Cell Membrane


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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.
  • 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. Why? 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
  • Cell Membrane

    1. 1. The Cell Membrane
    2. 2. Overview <ul><li>Cell membrane separates living cell from nonliving surroundings </li></ul><ul><ul><li>thin barrier = 8nm thick </li></ul></ul><ul><li>Controls traffic in & out of the cell </li></ul><ul><ul><li>selectively permeable </li></ul></ul><ul><ul><li>allows some substances to cross more easily than others </li></ul></ul><ul><ul><ul><li>hydrophobic vs hydrophilic </li></ul></ul></ul><ul><li>Made of phospholipids , proteins & other macromolecules </li></ul>
    3. 3. Phospholipids <ul><li>Fatty acid tails </li></ul><ul><ul><li>hydrophobic </li></ul></ul><ul><li>Phosphate group head </li></ul><ul><ul><li>hydrophilic </li></ul></ul><ul><li>Arranged as a bilayer </li></ul>Fatty acid Phosphate Aaaah, one of those structure–function examples
    4. 4. Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads
    5. 5. More than lipids… <ul><li>In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer </li></ul>It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model !
    6. 6. Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Extracellular fluid Cholesterol Cytoplasm Phospholipids Glycolipid Transmembrane proteins Filaments of cytoskeleton Peripheral protein Glycoprotein
    7. 7. Membrane fat composition varies <ul><li>Fat composition affects flexibility </li></ul><ul><ul><li>membrane must be fluid & flexible </li></ul></ul><ul><ul><ul><li>about as fluid as thick salad oil </li></ul></ul></ul><ul><ul><li>% unsaturated fatty acids in phospholipids </li></ul></ul><ul><ul><ul><li>keep membrane less viscous </li></ul></ul></ul><ul><ul><ul><li>cold-adapted organisms, like winter wheat </li></ul></ul></ul><ul><ul><ul><ul><li>increase % in autumn </li></ul></ul></ul></ul><ul><ul><li>cholesterol in membrane </li></ul></ul>
    8. 8. Membrane Proteins <ul><li>Proteins determine membrane’s specific functions </li></ul><ul><ul><li>cell membrane & organelle membranes each have unique collections of proteins </li></ul></ul><ul><li>Membrane proteins: </li></ul><ul><ul><li>peripheral proteins </li></ul></ul><ul><ul><ul><li>loosely bound to surface of membrane </li></ul></ul></ul><ul><ul><ul><li>cell surface identity marker ( antigens ) </li></ul></ul></ul><ul><ul><li>integral proteins </li></ul></ul><ul><ul><ul><li>penetrate lipid bilayer, usually across whole membrane </li></ul></ul></ul><ul><ul><ul><li>transmembrane protein </li></ul></ul></ul><ul><ul><ul><li>transport proteins </li></ul></ul></ul><ul><ul><ul><ul><li>channels, permeases (pumps) </li></ul></ul></ul></ul>
    9. 9. Why are proteins the perfect molecule to build structures in the cell membrane?
    10. 10. Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic
    11. 11. Classes of amino acids What do these amino acids have in common? polar & hydrophilic I like the polar ones the best !
    12. 12. Proteins domains anchor molecule <ul><li>Within membrane </li></ul><ul><ul><li>nonpolar amino acids </li></ul></ul><ul><ul><ul><li>hydrophobic </li></ul></ul></ul><ul><ul><ul><li>anchors protein into membrane </li></ul></ul></ul><ul><li>On outer surfaces of membrane </li></ul><ul><ul><li>polar amino acids </li></ul></ul><ul><ul><ul><li>hydrophilic </li></ul></ul></ul><ul><ul><ul><li>extend into extracellular fluid & into cytosol </li></ul></ul></ul>Polar areas of protein Nonpolar areas of protein
    13. 13. Examples proton pump channel in photosynthetic bacteria water channel in bacteria function through conformational change = shape change NH 2 H + COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic)  -helices in the cell membrane H + Porin monomer  -pleated sheets Bacterial outer membrane
    14. 14. Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Cell surface receptor Enzyme activity Cell surface identity marker Attachment to the cytoskeleton Cell adhesion
    15. 15. Membrane carbohydrates <ul><li>Play a key role in cell-cell recognition </li></ul><ul><ul><li>ability of a cell to distinguish one cell from another </li></ul></ul><ul><ul><ul><li>antigens </li></ul></ul></ul><ul><ul><li>important in organ & tissue development </li></ul></ul><ul><ul><li>basis for rejection of foreign cells by immune system </li></ul></ul>
    16. 16. Any Questions??
    17. 17. Movement across the Cell Membrane
    18. 18. Diffusion <ul><li>2nd Law of Thermodynamics governs biological systems </li></ul><ul><ul><li>universe tends towards disorder (entropy) </li></ul></ul><ul><li>Diffusion </li></ul><ul><ul><li>movement from high  low concentration </li></ul></ul>
    19. 19. Diffusion <ul><li>Move from HIGH to LOW concentration </li></ul><ul><ul><li>“ passive transport” </li></ul></ul><ul><ul><li>no energy needed </li></ul></ul>diffusion osmosis movement of water
    20. 20. Diffusion across cell membrane <ul><li>Cell membrane is the boundary between inside & outside… </li></ul><ul><ul><li>separates cell from its environment </li></ul></ul>IN food carbohydrates sugars, proteins amino acids lipids salts, O 2 , H 2 O OUT waste ammonia salts CO 2 H 2 O products cell needs materials in & products or waste out IN OUT Can it be an impenetrable boundary? NO!
    21. 21. Diffusion through phospholipid bilayer <ul><li>What molecules can get through directly? </li></ul><ul><ul><li>fats & other lipids </li></ul></ul>lipid salt aa H 2 O sugar NH 3 <ul><li>What molecules can NOT get through directly? </li></ul><ul><ul><li>polar molecules </li></ul></ul><ul><ul><ul><li>H 2 O </li></ul></ul></ul><ul><ul><li>ions </li></ul></ul><ul><ul><ul><li>salts, ammonia </li></ul></ul></ul><ul><ul><li>large molecules </li></ul></ul><ul><ul><ul><li>starches, proteins </li></ul></ul></ul>inside cell outside cell
    22. 22. Channels through cell membrane <ul><li>Membrane becomes semi-permeable with protein channels </li></ul><ul><ul><li>specific channels allow specific material across cell membrane </li></ul></ul>inside cell outside cell sugar aa H 2 O salt NH 3
    23. 23. Facilitated Diffusion <ul><li>Diffusion through protein channels </li></ul><ul><ul><li>channels move specific molecules across cell membrane </li></ul></ul><ul><ul><li>no energy needed </li></ul></ul>“ The Bouncer” open channel = fast transport facilitated = with help high low
    24. 24. Active Transport <ul><li>Cells may need to move molecules against concentration gradient </li></ul><ul><ul><li>shape change transports solute from one side of membrane to other </li></ul></ul><ul><ul><li>protein “pump” </li></ul></ul><ul><ul><li>“ costs” energy = ATP </li></ul></ul>“ The Doorman” conformational change ATP low high
    25. 25. Active transport <ul><li>Many models & mechanisms </li></ul>symport antiport ATP ATP
    26. 26. Getting through cell membrane <ul><li>Passive Transport </li></ul><ul><ul><li>Simple diffusion </li></ul></ul><ul><ul><ul><li>diffusion of nonpolar, hydrophobic molecules </li></ul></ul></ul><ul><ul><ul><ul><li>lipids </li></ul></ul></ul></ul><ul><ul><ul><ul><li>high  low concentration gradient </li></ul></ul></ul></ul><ul><ul><li>Facilitated transport </li></ul></ul><ul><ul><ul><li>diffusion of polar, hydrophilic molecules </li></ul></ul></ul><ul><ul><ul><li>through a protein channel </li></ul></ul></ul><ul><ul><ul><ul><li>high  low concentration gradient </li></ul></ul></ul></ul><ul><li>Active transport </li></ul><ul><ul><li>diffusion against concentration gradient </li></ul></ul><ul><ul><ul><li>low  high </li></ul></ul></ul><ul><ul><li>uses a protein pump </li></ul></ul><ul><ul><li>requires ATP </li></ul></ul>ATP
    27. 27. Transport summary simple diffusion facilitated diffusion active transport ATP
    28. 28. How about large molecules? <ul><li>Moving large molecules into & out of cell </li></ul><ul><ul><li>through vesicles & vacuoles </li></ul></ul><ul><ul><li>endocytosis </li></ul></ul><ul><ul><ul><li>phagocytosis = “cellular eating” </li></ul></ul></ul><ul><ul><ul><li>pinocytosis = “cellular drinking” </li></ul></ul></ul><ul><ul><li>exocytosis </li></ul></ul>exocytosis
    29. 29. Endocytosis phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by molecular signal
    30. 30. The Special Case of Water Movement of water across the cell membrane
    31. 31. Osmosis is diffusion of water <ul><li>Water is very important to life, so we talk about water separately </li></ul><ul><li>Diffusion of water from high concentration of water to low concentration of water </li></ul><ul><ul><li>across a semi-permeable membrane </li></ul></ul>
    32. 32. Concentration of water <ul><li>Direction of osmosis is determined by comparing total solute concentrations </li></ul><ul><ul><li>Hypertonic - more solute, less water </li></ul></ul><ul><ul><li>Hypotonic - less solute, more water </li></ul></ul><ul><ul><li>Isotonic - equal solute, equal water </li></ul></ul>water net movement of water hypotonic hypertonic
    33. 33. Managing water balance <ul><li>Cell survival depends on balancing water uptake & loss </li></ul>freshwater balanced saltwater
    34. 34. Managing water balance <ul><li>Isotonic </li></ul><ul><ul><li>animal cell immersed in mild salt solution </li></ul></ul><ul><ul><ul><li>example : blood cells in blood plasma </li></ul></ul></ul><ul><ul><ul><li>problem : none </li></ul></ul></ul><ul><ul><ul><ul><li>no net movement of water </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>flows across membrane equally, in both directions </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>volume of cell is stable </li></ul></ul></ul></ul>balanced
    35. 35. Managing water balance <ul><li>Hypotonic </li></ul><ul><ul><li>a cell in fresh water </li></ul></ul><ul><ul><ul><li>example : Paramecium </li></ul></ul></ul><ul><ul><ul><li>problem : gains water , swells & can burst </li></ul></ul></ul><ul><ul><ul><ul><li>water continually enters Paramecium cell </li></ul></ul></ul></ul><ul><ul><ul><li>solution : contractile vacuole </li></ul></ul></ul><ul><ul><ul><ul><li>pumps water out of cell </li></ul></ul></ul></ul><ul><ul><ul><ul><li>ATP </li></ul></ul></ul></ul><ul><ul><li>plant cells </li></ul></ul><ul><ul><ul><li>turgid </li></ul></ul></ul>freshwater ATP
    36. 36. Water regulation <ul><li>Contractile vacuole in Paramecium </li></ul>ATP
    37. 37. Managing water balance <ul><li>Hypertonic </li></ul><ul><ul><li>a cell in salt water </li></ul></ul><ul><ul><ul><li>example : shellfish </li></ul></ul></ul><ul><ul><ul><li>problem : lose water & die </li></ul></ul></ul><ul><ul><ul><li>solution : take up water or pump out salt </li></ul></ul></ul><ul><ul><li>plant cells </li></ul></ul><ul><ul><ul><li>plasmolysis = wilt </li></ul></ul></ul>saltwater
    38. 38. Aquaporins <ul><li>Water moves rapidly into & out of cells </li></ul><ul><ul><li>evidence that there were water channels </li></ul></ul>1991 | 2003 Peter Agre John Hopkins Roderick MacKinnon Rockefeller
    39. 39. Osmosis… Cell (compared to beaker)  hypertonic or hypotonic Beaker (compared to cell)  hypertonic or hypotonic Which way does the water flow?  in or out of cell .05 M .03 M
    40. 40. Any Questions??