2. AP Biology 2007-2008 The Cell Membrane

3. AP Biology Phospholipids Fatty acid Phosphate  Phosphate head  hydrophilic  Fatty acid tails  hydrophobic  Arranged as a bilayer Aaaah, one of those structure–function examples “repelled by water” “attracted to water”

4. AP Biology Arranged as a Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads  Serves as a cellular barrier / border H2OH2O sugar lipids salt waste impermeable to polar molecules

5. AP Biology Cell membrane defines cell  Cell membrane separates living cell from aqueous environment  thin barrier = 8nm thick  Controls traffic in & out of the cell  allows some substances to cross more easily than others  hydrophobic (nonpolar) vs. hydrophilic (polar)

6. AP Biology Permeability to polar molecules?  Membrane becomes semi-permeable via protein channels  specific channels allow specific material across cell membrane inside cell outside cell sugaraa H2OH2O salt NH 3

7. AP Biology Cell membrane is more than lipids…  Transmembrane proteins embedded in phospholipid bilayer  create semi-permeabe channels lipid bilayer membrane protein channels in lipid bilyer membrane

8. AP Biology 2007-2008 Why are proteins the perfect molecule to build structures in the cell membrane?

9. AP Biology Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic

10. AP Biology Classes of amino acids What do these amino acids have in common? polar & hydrophilic I like the polar ones the best!

11. AP Biology Proteins domains anchor molecule  Within membrane  nonpolar amino acids  hydrophobic  anchors protein into membrane  On outer surfaces of membrane in fluid  polar amino acids  hydrophilic  extend into extracellular fluid & into cytosol Polar areas of protein Nonpolar areas of protein

12. AP Biology NH 2 H+H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic)  -helices in the cell membrane H+H+ Porin monomer  -pleated sheets Bacterial outer membrane proton pump channel in photosynthetic bacteria aquaporin = water channel in bacteria function through conformational change = protein changes shape Examples H2OH2O H2OH2O H+H+ H+H+

13. AP Biology 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 “Antigen” “Channel”

14. AP Biology Membrane Proteins  Proteins determine membrane’s specific functions  cell membrane & organelle membranes each have unique collections of proteins  Classes of membrane proteins:  peripheral proteins  loosely bound to surface of membrane  ex: cell surface identity marker (antigens)  integral proteins  penetrate lipid bilayer, usually across whole membrane  transmembrane protein  ex: transport proteins  channels, permeases (pumps)

15. AP Biology Cell membrane must be 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!

16. AP Biology Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Extracellular fluid Cholesterol Cytoplasm Glycolipid Transmembrane proteins Filaments of cytoskeleton Peripheral protein Glycoprotein Phospholipids 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

17. AP Biology 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

18. AP Biology Fluid Mosaic Model fluid structure  A membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top  Phospholipidslaterally  Phospholipids can move laterally a small amount and can “flex” their tails  Membrane proteins laterall  Membrane proteins also move side to side or laterally making the membrane fluid

19. AP Biology  Freeze-fracture  Freeze-fracture studies of the plasma membrane support the fluid mosaic model of membrane structure fracture plane often follows the hydrophobic interior of a membrane two separated layers membrane proteins go wholly with one of the layers A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

20. AP Biology The Fluidity of Membranes  Phospholipids in the plasma membrane Can move within the bilayer two ways Lateral movement (~10 7 times per second) Flip-flop (~ once per month)

21. AP Biology type of hydrocarbon tails  The type of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane FluidViscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails

22. AP Biology 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 membrane

23. AP Biology The Fluidity of Membranes steroid cholesterol  The steroid cholesterol Has different effects on membrane fluidity at different temperatures Figure 7.5 Cholesterol

24. AP Biology Any Questions??

25. AP Biology 2007-2008 Movement across the Cell Membrane

26. AP Biology Diffusion  2nd Law of Thermodynamics governs biological systems  universe tends towards disorder (entropy)  Diffusion  movement from HIGH  LOW concentration  Diffusion  movement from HIGH  LOW concentration

27. AP Biology Simple Diffusion  Move from HIGH to LOW concentration  “passive transport”  no energy needed diffusionosmosis movement of water

28. AP Biology Facilitated Diffusion  Diffusion through protein channels  channels move specific molecules across cell membrane  no energy needed “The Bouncer” open channel = fast transport facilitated = with help HIGH LOW

29. AP Biology Active Transport “The Doorman” conformational change  Cells may need to move molecules against concentration gradient  conformational shape change transports solute from one side of membrane to other  protein “pump”  “costs” energy = ATP ATP LOW HIGH

30. AP Biology symportantiport Active transport  Many models & mechanisms ATP

31. AP Biology 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  requires ATP ATP

32. AP Biology Transport summary simple diffusion facilitated diffusion active transport ATP

33. AP Biology How about large molecules?  Moving large molecules into & out of cell  through vesicles & vacuoles  endocytosis  phagocytosis = “cellular eating”  pinocytosis = “cellular drinking”  exocytosis exocytosis

34. AP Biology Endocytosis phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by molecular signal

35. AP Biology 2007-2008 The Special Case of Water Movement of water across the cell membrane

36. AP Biology Osmosis is just 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 membrane

37. AP Biology 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 hypotonichypertonic water net movement of water

38. AP Biology freshwaterbalancedsaltwater Managing water balance  Cell survival depends on balancing water uptake & loss

39. AP Biology Managing water balance  Hypotonic  a cell in fresh water  high concentration of water around cell  problem: cell gains water, swells & can burst  example: Paramecium  ex: water continually enters Paramecium cell  solution: contractile vacuole  pumps water out of cell  ATP  plant cells  turgid = full  cell wall protects from bursting freshwater ATP 1 No problem, here KABOOM!

40. AP Biology Pumping water out  Contractile vacuole in Paramecium ATP

41. AP Biology Managing water balance  Hypertonic  a cell in salt water  low concentration of water around cell  problem: cell loses water & can die  example: shellfish  solution: take up water or pump out salt  plant cells  plasmolysis = wilt  can recover saltwater 2 I will survive! I’m shrinking, I’m shrinking!

42. AP Biology Managing water balance  Isotonic  animal cell immersed in mild salt solution  no difference in concentration of water between cell & environment  problem: none  no net movement of water flows across membrane equally, in both directions  cell in equilibrium  volume of cell is stable  example: blood cells in blood plasma  slightly salty IV solution in hospital balanced 3 I could be better… That’s perfect!

43. AP Biology Aquaporins  Water moves rapidly into & out of cells  evidence that there were water channels  protein channels allowing flow of water across cell membrane 1991 | 2003 Peter Agre John Hopkins Roderick MacKinnon Rockefeller

44. AP Biology 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 Do you understand Osmosis…

45. AP Biology Water Potential  Water potential is the tendency of water to leave one place in favor of another place.  Water will always move from and area of higher water potential to an area of lower water potential.  The water potential of water at atmospheric pressure is zero.

46. AP Biology Physical Factors That Affect Water Potential  Solute Potential  Movement of water into and out of the cell is dependent on solute potential.  Adding solute to a solution lowers water potential.  Solute potential is always negative because the solute potential of pure water is zero.

47. AP Biology Physical Factors That Affect Water Potential  Pressure Potential  Constriction of the cell wall expansion causes pressure to be exerted in the opposite direction to water coming into the cell.  Pressure potential is usually positive in living cells.  Pressure potential can be:  positive(pushing effect)  negative(pulling effect)  Water moving up the xylem of the plant

48. AP Biology Water Potential and Living Cells  Water Potential is the sum of pressure potential and solute potential.  Plant cells have a lower water potential than the surrounding environment.  Animal cells have the same water potentials as the fluid that surrounds them.

49. AP Biology

50. Any Questions??

51. AP Biology 2007-2008 Ghosts of Lectures Past (storage)

52. AP Biology Diffusion through phospholipid bilayer  What molecules can get through directly?  fats & other lipids inside cell outside cell lipid salt aa H2OH2O sugar NH 3  What molecules can NOT get through directly?  polar molecules H2OH2O  ions (charged)  salts, ammonia  large molecules  starches, proteins

53. AP Biology Diffusion across cell membrane  Cell membrane is the boundary between inside & outside…  separates cell from its environment IN food carbohydrates sugars, proteins amino acids lipids salts, O 2, H 2 O OUT waste ammonia salts CO 2 H2OH2O products cell needs materials in & products or waste out IN OUT Can it be an impenetrable boundary? NO!