Ch 6

• At the end of this lecture topic, you should be able to

  • List the components of the plasma membrane and explain the role each component plays
  • Explain why the properties of phospholipids cause them to form a bilayer
  • Explain how membrane fluidity is influenced by temperature and membrane composition
  • Describe how proteins and carbohydrates are spatially arranged in cell membranes
  • Distinguish between integral and peripheral membrane proteins
  • Describe the two ways carbohydrates can be attached to the membrane
  • Explain why the membrane is permeable to some substances but not to others
  • Determine how a molecule will move across the cell membrane based on its properties (size, polarity, and charge)
  • Describe and distinguish between passive and active transport
  • Describe and distinguish between simple diffusion and facilitated diffusion
  • Describe the two classes of proteins that may be involved in facilitated diffusion
  • Describe and distinguish between diffusion and osmosis
  • Distinguish between hypertonic, hypotonic, and isotonic solutions, and describe their effects on cells with and without cell walls
  • Define membrane potential and explain how it affects the movement of ions in and out of the cell
  • Describe the ways in which active transport can be powered
  • Describe and distinguish between primary and secondary active transport
  • Describe the mechanism of function of proton pumps, sodium-potassium pumps, and cotransporters. Identify which of these transporters help to establish membrane potential
  • Define and distinguish between exocytosis and endocytosis
  • Describe and distinguish between phagocytosis, pinocytosis, and receptor-mediated endocytosis

  
• The information covered in these lectures is based on your textbook: Biology: Exploring the Diversity of Life Chapter 4, Sections 4.1 through 4.6.
• For additional supplementary reading on these topics, you can consult the following websites:
• Websites


  • https://openstax.org/books/biology-2e/pages/5-1-components-and-structure
  • https://openstax.org/books/biology-2e/pages/5-2-passive-transport
  • https://openstax.org/books/biology-2e/pages/5-3-active-transport
  • https://openstax.org/books/biology-2e/pages/5-4-bulk-transport
• Be sure to complete Topic 6 Reading Activity, Part 1 (available on Mêskanâs).
• All membranes contain


  • Phospholipids
  • Proteins
  • Carbohydrates
  • Sterols

  
• Amphipathic


  Phospholipids form a bilayer that faces H2O on both sides
• As temperature increases


  Membrane fluidity increases
• As temperature decreases


  Membrane fluidity decreases
• Unsaturated fatty acids


  More fluid than saturated ones
• Double bonds in fatty acids increase membrane fluidity
• Some cells can alter fatty acid composition in response to temperature changes
• Maintaining membrane fluidity


  1. Add double bonds to fatty acids at lower temp

  2. Catalyzed by desaturase enzyme
• Sterols


  Regulate fluidity in animal membranes
• At low temperature


  Sterols prevent close packing of phospholipids, decreasing solidification
• At high temperature


  Sterols restrain movement in membrane, decreasing disruption

  
• Integral membrane proteins


  Pass through the lipid bilayer, with hydrophobic regions interacting with the membrane interior and hydrophilic regions interacting with the surface

  
• Peripheral membrane proteins


  Attach to the surface of the membrane, binding to phospholipids or integral proteins through hydrogen bonds or ionic bonds

  
• Carbohydrates


  Short, branched chains of sugar monomers, 15 or fewer, with a mixture of different monomers
• Glycoprotein


  Carbohydrate attached to a protein
• Glycolipid


  Carbohydrate attached to a lipid
• Cell-cell recognition


  Function of carbohydrates on the cell surface
• Examples of cell-cell recognition


  • Immune system detecting pathogens
  • Human blood types
• Membrane asymmetry


  • Membrane layers have different compositions
  • Membrane components have a specific orientation
  • Outside face has carbohydrates attached to proteins that anchor the membrane to the cytoskeleton
  • Inside face has proteins that anchor the membrane to the cytoskeleton

  
• Passive transport


  Movement across the membrane that does not require additional energy input
• Active transport


  Movement across the membrane that requires additional energy input
• Diffusion


  The tendency of molecules to move down a concentration gradient, from high to low concentration, releasing energy

  
• Simple diffusion


  Diffusion of small, non-polar molecules across the lipid bilayer

  
• Small, non-polar molecules can move through the hydrophobic core of the membrane between phospholipids

  
• Large, polar, and/or charged molecules require membrane transport proteins for diffusion across the membrane</b>

  
• Facilitated diffusion


  Diffusion of substances aided by membrane transport proteins, allowing diffusion of molecules that can't cross the membrane on their own

  
• Proteins used in facilitated diffusion

  • Channel proteins that form specific channels
  • Carrier proteins that change shape to carry molecules across

  
• Osmosis


  Diffusion of water across a selectively permeable membrane

  
• Comparison of solutions


  • Hypertonic solution: Contains higher solute concentration
  • Hypotonic solution: Contains lower solute concentration
  • Isotonic solution: Equal solute concentrations
• Water movement


  Moves from hypotonic to hypertonic solution
• No net water movement


  Between isotonic solutions
• Organisms with cell walls respond differently to osmosis than those without

  
• In a hypotonic solution

  Water moves into the cell, causing it to swell but not burst due to the cell wall
• In an isotonic solution


  No pressure is exerted on the cell wall, causing the cell to be flaccid
• In a hypertonic solution


  Water moves out of the cell, causing the cell membrane to separate from the cell wall (plasmolysis)