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01 Cell Membrane.pdf

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Michelle French

PSL302: Lecture 01, by Prof. MacKay▯ Monday, Sept. 13, 2010 Cell Membrane Fluid Mosaic Model of Cell Membrane Overview -Phospholipid bilayer -Gases and lipid-soluble molecules diffuse thru readily -Impermeable to organic anions (i.e. proteins) -Permeability depends on molecular size, lipid solubility and charge Membrane Transport September 13, 2010 -Water diffuses thru aquaporin channels -Fluid Mosaic Model of Cell F5-2 Membrane Membrane Lipids Polar end (hydrophilic): glycerol + phosphate + choline/serine/etc. Non-polar end (hydrophobic): normally 1 fatty acid (FA) tail is saturated, other polyunsaturated -> Cholesterol inserted among FA chains 4 Membrane Fluidity: to remain intact and flexible - Mechanical properties of membrane determined by lipid composition - Essential for efficient carrier and channel protein fxn - Cholesterol stabilizes membrane & prevents diffusion of polar molecules across it. - Reduced by agin & poor diet (FA sourced from food) Membrane Transport September 13, 2010 Membrane Proteins - Structural support for cytoskeleton - Transmembrane (TM) channels for diffusion of selected molecules/ions - Selective transporters (carriers) of molecules across membrane - Enzymes/ion pumps for active transport Membrane-Spanning Proteins - Receptors for signaling molecules (part of endocrine system) Membrane-spanning proteins: diagram -Crossing membrane 7x common: one end of peptide chain on either side -Sections of protein crossing membrane usually are helical, and outside the membrane, sections are usually beta sheets F5-5 • 1 of 5 PSL302: Lecture 01, by Prof. MacKay▯ Monday, Sept. 13, 2010 Glycocalyx - Matrix of long carb filaments that fill extracellular space (ECS, or F for fluid), slowing down diffusion of molecules into cell (cell’s “fur coat”) - Anchored to membrane proteins (glycoproteins) or lipids (glycolipids) - Contain hyaluronic acid & chondroitin sulfate - Net -ve charge: attracts & sequesters Ca 2+on carb filament - Important for cell identification, triggering immune responses, cell adhesion (to surfaces & each other) Membrane Permeability Membrane Transport September 13, 2010 - If a substance an cross the membrane by any means, the membrane is permeable to that substance, e.g. Gases - Polar molecules and ions need the help of proteins (channels or carriers) to cross. Channels -4-5 protein subunits s.t. central pore created thru membrane -“Pore loops” of protein molecules dangling inside channel = selectivity filter: only specific molecules can physically pass thru the pore Aquaporins (membrane-spanning proteins) - Family of channels spec. for water & small non-ionic molecules, ex. glycerol, urea - Specificity of ‘pore’ based on size exclusion - Narrow part of pore diameter = water molecule’s diameter = ~4Å 14 - Diffusion of water 3-5 fold slower than bulk water diffusion (Membrane Transportround September 13, 2010 otherwise undisturbed in a beaker) Gated Channels - Membrane channels generally not kept perpetually open - Protein components switch btwn 2 shapes: one creates open pore, other blocks pore - Factors determining channel shape: 1) binding of chemical agent, or 2) voltage across membrane - Channel open only when no net charge across membrane (depolarized) - S4 electrostatically attracted to interior side of membrane (-vely charged) = channel closed w/ net -ve charge across membrane (polarized) Active Transport - Mech to move selected molecules across membrane against concentration gradient - Requires energy from ATP hydrolysis to change protein conformation 1) Substrate binds to protein carrier 2) ATP attaches on: conformation changes, occluding substrate to binding site 3) ATP hydrolyzed (Pi attached): substrate passes thru using hydrolysis energy 18 4) Pi falls off: protein returns to original conformation, releasing substrate 2 of 5 Membrane Transport September 13, 2010 PSL302: Lecture 01, by Prof. MacKay▯ Monday, Sept. 13, 2010 ATPases Ex. Na -K pump, CSodium Pump+ pump, H pump (H -K ATPase) in stomach Sodium Pump + Na+ + K+ ecf K occlusionATP attaches to pump: cytoplasmic Na enters pore, triggering...
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