Membrane structure


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Membrane structure

  1. 1. Membrane StructureFluid Mosaic Model, MembraneCharacteristic and Composition
  2. 2. Fluid Mosaic Model Model developed by Singer and Nicolson (1972) to understand membrane structure Fluid implies movement on membrane Mosaic implies that the membrane consists of many different molecules
  3. 3. Membrane Characteristics Bilayer Fluidity Asymmetry
  4. 4. Phospholipid Bilayer Bilayer = 2 layers Each layer is called a leaflet and composed of phospholipids
  5. 5. Phospholipid Structure
  6. 6. Phospholipid Bilayer Phospholipid arranged so that hydrophobic tails do not face water Water is on the intracellular and extracellular side Hydrophobic tails face inwards forming a hydrophobic core Hydrophilic heads face outwards
  7. 7. Membrane FluidityMembrane fluidity is affected by: Saturation of fatty acid  Double bonds bends fatty acid chains preventing phospholipids from tight packing Hydrophobic restrictions  Lateral movement: across same leaflet  Flip-flop movement is rare Cholesterol and temperature  (see later slide)
  8. 8. Asymmetry Each leaflet has a different composition Leaflet facing the intracellular side has different components compared to the extracellular side Restrictions in the flip-flop motion help to maintain this asymmetry
  9. 9. Membrane Composition Membrane Proteins Carbohydrates Cholesterol
  10. 10. Membrane Proteins Integral: embedded in the membrane and spans the entire width of the bilayer  Classified by type of transport: active or passive  Classified by direction of transport: Uniport, symport, antiport Peripheral: bound non-covalently to either surface of the membrane
  11. 11. Integral Proteins Function: transport Assist in movement of molecules into and out of cell
  12. 12. Type of Transport Passive transport:  Does not require energy  Movement of molecules from area of high to low concentration  2 types of passive protein transporters: channel or carrier Active transport:  Requires energy  Movement of molecules from area of low to high concentration  All protein pumps are active transporters
  13. 13. Channel Proteins move small charged molecules (e.g. ions) Charged particles need help crossing the hydrophobic core Particles transported have the opposite charge to that of the protein Acts like a tunnel
  14. 14. Carrier Proteins moves large uncharged molecules (e.g. glucose) Proteins change shape (conformational change) to allow molecules through Acts like a gate/door
  15. 15. Direction of Transport Uniport: movement of a single molecule in one direction  E.g. all channel proteins Symport: movement of 2 molecules in the same direction  E.g. Na/glucose symporter Antiport: movement of 2 molecules in opposite directions  E.g. Na/K pump
  16. 16. Direction of Transport
  17. 17. Antiport Animation
  18. 18. Peripheral Proteins Function  Extracellular side: communication  Intracellular side: structural support
  19. 19. Extracellular Peripheral Protein Receptor and recognition proteins Located on outer leaflet and surface Example: antigens, glycoproteins
  20. 20. Intracellular Peripheral Protein Cytoskeletal protein Located on inner membrane surface Attached to cytoskeleton of cell Immobilized (anchored) on membrane
  21. 21. Carbohydrates Glycoprotein = carbohydrate + protein Glycolipid = carbohydate + lipid (phospholipid) Extracellular side Function of cell surface carbohydrates:  identifies the cell (like a name) helping other cells recognize it  acts as a signal for communication
  22. 22. Cholesterol Low Temperature:  Cholesterol’s large molecular size interrupts the hydrophobic interactions of the phospholipid  Makes the membrane more flexible High temperatures  Phospholipids are more fluid  Nonpolar property of cholesterol stabilizes the interactions between the fatty acid chains of phospholipid
  23. 23. CholesterolClarification Both of cholesterol’s properties (large molecular size and nonpolar interactions) are present at both low and high temperatures However, certain properties play a more important role at certain temperatures (as explained on the previous slide)
  24. 24. Cholesterol How is having cholesterol on the cell membrane advantageous?
  25. 25. Phospholipid Bilayer
  26. 26. Phospholipid Bilayer