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Unformatted text preview: Chapter 5 What Is the Structure of a Biological Membrane? Cell membranes are bilayered, dynamic structures that: Perform vital physiological roles Form boundaries between cells and their environments Regulate movement of molecules into and out of cells Lipids, proteins, and carbohydrates in various combinations make these tasks possible. The lipid portion of a cell membrane is a barrier for water-soluble molecules. The general design is called the fluid mosaic model The phospholipid bilayer is like a lake in which a variety of proteins float. Membrane proteins embedded in phospholipid bilayer Carbohydrates attach to lipid or protein molecules on the membrane. Membrane core is hydrophobic region Exterior and interior are hydrophilic regions Membranes vary in lipid composition Phospholipids vary fatty acid chain length, degree of saturation, phosphate groups Membranes may be up to 25% cholesterol Cholesterol concentration varies to increase or decrease fluidity Fluidity also depends on temperature and the degree of saturation of fatty acids Membranes of cells that live at low temperatures tend to be high in unsaturated (kinked) and short-chain fatty acids to maintain optimal fluidity. All biological membranes contain proteins Protein to phospholipid ratio varies depending on membrane function Many membrane proteins have hydrophilic and hydrophobic regions (or domains) Membrane proteins and lipids are independent only interact non covalently Both lipids and proteins are free to move around laterally , but do not flip-flop Integral membrane proteins have hydrophobic regions that penetrate or entirely cross phospholipid bilayer Transmembrane proteins have a specific orientation, showing different faces on either side of membrane, each with different properties Peripheral membrane proteins lack hydrophobic regions and are not embedded in bilayer Some membrane proteins can move freely within bilayer Others are anchored to a specific region Some can be anchored by cytoskeleton elements, or lipid rafts lipids in semisolid state Membranes are dynamic and are constantly forming, transforming, fusing, and breaking down How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? Some cells have carbohydrates on the external surface Glycolipid carbohydrate bound to lipid Glycoprotein c arbo-hydrates bound to protein Plasma membrane glycoproteins enable cells to be recognized by other cells and proteins....
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- Spring '08