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Unformatted text preview: Chapter 10: Membrane Structure The Lipid Bilayer Provides structure for all cell membranes Phosphoglycerides, sphingolipids, and sterols are the major lipids in cells Constitute 50% of the mass of most animal membranes They are amphiphilic; meaning they have both hydrophilic(polar end) and a hydrophobic(nonpolar end) Three Types: o Phospholipids Most abundant membrane lipids, have a polar head group, and 2 nonpolar tails 1 tail usually has a cis double bond, creating a kink. o Phosphoglycerides Main phospholipid in most animal cell membranes 3 carbon glycerol backbone; 2 longchain fatty acids are ester bonded to C-atoms o Sphingomylein Built from sphingosine rather than glycerol In addition to phospholipids, lipid bilayers contain cholesterol & glycolipids Phospholipids spontaneously form bilayers Caused by shape and amphiphilic nature Sponatensouly aggregate into micelles or bilayers because The lipid bilayer is a 2D fluid readily exchange places with their neighbors within a leaflet Fluidity of a lipid bilayer depends on its composition depends on composition and temperature shorter chain length reduces the tendency of the hydrocarbon tails to interact w/ each other cis-DB produce kinks in the chains that make them more difficult to pack together Cholesterol: modulates the properties of lipid bilayers o I t enhances the permeability properties of the bilayer o Inserts itself into the bilayer with its OH group close to the polar head so that its rigid steroid rings interact with the hydrocarbon chains and partly immobilize it o Decreases the mobility of few CH2 groups of the chain and makes the bilayer less deformable and therefore decreases the permeability of the layer to water-soluble molecules o Also prevents the chains from coming together and crystallizing Despite their fluidity, bilayers can form domains of different compositions Different lipids can come together transiently creating a dynamic patchwork of different domains Special domains called lipid rafts have been debated by scientists for a while Lipid D roplets are surrdouned by a phospholipid monolayer Most cells store an excess of lipids in lipid droplets, from where they can be retrieved as building blocks from membrane synthesis or as a food source Fat cells, adipocytes, are speciailized for lipid storage and contain vast #s of lipid droplets They store neutral lipids such as t riaglycerides and cholesterol esters They are unique because they are surrounded by a single monolayer of phospholipids, which contains a large variety of proteins They form rapidly when cells are exposed to high concentrations of fatty acids from discrete regions of the ER where many enzymes of lipid metabolism are concentrated....
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- Spring '08