notes 2 - CELLMEMBRANE MODELS(plasmamembrane...

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CELL  MEMBRANE  MODELS  (plasma membrane): All cells have a thin outer covering Plasma membrane or cell membrane Early models inferred the structure from chemical and permeability properties of the membrane Fluid-Mosaic Model 2 layers of phospholipids as before, but now exposed proteins in a mosaic patterns, not a  continuous layer pores bounded by protein as before fluidity: proteins and phospholipids can move (dynamic structure), evidence from hybrid cells Cell Coats:  (not a separate entity from the cell membrane) Composition: glycocalyx – composed of glycolipids, glycoproteins
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Function: cell recognition Why do cells have a membrane? Structural – holds the cell together Function – important functional roles Functional Properties: Maintain concentrations of molecules and ions (dynamic barrier) Create free energy gradients Orient molecules into functional complexes Membrane Permeability: size and shape of molecules solubility in lipids  o non-polar and lipids pass right through o receptors of proteins are on the outer surface net electric charge chemical properties ------But water moves freely across the membrane------ important to study the movement of water ** water is the universal solvent in biological systems Cystic Fibrosis: Lacks Cl- channels in cell membrane Based on faulty protein Symptoms: salty skin, mucus build up in lungs --  Simple problem of cell membrane permeability No cure Key to understanding CF and treatment is diffusion Diffusion:  Movement of small molecules as a consequence of thermal agitation   (thermal energy) physical property of matter seeming contradiction: movement of individual molecules randomly, but the net effect is non- random movement from a region of high free energy to a region of low free energy (or down a free  energy gradient) Free energy = force of movement (Δ G =  Δ H – T Δ S) Biological Systems: (with membranes)
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Osmosis: net movement of water across a semi-permeable membrane (ie permeable to water, but  not to solutes) Terminology : Hypertonic: more concentrated Isotonic: equal concentration Hypotonic: less concentrated A single-celled freshwater organism, such as a paramecium, is transferred to salt water. What is likely to  happen? The cell shrinks. Salt water I more concentrated; it will leave the paramecium and the cell will shrink.
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