3b B130_03b_Membrane_Transport_

3b B130_03b_Membrane_Transport_ - Unit 3b Membrane...

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Unit 3b Membrane Transport What determines what can cross a membrane? What drives movement of solutes? diffusion, osmosis, tonicity, turgor pressure for substances that can’t diffuse through bilayer: membrane transport proteins channels – chemical and electrical gradients, gating carriers – down gradient - facilitated diffusion against gradient - active transport ATP-driven – Na+/K+-ATPase coupled (secondary active transport) membrane proteins and gene expression 1 still not on midterm
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Membrane Transport: Basic Principles need to allow passage of certain substances in / out of cell gases, ions, nutrients / waste products lipid bilayers tend to block passage of polar (water-soluble) molecules substances can enter a cell by … passing directly through lipid bilayer being transported across bilayer by membrane proteins acting as carriers or channels being engulfed by the cell, avoiding passing through the membrane
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What molecules can pass through membranes? depends on polarity (charge), size 3 relative permeability O2, CO2, N2 H2O, urea, glycerol glucose, fructose Cl– , K+, Na+ O2, CO2 H2O glycerol, urea glucose Cl– K + Na+ high low hydrophobic molecules small, uncharged polar larger, uncharged, polar ions Fig 6.8 Based on what you know about their structure, what permeability would you predict for: proteins, amino acids, nucleic acids, fats, ethanol
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Diffusion 4 dissolved solutes (molecules / ions) solution are in constant, random motion solutes will spontaneously ‘spread out’ (↑ entropy) until concentrations on all regions are equal at that point – no NET flux
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Diffusion across a Semi-Permeable Membrane 5 Diffusion of two solutes Diffusion of one solute key terms: concentration gradient, potential energy, dynamic equilibrium
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Water also moves down its concentration gradient. 6 starting situation : more solute on right side of membrane, solute CANNOT cross Fig 6.15 outcomes: ‘concentration’ of water equal on both sides concentration of total solute equal on both sides as long as water allowed to cross water moves across membrane to attempt to equalize its concentration
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Osmosis diffusion of water across a semi-permeable membrane down its concentration gradient (toward a higher solute concentration) once ‘water concentration’ equal on both sides, no net movement of water water concentration depends on total concentration of osmotically active particles (solutes) all ions, molecules dissolved in fluid 7
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Osmosis and Cells water is constantly moving through cell membrane in both directions ideally, ‘osmotic tone’ (concentration of osmotically active
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This note was uploaded on 03/01/2012 for the course BIOL 130 taught by Professor Bols during the Spring '08 term at Waterloo.

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3b B130_03b_Membrane_Transport_ - Unit 3b Membrane...

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