STPM Form 6 Biology Cell membrane

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STPM Form 6 Biology Cell membrane

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STPM Form 6 Biology Cell membrane

  1. 1. The Cell MembraneAP Biology
  2. 2. Overview  Cell membrane separates living cell from nonliving surroundings  thin barrier = 8nm thick  Controls traffic in & out of the cell  selectively permeable  allows some substances to cross more easily than others  hydrophobic vs hydrophilic  Made of phospholipids, proteins & other macromoleculesAP Biology
  3. 3. Phosphate Phospholipids  Fatty acid tails  hydrophobic  Phosphate group head Fatty acid  hydrophilic  Arranged as a bilayer Aaaah, one of those structure–function examplesAP Biology
  4. 4. Phospholipid bilayer polarhydrophilic heads nonpolarhydrophobic tails polarhydrophilic headsAP Biology
  5. 5. More than lipids…  In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model!AP Biology
  6. 6. Membrane is a collage of proteins & other moleculesembedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid GlycolipidPhospholipids Cholesterol Transmembrane proteins Peripheral protein Cytoplasm Filaments of cytoskeleton AP Biology
  7. 7. Membrane fat composition varies  Fat composition affects flexibility  membrane must be fluid & flexible  about as fluid as thick salad oil  % unsaturated fatty acids in phospholipids  keep membrane less viscous  cold-adapted organisms, like winter wheat  increase % in autumn  cholesterol in membraneAP Biology
  8. 8. Membrane Proteins  Proteins determine membrane’s specific functions  cell membrane & organelle membranes each have unique collections of proteins  Membrane proteins:  peripheral proteins  loosely bound to surface of membrane  cell surface identity marker (antigens)  integral proteins  penetrate lipid bilayer, usually across whole membrane  transmembrane protein  transport proteins  channels, permeases (pumps)AP Biology
  9. 9. Fluid Mosaic ModelAP Biology
  10. 10. AP Biology
  11. 11. Membrane FluidityMore kinks1. More difficult to pack phospholipid together – membrane staysfluid at lower temp(Bacteria, yeast adjust the fatty acid composition according totemp, to maintain membrane fluidity)2. The kinks shorten the length of hydrocarbon chains, so that themembrane is thinner.AP Biology
  12. 12. 1. Lateral movement in the plane of the lipid layer 2. Rotational movement around the longitudinal axis of the molecule 3.Transversion or “flip-flop” movement of phospholipids from one layer to the other. This movement is rare and requires energy.AP Biology
  13. 13. Cholesterol in Plasma Mambrane Unique to plasma membrane Stabilize membraneAP Biology
  14. 14. AP Biology
  15. 15. The stereochemistry has a profound effect on the lateral packing of lipids in the membrane. Unsaturated fatty acids decrease the close packing of molecules in the membraneAP Biology
  16. 16. Cholesterol  Makes the bilayer – Stronger – More flexible – Less fluid at high temperatures – Less solid at low temperatures – Slows phospholipid movement – Less permeable to water-soluble substancesAP Biology
  17. 17. If the membrane consists mainly of saturated fatty acid chains, it interdigitates with the hydrocarbon chains making them more loosely packed, thus increasing fluidity. If the membrane contains several unsaturated fatty acids, it fits into the gaps caused by bending at the double bonds and thus stabilizes the membrane.AP Biology
  18. 18.  Brain cell membranes in ground squirrels become more solid during hibernation.  Phospholipids containing more polyunsaturated fatty acids are more fluid than those with fewer polyunsaturated fatty acids.  Cholesterol in membranes reduces fluidity as well.  There are times when membranes need more or less fluidity, and molecular composition provides for sure membrane flexibility.AP Biology
  19. 19. Why are proteins the perfect molecule to build structures in the cell membrane?AP Biology 2007-2008
  20. 20. Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobicAP Biology
  21. 21. Classes of amino acids What do these amino acids have in common? I like the polar ones the best!AP Biology polar & hydrophilic
  22. 22. Proteins domains anchor molecule  Within membrane Polar areas of protein  nonpolar amino acids  hydrophobic  anchors protein into membrane  On outer surfaces of membrane  polar amino acids  hydrophilic  extend into extracellular fluid & Nonpolar areas of proteinAP Biology into cytosol
  23. 23. H+ Examples Retinal chromophore NH2water channelin bacteriaPorin monomer b-pleated sheets Bacterial Nonpolar outer (hydrophobic) COOH membrane a-helices in the cell membrane H+ Cytoplasm proton pump channel in photosynthetic bacteria function through conformational change =AP Biology shape change
  24. 24. Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Enzyme Cell surface activity receptor Cell surface Cell adhesion Attachment to theAP Biology identity marker cytoskeleton
  25. 25. Membrane carbohydrates  Play a key role in cell-cell recognition  ability of a cell to distinguish one cell from another  antigens  important in organ & tissue development  basis for rejection of foreign cells by immune systemAP Biology
  26. 26. Glycoprotein Glycoprotein, a membrane protein with a carbohydrate group attached to the extracellular side. • The carbohydrate acts as an identification tag for the cell. • Examples: – Immune system and bacteria – Red blood cellsAP Biology
  27. 27. Any Questions??AP Biology
  28. 28. Movement across the Cell MembraneAP Biology 2007-2008
  29. 29. Diffusion  2nd Law of Thermodynamics governs biological systems  universe tends towards disorder (entropy)  DiffusionAP Biology  movement from high  low concentration
  30. 30. Diffusion  Move from HIGH to LOW concentration  “passive transport”  no energy needed movement of waterAP Biology diffusion osmosis
  31. 31. Diffusion across cell membrane  Cell membrane is the boundary between inside & outside…  separates cell from its environment Can it be an impenetrable boundary? NO! IN OUT food waste carbohydrates OUT ammonia sugars, proteins salts amino acids CO2 IN lipids H2O salts, O2, H2O productsAP Biology cell needs materials in & products or waste out
  32. 32. Diffusion through phospholipid bilayer  What molecules can get through directly?  fats & other lipids lipid  What molecules can inside cell NOT get through NH3 salt directly?  polar molecules  H 2O  ions  salts, ammonia sugar aa H 2O outside cell  large molecules  starches, proteinsAP Biology
  33. 33. Channels through cell membrane  Membrane becomes semi-permeable with protein channels  specific channels allow specific material across cell membraneinside cell H 2O aa sugar NH3AP Biology salt outside cell
  34. 34. Facilitated Diffusion  Diffusion through protein channels  channels move specific molecules across cell membrane facilitated = with help  no energy needed open channel = fast transport high lowAP Biology “The Bouncer”
  35. 35. Active Transport  Cells may need to move molecules against concentration gradient  shape change transports solute from one side of membrane to other  protein “pump” conformational change  “costs” energy = ATP low ATP highAP Biology “The Doorman”
  36. 36. Active transport  Many models & mechanisms ATP ATP antiport symportAP Biology
  37. 37. Getting through cell membrane  Passive Transport  Simple diffusion  diffusion of nonpolar, hydrophobic molecules  lipids  high  low concentration gradient  Facilitated transport  diffusion of polar, hydrophilic molecules  through a protein channel  high  low concentration gradient  Active transport  diffusion against concentration gradient  low  high  uses a protein pump ATP  requires ATPAP Biology
  38. 38. Transport summary simple diffusion facilitated diffusion active ATP transportAP Biology
  39. 39. How about large molecules?  Moving large molecules into & out of cell  through vesicles & vacuoles  endocytosis  phagocytosis = “cellular eating”  pinocytosis = “cellular drinking”  exocytosisAP Biology exocytosis
  40. 40. Endocytosis fuse with phagocytosis lysosome for digestion pinocytosis non-specific process triggered by receptor-mediated molecular endocytosis signalAP Biology
  41. 41. The Special Case of Water Movement of water across the cell membraneAP Biology 2007-2008
  42. 42. Osmosis is diffusion of water  Water is very important to life, so we talk about water separately  Diffusion of water from high concentration of water to low concentration of water  across a semi-permeable membraneAP Biology
  43. 43. Concentration of water  Direction of osmosis is determined by comparing total solute concentrations  Hypertonic - more solute, less water  Hypotonic - less solute, more water  Isotonic - equal solute, equal water water hypotonic hypertonicAP Biology net movement of water
  44. 44. Managing water balance  Cell survival depends on balancing water uptake & lossAP Biology freshwater balanced saltwater
  45. 45. Managing water balance  Isotonic  animal cell immersed in mild salt solution  example: blood cells in blood plasma  problem: none  no net movement of water  flows across membrane equally, in both directions  volume of cell is stableAP Biology balanced
  46. 46. Managing water balance  Hypotonic  a cell in fresh water  example: Paramecium  problem: gains water, swells & can burst  water continually enters Paramecium cell  solution: contractile vacuole ATP  pumps water out of cell  ATP  plant cells  turgidAP Biology freshwater
  47. 47. Water regulation  Contractile vacuole in Paramecium ATPAP Biology
  48. 48. Managing water balance  Hypertonic  a cell in salt water  example: shellfish  problem: lose water & die  solution: take up water or pump out salt  plant cells  plasmolysis = wiltAP Biology saltwater
  49. 49. 1991 | 2003 Aquaporins  Water moves rapidly into & out of cells  evidence that there were water channels Peter Agre Roderick MacKinnon John Hopkins RockefellerAP Biology
  50. 50. Osmosis… .05 M .03 M Cell (compared to beaker)  hypertonic or hypotonic Beaker (compared to cell)  hypertonic or hypotonic Which way does the water flow?  in or out of cellAP Biology
  51. 51. Any Questions??AP Biology
  52. 52. Cytoplasm Cytoplasm Cytosol Organelles Solution Microbody CompartmentsAP Biology
  53. 53. AP Biology

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