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Chapter 7 Membrane structure and function Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein Transmembrane proteins Cytoplasm Filaments of cytoskeleton 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model • 2nd Law of Thermodynamics governs biological systems – universe tends towards disorder (entropy) Diffusion movement from HIGH LOW concentration Passive Transport: DIFFUSION • PASSIVE – Requires NO ENERGY • Automatic due to kinetic energy of molecules • Moves DOWN CONCENTRATION GRADIENT from [HIGH] → [LOW] until reaches equilibrium • Ex: Oxygen/CO2 cross capillary cell membranes FACILITATED DIFFUSION with CARRIER PROTEINS • PASSIVE- Requires NO ENERGY Trans-membrane proteins assist in movement Grab molecule, change shape, flip to other side Moves from [HIGH] → [LOW] facilitated = with help open channel = fast transport HIGH LOW “The Bouncer” FACILITATED DIFFUSION with ION CHANNELS • transmembrane proteins form “tunnels” across membrane Moves from [HIGH] → [LOW] • Moves charged ions (Na+ , K+, Ca++ Cl-) past hydrophobic tails in center • Can be “gated” or not Gates can open/close in response to electrical/chemical signals The Special Case of Water Movement of water across the cell membrane FACILITATED DIFFUSION with AQUAPORINS • OSMOSIS= Diffusion of water across a semipermeable membrane AQUAPORIN proteins move POLAR WATER molecules past phobic tails [HIGH] → [Low] Concentration of water • Direction of osmosis is determined by comparing total solute concentrations – Hypertonic - more solute, less water – Hypotonic - less solute, more water – Isotonic - equal solute, equal water water hypotonic hypertonic net movement of water 1 Managing water balance • Hypotonic Surrounding – a cell in fresh water (low solute) – high concentration of water around cell • problem: cell gains water, swells & can burst KABOOM! • example: Paramecium – ex: water continually enters Paramecium cell • solution: contractile vacuole ATP – pumps water out of cell – ATP – plant cells No problem, here • turgid = full • cell wall protects from bursting freshwater 2 Managing water balance • Hypertonic Surrounding – a cell in salt water (high Solute)_ – low concentration of water around cell I’m shrinking, I’m shrinking! • problem: cell loses water & can die • example: shellfish • solution: take up water or pump out salt – plant cells • plasmolysis = wilt • can recover I will survive! saltwater 3 Managing water balance • Isotonic With Surrounding That’s perfect! – animal cell immersed in mild salt solution (equal salt solution) – no difference in concentration of water between cell & environment • problem: none – no net movement of water » flows across membrane equally, in both directions – cell in equilibrium – volume of cell is stable • example: blood cells in blood plasma I could be better… – slightly salty IV solution in hospital balanced Practice (not in notes) - Do you understand Osmosis… .05 M .03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell Managing water balance • Cell survival depends on balancing water uptake & loss freshwater balanced saltwater If there is a concentration difference on opposite sides of a membrane and solute can’t move, water will Hypotonic Environment Isotonic Environment Hypertonic Environment [solute] outside ‹ inside [solute] outside = inside [solute] outside > inside Net movement of water into cell Net movement of water is equal Net movement of water out Animal cells: swell & burst = CYTOLYSIS No change in size Animal cells: shrink = CRENATION Plant cells increase TURGOR PRESSURE Plant cells: can’t shrink due to cell wall PLASMOLYSIS Link Osmosis is just diffusion of water • Water is very important to life, so we talk about water separately • Diffusion of water from HIGH concentration of water to LOW concentration of water – across a semi-permeable membrane Active transport • Many models & mechanisms • Requires energy ATP ATP antiport symport PUMPS • Can move against concentration gradient [low] → [high] • Used to create electrochemical gradients across cell membranes PROTON PUMP • Main electrogenic pump in PLANTS • ATP provides energy to pump H+ ions across a membrane • Stored H+ = potential energy to do work EX: COTRANSPORT (see below) ATP PRODUCTION during cellular respiration/photosynthesis SODIUM-POTASSIUM (Na+-K+) PUMP • Moves 3 Na+ ions in and 2 K+ ions out Main electro-genic pump in ANIMALS • EX: Na+-K+ pump sets up membrane potential Nerve signal results when Na+ and K+ exchange places Then pump resets membrane for next signal CO - TRANSPORT Na+-K+/ Proton pumps use ATP to create concentration gradient Movement of substance is linked to return of Na+/H+ as it flows back down its concentration gradient EX: sucrose is linked to H+ transport TYPES OF ACTIVE TRANSPORTrequires energy ENDOCYTOSIS • cell membrane engulfs substance brings it into cell in a VESICLE • PHAGOCYTOSIS- “phage” = cell eating • large molecules/ whole cells • PINOCYTOSIS- “pino” = cell drinking • small molecules, fluids • Ex: White blood cell eating bacteria RECEPTOR MEDIATED ENDOCYTOSIS • Substances (=LILGANDS) bind to specific RECEPTORS in membrane • Vesicle forms from area with receptors • Often clustered in coated pits • EX: uptake of LDL-cholesterol carrier requires receptor on cell surface Endocytosis - Summary phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by molecular signal EXOCYTOSIS • VESICLES fuse with cell membrane and release substances outside cell • Ex; Golgi export