Passive Transport: Diffusion, Osmosis, & Facilitated Diffusion [This lecture is online] Abramsbiology.weebly.com Main idea of this lecture: How do molecules, big and small, get in OR out of a cell? 2 Main ways: Passive Transport (Does not require energy)– Day 1 AcCve Transport (Requires energy)– Day 2 Plasma membrane Controls what goes in or comes out of a cell -­‐ “Semi-­‐permeable” -­‐Composed of a phospholipid bilayer Fluid mosaic model -­‐Singer and Nicholson, 1972 Passive transport: ConcentraOon Gradients Molecules are always trying to spread out as much as possible (“entropy”– 2nd law of thermodynamics) Molecules always move from areas of high concentraOon to areas of low concentraOon -­‐This is known as diffusion -­‐If there is a membrane, the molecules must be able to pass through (i.e. “semi-­‐permeable”) Passive Transport: Diffusion Molecules moves from an area of high concentraCon to an area of low concentraCon. (They are moving down the concentraCon gradient) No energy is required Substances that enter via diffusion: Oxygen (O2), Water (H2O), Carbon Dioxide (CO2), Diffusion Diffusion example #1 In which direcOon will the solute move? 100 mM (millimolar) of Carbon Dioxide (CO2) 10 mM (millimolar) of Carbon Dioxide (CO2) Diffusion example #2 In which direcOon will the solute move? 50 mM (millimolar) of Oxygen (O2) 80 mM (millimolar) of Oxygen (O2) Diffusion example #3 In which direcOon will the solute move? 90 mM (millimolar) of Sodium 150 mM (millimolar) of glucose Diffusion example #4 In which direcOon will the solute move? 70 mM (millimolar) of Oxygen 60 mM (millimolar) of Oxygen Passive Transport: Osmosis Water molecules can freely pass through the plasma membrane. The movement (diffusion) of water across a membrane is known as osmosis ***Water moves across a membrane to balance out the amount of solute on each side –  Solute: Something dissolved is a liquid (for example, sugar, salt, iced tea mix) Osmosis Osmosis example #1 In which direcOon will the water move? 100 mM (millimolar) of glucose 10 mM (millimolar) of glucose Osmosis example #2 In which direcOon will the water move? 75 mM (millimolar) of glucose 70 mM (millimolar) of glucose Osmosis example #3 In which direcOon will the water move? 80 mM (millimolar) of glucose 95 mM (millimolar) of glucose Osmosis example #4 In which direcOon will the water move? 50 mM (millimolar) of glucose 50 mM (millimolar) of fructose Passive transport: Facilitated diffusion For various reasons (size, charge, polarity, etc.), some molecules can’t freely diffuse across the plasma membrane If these molecules are moving down a concentraOon gradient ([high] to [low]), they may use: -­‐Ion Channels -­‐Carrier Proteins (Both of these fall under the umbrella of “Facilitated diffusion) SIDENOTE: Peripheral proteins -­‐Signaling (communicaOon between cells; intracellular communicaOon) Integral proteins -­‐TransportaOon of molecules Passive transport: Facilitated Diffusion: Ion Channels Integral proteins that form a channel through the membrane and allow specific molecules to pass -­‐They may be open all the Ome or only open given specific sOmuli (voltage, ligand, mechanical) Ion channel Passive transport: Facilitated Diffusion: Carrier Proteins -­‐Molecule binds to carrier protein -­‐The binding of the molecules causes the protein to change shape (conformaOon), thereby exposing the other side of the membrane -­‐It’s like a revolving door This process is highly selecCve and reversible Carrier proteins Equilibrium The concentraOon of a solute inside a cell is equal to the concentraOon outside the cell MOLECULES CONTINUE TO MOVE ACROSS THE PLASMA MEMBRANE, HOWEVER THERE IS NO NET (OVERALL) CHANGE IN CONCENTRATION