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Unformatted text preview: Chapter 5: The Dynamic Cell Membrane 5.1: What is the structure of a biological membrane? Fluid mosaic model: molecular model of biological membranes consisting of:- Lipids: establish physical integrity of membrane and create an effective barrier to rapid passage of hydrophilic materials. Phospholipid bilayer: lipid lake in which a variety of proteins float. - Proteins: embedded in the phospholipids bilayer; move materials through the membrane and receive chemical signals from cells external environment. - Carbohydrates: attached to lipid or protein molecules; in plasma membranes, located on outside and protrude into the environment away from the cell. Crucial in recognizing specific molecules. I. Lipids constitute the bulk of a membrane A. Lipids in biological membranes are usually phospholipids: 1. Hydrophilic regions : phosphorous-containing headelectrically charged, associates with polar water. 2. Hydrophobic regions : long, nonpolar fatty acid tailsassociate with other nonpolar materials, do not dissolve in water or associate with hydrophilic substances. B. To coexist with water, phospholipids form a phospholipid bilayer : fatty acid tails interacting with eachother and polar heads facing outside aqueous environment. 1. Stabilizes membrane structure, but leaves it flexible. 2. Fatty acids make hydrophobic interior somewhat fluid permitting some molecules to move laterally within the plane of the membrane. C. Phospholipids can differ in fatty acid chain length, degree of fatty acid unsaturation, and polar phosphate groups present. D. 25% of lipid content may be cholesterol: important to membrane integrity; not hazardous; commonly situated next to an unsaturated fatty acid. E. Fluidity is affected by lipid composition and temperature: shorter-chain fatty acids, unsaturated fatty acids, and less cholesterol result in more fluid membranes. II. Membrane proteins are asymmetrically distributed A. Proteins are embedded in, or extend across the phospholipid bilayer: 1. Hydrophilic regions : amino acids with hydrophilic side chains; polar character; interact with water. Stick out into aqueous extracellular environment or cytoplasm. 2. Hydrophobic regions : amino acids with hydrophobic side chains; nonpolar character; interact with interior fatty acid chains. B. Proteins and lipids in a membrane are independent of each other and interact only non-covalently. (Polar protein ends can interact with polar lipid ends; nonpolar regions of both interact hydrohobically). C. Integral membrane proteins: hydrophobic domains; penetrate bilayer. Long hydrophobic -helical regions span the core of the bilayer; hydrophilic ends protrude aqueous environments on either side....
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This note was uploaded on 05/07/2008 for the course LIFESCI 2 taught by Professor Pires during the Spring '08 term at UCLA.
- Spring '08