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Unformatted text preview: Group: A6.5 Chapter 7: Membrane Structure and Function • Overview: Life at the Edge • The plasma membrane – Is the boundary that separates the living cell from its nonliving surroundings • The plasma membrane exhibits selective permeability – It allows some substances to cross it more easily than others • Cellular membranes are fluid mosaics of lipids and proteins • Phospholipids – Are the most abundant lipid in the plasma membrane – Are amphipathic, containing both hydrophobic and hydrophilic regions • The fluid mosaic model of membrane structure – States that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it • Membranes have been chemically analyzed – And found to be composed of proteins and lipids • Scientists studying the plasma membrane – Reasoned that it must be a phospholipid bilayer • The Davson-Danielli sandwich model of membrane structure – Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers – Was supported by electron microscope pictures of membranes • In 1972, Singer and Nicolson – Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer • Freeze-fracture studies of the plasma membrane – Supported the fluid mosaic model of membrane structure • Phospholipids in the plasma membrane – Can move within the bilayer • Proteins in the plasma membrane – Can drift within the bilayer • The type of hydrocarbon tails in phospholipids – Affects the fluidity of the plasma membrane • The steroid cholesterol – Has different effects on membrane fluidity at different temperatures • A membrane – Is a collage of different proteins embedded in the fluid matrix of the lipid bilayer • Integral proteins – Penetrate the hydrophobic core of the lipid bilayer – Are often transmembrane proteins, completely spanning the membrane • Peripheral proteins – Are appendages loosely bound to the surface of the membrane • An overview of six major functions of membrane proteins • Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.an energy source to actively pump substances across the membrane....
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- Spring '07