails (non-polar) are excluded by water moleculesThree classes of membrane lipidsPacking arrangements of lipid molecules in an aqueous environmentCone-shaped lipid molecules for micelles, cylinder-shaped lipids form bilayersPhospholipid bilayers spontaneously close to form a sealed compartmentThe membrane bilayer is a fluid•Lateral diffusion occurs rapidly within the plane of the membrane•Individual phospholipids may rotate axially•Flip-flopping from one side to the other is very rare as it is energetically unfavorableThe membrane bilayer is a fluid: FRAP1. A fluorescent probe is used to label membrane proteins2. The probe is destroyed in a small region using intense laser light3. Fluorescence microscopy is used to observe behavior of the unbleached probe•Fluorescence microscopy of single GFP-labeled membrane proteins•Diffusive movement within the plane of the lipid bilayerQuickTime™ and aYUV420 codec decompressorare needed to see this picture.The membrane bilayer is a fluid: single molecule imagingQuickTime™ and aMPEG-4 Video decompressorare needed to see this picture.The membrane bilayer is a fluid: laser tweezersBeads trapped in a laser may be used to exert pulling forces on membranesThe composition of a membrane regulates the degree of its fluidity •Membrane lipids with fatty acyl side chains that are saturated (no double bonds) pack tightly in the membrane and make it less fluid•Lipids that are unsaturated (1, 2, or 3 double bonds) pack loosely and make it more fluidThe composition of a membrane regulates the degree of its fluidity The presence of cholesterol in the membrane stiffens the bilayer making it more rigidCellular membranes are asymmetric1. All lipids are synthesized on the cytosolic surface of the ER2. Lipids in the outer leaflets are transported there by flippases3. Continuity between organelle lumen & extracellular spaceProteins are major components of biological membranes•Proteins compose ~50% of the membrane•~1 protein:50 lipid molecules•Membrane proteins perform many functionsMembrane proteins associate with the bilayer in different ways •Transmembrane proteins span the bilayer•Peripheral membrane proteins associate with one sideTransmembrane proteins usually span the bilayer using alpha-helicesSome membrane proteins use beta-sheets to cross the bilayer•Beta-sheets arranged in this cylindrical conformation are known as a “beta-barell”•Hydrophilic amino acid residues face towards the pore, hydrophobics face the bilayerThe cytoplasmic side of the membrane is called the cell cortex•Meshwork of transmembrane proteins and filaments (spectrin)•Mechanical support for the membrane and cell shapeThe extracellular surface of the membrane is coated with carbohydrateExtracellular glycoproteins perform numerous functions•Carbohydrate layer protects cells from chemical and mechanical damage•Different cell types present different combinations of glycoproteins and proteoglycan on their surface - molecular signature•Information in the carbohydrate layer aids in cell-cell recognition and communicationCells use different mechanisms to restrict membrane protein movementsA. By tethering to elements inside of the cell (cortex)B. By tethering to elements outside of the cellC. By interacting with proteins on the surface of another cellD. By diffusion barriers established within polarized


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UNC-Chapel Hill BIOL 205 - Cellular Building Blocks- Lipids and Membranes

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