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Chapter 7 ~ Membrane Structure & Function Membrane structure, I Selective permeability – only some substances can cross Amphipathic – has both hydrophobic & hydrophilic regions Singer-Nicolson: fluid mosaic model Fluid structure w/ various proteins embedded Membrane structure, II Phospholipids – membrane fluidity Lipids move along membrane lateral May move in/out – rare – flipflop Unsaturated – kinked tails – increase fluidity Cholesterol – membrane stabilization “Mosaic” Structure – collection of different proteins – unique to each cell Integral proteins – transmembrane proteins – cross entire membrane Peripheral proteins – surface of membrane Membrane carbohydrates – cell to cell recognition Oligosaccharides (cell markers); glycolipids; glycoproteins Immune system, blood types Membrane structure, III Membrane protein function: Transport – channel, shuttle Enzymatic activity – active site exposed Signal transduction (receptor) – binds to chem messenger – sends message inside Intercellular joining – gap/tight junctions Cell-cell recognition ECM attachment – maintain cell shape and stability Membrane traffic Passive transport~ diffusion of a substance across a biological membrane – no energy Diffusion~ tendency of any molecule to spread out into available space Moves down its concentration gradient Osmosis~ the diffusion of water across a selectively permeable membrane Water balance Osmoregulation – control of water balance Hypertonic – higher concentration of solutes – cell is hypotonic – cell shrinks Hypotonic – lower concentration of solutes – cell is hypertonic – cell swells Isotonic – equal concentrations of solutes Cells with Walls: Turgid (very firm) - hypo Flaccid (limp) - iso Plasmolysis – plasma membrane pulls away from cell wall - hyper Water Balance cont. Water potential (Ψ) - movement of water from an area of high concen. to low concen. Ψ = ΨP + ΨS Water Potential = Pressure Potential + Solute Potential At atmospheric pressure - Ψp = 0 Water potential of pure water in an open beaker is zero (ψ = 0) Water will move from an area of high Ψ to an area of low Ψ Tonic refers to solute concen., potential refers to water concen. The Solute Potential of the Solution Ψs = – iCRT i = ionization constant (For sucrose this is 1.0 because sucrose does not ionize in water.) C = molar concentration R = pressure constant (R = 0.0831 liter bars/mole K) T = temperature in Kelvin (273 + °C) Specialized Transport Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient Channel proteins: ion channels, aquaporins, gated proteins, carrier proteins Active transport~ movement of a substance against its concentration gradient with the help of cellular energy Types of Active Transport Sodium-potassium pump – electrochemical gradient – nerve cells Cotransport – solute pumped out can do work as it diffuses back Exocytosis~ secretion of macromolecules by the fusion of transport vesicles with the plasma membrane Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane Phagocytosis Pinocytosis Receptor-mediated • Ligands – bind to receptor