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Chapter 6

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CAS BI 108
Francis Monette

Savan Shah CH 110 Outline Chapter 6 Chapter 6: Cell Membranes I. What are the Passive Processes of Membrane Transport? - Selective Permeability Allows the membrane to determine what substances enter or leave cell or membrane • Passive Transport  Do not require the input of chemical energy • Active Transport  Require the input of chemical energy • Concentration Gradient  Difference between its concentration on one side of membrane and its concentration on the other • Diffusion  Process of random movement toward state of equilibrium oFactors that affect diffusion:  Diameter of molecules  Smaller molecules diffuse faster  Temperature  Higher temperatures lead to faster diffusion (more energy)  Concentration Gradient  The change in solute concentration with distance in a given direction (greater concentration gradient, more rapid diffusion) oUsefulness of diffusion as transport mechanism declines greatly as distances become greater (greater distance, takes longer to diffuse) oSolution without barriers  Solutes diffuse at rates determined by temperature, physical properties, and concentration gradients oMolecules to which membrane is permeable, diffuse from one compartment to other until concentrations are equal on both sides oSimple Diffusion  Small molecules pass through phospholipid bilayer of membrane  More lipid-soluble  More rapidly it diffuses through membrane bilayer  Polar molecules do not pass readily through a membrane  Not very soluble in hydrophobic interior  Substance form many hydrogen bonds with water and ions oOsmosis  Water molecules pass through specialized channels in membranes by diffusion process (passive process)  Higher the total solute concentration, the lower the concentration of water molecules  Hypertonic  Higher solute concentration outside cell (Shrivels)  Isotonic  Solutions have equal solute concentrations  Hypotonic  Lower solute concentration than the other solution (Swells)  Water moves from hypotonic solution across membrane to hypertonic solution oTurgor Pressure  Pressure within a cell (Keeps plant rigid) • Channel Proteins  Integral membrane proteins that form channels across membrane through which certain substances can pass • Carrier Proteins  Bind substances and speed up their diffusion through phospholipid • Facilitated Diffusion  Substances diffuse according to their concentration gradients • Ion Channels  Movement of ions across membranes oGated Channels  Opens when stimulus causes change in 3D shape of channel oStimulus is binding of chemical signal  Ligand oLigand-Gated Channels Savan Shah CH 110 Outline Chapter 6 oVoltage-Gated Channels  Stimulated to open or close by change in voltage • Specificity of Ion Channels oBoth Na and K are attracted to water molecules  Surrounded by water “shells”, held by attraction of their positive charges to the negatively charged oxygen atoms on water molecules oPotassium channel contains highly polar oxygen atoms at its opening + oWhen K ion approaches opening, it more strongly attached to oxygen atoms those of water molecules in its shell  Sheds its water shell, passes through the channel oSmaller Na ion is kept a bit more distant from oxygen atoms at opening of channel because extra water molecules fit between ion and oxygen atoms at opening
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