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BIS 104 membrane concepts 05

BIS 104 membrane concepts 05 - Different cellular membranes...

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Biomembranes: Key Concepts* The eukaryotic cell is demarcated from the external environment by the plasma membrane and organized into membrane-limited internal compartments (organelles and vesicles). The total surface area of internal membranes far exceeds that of the plasma membrane. The phospholipid bilayer, the basic structural unit of all biomembrnes, is a two- dimensional lipid sheet with hydrophilic faces and a hydrophobic core, which is impermeable to water-soluble molecules and ions. Certain proteins present in biomembranes make them selectively permeable to water-soluble molecules and ions. The primary lipid components of biomembranes are phosphoglycerides, sphingolipids, and steroids. Most lipids and many proteins are laterally mobile in biomembranes.
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Unformatted text preview: Different cellular membranes vary in lipid composition. Phospholipids and sphingolipids are asymmetrically distributed in the two leaflets of the bilayer, whereas cholesterol is fairly evenly distributed in both leaflets. Natural biomembranes generally have a fluid-like consistency. In general, membrane fluidity is decreased by sphingolipids and cholesterol and increased by phosphoglycerides. The lipid composition of a membrane also influences its thickness and curvature. Lipid rafts are microdomains containing cholesterol, sphingolipids, and certain membrane proteins that form in the plane of the bilayer. These aggregates are sites for signaling across the plasma membrane. *From: Molecular Cell Biology , Lodish, et al. W. H. Freeman & Co. 2003...
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