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Unformatted text preview: Chapter 7: Membrane Structure and Function The Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognition, a cells ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units o Some of these carbohydrates are covalently bonded to lipids, forming molecules called glycolipids o Most, h/e, are covalently bonded to proteins glycoproteins The carbohydrates on the external side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces The two lipid bilayers may differ in specific liquid composition, and each protein has directional orientation in the membrane When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the cytoplasmic layer of the plasma membrane o Therefore, molecules that start out on the inside face of the ER end up on the outside face of the plasma membrane The process starts with o 1) the synthesis of membrane proteins and lipids in the ER carbohydrates are added to the proteins, making them glycoproteins The carbohydrate portions may then be modified o 2) inside the Golgi apparatus, the glycoproteins undergo further carbohydrate modification, and lipids acquire carbohydrates, becoming glycolipids o 3) Transmembrane proteins, membrane glycolipids and secretory proteins are transported in vesicles to the plasma membrane o 4) There the vesicles fuse with the membrane, releasing secretory proteins from the cell Vesicle fusion positions the carbohydrates of membrane glycoproteins and glycolipids on the outside of the plasma membrane Thus, the asymmetrical distribution of proteins, lipids, and their associated carbohydrates in the plasma membrane is determined as the membrane is being built by the ER and Golgi apparatus 7.2 Membrane structure results in selective permeability The biological membrane is an exquisite example of a supra-molecular structure many molecules ordered into a higher level of organization with emergent properties beyond those of the individual molecules The fluid mosaic model helps explain how membranes regulate the cells molecular traffic The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules, such as hydrocarbons, carbon dioxide, & oxygen, can dissolve in the lipid bilayer of the membrane & cross it with ease, w/o the aide of membrane proteins However, the hydrophobic core of the membrane impedes the direct passage of ions and polar molecules, which are hydrophilic, through the membrane Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water, an extremely small polar molecule, does not cross very rapidly...
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This note was uploaded on 11/16/2008 for the course BILD BILD1 taught by Professor Bever during the Winter '08 term at UCSD.
- Winter '08