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The cell membrane - A micro-insight of life.
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The cell membrane - A micro-insight of life.


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  • The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
  • <number>
  • <number>
  • Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.
  • The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.
  • Movement from high concentration of that substance to low concentration of that substance.
  • Donuts!
    Each transport protein is specific as to the substances that it will translocate (move).
    For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.
    Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.
    These channel proteins allow fast transport.
    For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion.
  • Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport.
  • Plants: nitrate & phosphate pumps in roots.
    Nitrate for amino acids
    Phosphate for DNA & membranes
    Not coincidentally these are the main constituents of fertilizer
    Supplying these nutrients to plants
    Replenishing the soil since plants are depleting it
  • Transcript

    • 1. The Cell Membrane AP Biology 2007-2008
    • 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 macromolecules AP Biology
    • 3. Phosphate Phospholipids  Fatty acid tails  hydrophobic  Phosphate group head  hydrophilic Fatty acid  Arranged as a bilayer Aaaah, one of those structure–function examples AP Biology
    • 4. Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads AP Biology
    • 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. Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein AP Biology Cytoplasm Transmembrane proteins Filaments of cytoskeleton
    • 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  AP Biology cholesterol in membrane
    • 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 AP Biology  transport proteins  channels, permeases (pumps)
    • 9. Why are proteins the perfect molecule to build structures in the cell membrane? AP Biology 2007-2008
    • 10. Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic AP Biology
    • 11. Classes of amino acids What do these amino acids have in common? I like the polar ones the best! AP Biology polar & hydrophilic
    • 12. 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 AP Biology extracellular fluid & into cytosol Nonpolar areas of protein
    • 13. H+ Examples Retinal chromophore NH2 water channel in bacteria Porin monomer β-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) α-helices in the cell membrane COOH H+ Cytoplasm proton pump channel in photosynthetic bacteria AP Biology function through conformational change = shape change
    • 14. Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter AP Biology Enzyme activity Cell surface receptor Cell surface identity marker Cell adhesion Attachment to the cytoskeleton
    • 15. 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 system  AP Biology
    • 16. Any Questions?? AP Biology
    • 17. Movement across the Cell Membrane AP Biology 2007-2008
    • 18. Diffusion  2nd Law of Thermodynamics governs biological systems  universe tends towards disorder (entropy)  Diffusion  AP Biology movement from high → low concentration
    • 19. Diffusion  Move from HIGH to LOW concentration “passive transport”  no energy needed  AP Biology diffusion movement of water osmosis
    • 20. Diffusion across cell membrane  Cell membrane is the boundary between inside & outside…  separates cell from its environment Can it be an impenetrable boundary? NO! OUT IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O AP Biology OUT IN waste ammonia salts CO2 H2O products cell needs materials in & products or waste out
    • 21. Diffusion through phospholipid bilayer  What molecules can get through directly?  fats & other lipids inside cell NH3  What molecules can lipid salt NOT get through directly?  polar molecules  H2O  outside cell sugar aa H2O ions  salts, ammonia  large molecules  starches, proteins AP Biology
    • 22. Channels through cell membrane  Membrane becomes semi-permeable with protein channels  specific channels allow specific material across cell membrane inside cell NH3 AP Biology salt H2O aa sugar outside cell
    • 23. Facilitated Diffusion  Diffusion through protein channels   channels move specific molecules across cell membrane facilitated = with help no energy needed open channel = fast transport high low AP Biology “The Bouncer”
    • 24. 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 high AP Biology “The Doorman”
    • 25. Active transport  Many models & mechanisms ATP AP Biology ATP antiport symport
    • 26. 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   AP Biology uses a protein pump requires ATP ATP
    • 27. Transport summary simple diffusion facilitated diffusion active transport AP Biology ATP
    • 28. How about large molecules?  Moving large molecules into & out of cell through vesicles & vacuoles  endocytosis   phagocytosis = “cellular eating”  pinocytosis = “cellular drinking”  AP Biology exocytosis exocytosis
    • 29. Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by molecular signal AP Biology
    • 30. The Special Case of Water Movement of water across the cell membrane AP Biology 2007-2008
    • 31. 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  AP Biology across a semi-permeable membrane
    • 32. 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 AP Biology hypotonic hypertonic net movement of water
    • 33. Managing water balance  Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater
    • 34. 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 stable AP Biology balanced
    • 35. 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  pumps water out of cell ATP  ATP  AP Biology plant cells  turgid freshwater
    • 36. Water regulation  Contractile vacuole in Paramecium ATP AP Biology
    • 37. 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 = wilt AP Biology saltwater
    • 38. 1991 | 2003 Aquaporins  Water moves rapidly into & out of cells  evidence that there were water channels Peter Agre AP Biology Roderick MacKinnon John Hopkins Rockefeller
    • 39. Osmosis… .05 M .03 M Cell (compared to beaker) → hypertonic or hypotonic Beaker (compared to cell) → hypertonic or hypotonic AP Biology Which way does the water flow? → in or out of cell
    • 40. Any Questions?? AP Biology