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Membrane Structure & Function RAVEN & JOHNSON CHAPTER 5 CAMPBELL CHAPTER 8 Membrane Structure Lipids (phospholipids) & proteins They are amphipathic – both hydrophobic & hydrophilic Fluid-mosaic model (Singer & Nicolson, 1972) Fluidity Held by weak hydrophobic interactions Movement is lateral Fluidity Membrane must be fluid to work properly Cholesterol is wedged into phospholipid tails – restrains movement in warm temp (less fluid), but also lowers solidifying temp (more fluid) Fluidity Mosaic Embedded proteins (over 50 types) determine specific function of the membrane Integral proteins Transmembrane (-helical amino acids) & hydrophilic regions Hydrophobic Mosaic Peripheral proteins On the surface of membrane Protein Functions Cell Recognition Crucial Determined by surface molecules (carbohydrates) on plasma membrane Cell Recognition Oligosaccharides (polysaccharide fewer than 15 units) vary for different types of cells Used as markers to distinguish Example – A, B, AB, O Molecular Movement Hydrophobic molecules move across easily Hydrocarbons, CO2, O2 Hydrophilic have trouble moving through hydrophobic core (ions, polar molecules, water, glucose, sugars) Need transport proteins (facilitated diffusion) Molecular Movement Proteins are specific to molecule it transports Function: move molecules across the membrane that would not be able to do so otherwise Two types: Molecular Movement Molecular Movement 1) Channel Proteins Water or small ions If water, called aquaporins If needs electrical or chemical stimulus to open & close, called gated channel Molecular Movement 2) Other carrier proteins Some transport proteins change shape to transport molecules Molecular Movement Review: Passive transport (diffusion) Active transport Concentration gradient Equilibrium Osmosis Hypertonic, Hypotonic, Isotonic ( & what happens to the cell) Molecular Movement Molecular Movement Molecular Movement Animal cells need isotonic environment If not, cells must adapt for osmoregulation (water balance) Ex – Paramecium’s contractile vacuole Active Transport Against concentration gradient – requires energy (ATP) Ability to have cell with concentrations very different from surroundings Ex) Animal cell has much K+ and little Na+ compared to surroundings Active Transport Accomplished by membrane proteins ATP transfers P to transport protein, protein changes shape, molecule moves in or out Ex) Sodium-Potassium pump One ATP pumps three Na+ out and two K+ in Active Transport Cotransport Coupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient Exocytosis & Endocytosis For macromolecules Exocytosis – cell secretes macromolecules by fusion of vesicles with plasma membrane Exocytosis & Endocytosis Endocytosis – cells takes in macromolecules by forming new vesicles Three types: Phagocytosis (particles) Pinocytosis (any liquid) Receptor-mediated endocytosis (specific) Exocytosis & Endocytosis Exocytosis & Endocytosis Receptor-mediated endocytosis Proteins embedded in membrane – specific receptor sites (area where they are – coated pits) Extracellular substances (ligands) bind to sites – triggers vesicle formation Process enables cell to acquire bulk quantities of specific substances (cholesterol) Review Review Website (http://www.wiley.com/college/pratt/0471393878 /student/animations/membrane_transport/index. html)