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Transcript
Membrane Structure & Function Functions of the Cell Membrane Isolates the cytoplasm from the external environment Regulates the flow of materials into and out of the cell Communicates with other cells The Fluid Mosaic Model Currently accepted model of the cell membrane Proposed by Singer and Nicolson in 1972 Phospholipid bilayer Hydrophilic “head” – exposed to the outside Hydrophobic “tail” – hides inside Membrane proteins are randomly dispersed in phospholipid bilayer Fluid Mosaic Model Fluidity of the Membrane The lipids and proteins can drift throughout the membrane Membrane is NOT stiff/rigid Cholesterol makes the membrane stronger by limiting the movement of phospholipids Membrane as a Mosaic Lipid bilayer has membrane proteins “stuck” in it Integral proteins Go through the membrane (both sides) Peripheral proteins attached to the surface of the membrane Selective Permeability of the Cell Membrane The cell membrane can “choose” what enters and exits a cell “Gatekeeper of the Cell” Passive Transport Def’n: Diffusion of a substance that does NOT require the input of energy by the cell 3 types of passive transport: Diffusion Osmosis Facilitated diffusion Diffusion Movement of molecules from high to low concentration until equilibrium is reached. Passive Transport= no energy required What substances may diffuse across membrane? Nonpolar (noncharged) molecules; small polar molecules Diffusion Each substance diffuses down its OWN concentration gradient and is unaffected by concentration gradients of other substances Diffusion Does all movement stop once equilibrium is reached?? NO!! Equal rates in all directions Osmosis Def’n: The passive transport of water across a selectively permeable membrane Hyper-, hypo-, iso- tonic RELATIVE TERMS!! Always referring to solute concentration Water moves from areas of lower concentration of solutes (hypotonic) to areas of higher solute concentration (hypertonic) Osmosis in Plant and Animal Cells Animal Cells: Plasmolysis Occurs when a cell is in a hypertonic solution • Water goes from cell into solution Cytolysis Occurs when a cell is in a hypotonic solution • Water goes from solution into cell Plant Cells: Turgid Flaccid Osmosis in Plant and Animal Cells