Chapter 5 The Plasma Membrane

1. Chapter 5The Plasma Membrane

2. Diffusion • 2nd Law of Thermodynamicsgoverns biological systems • Universe tends towards disorder—ENTROPY! • Diffusion • movement from highlow concentration

3. Diffusion of 2 solutes • Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances

4. Diffusion • Move from HIGHto LOW concentration • “passive transport” • no energy needed diffusion osmosis

5. NO! Cell (plasma) membrane • Cells need an inside & an outside… • separate cell from its environment • cell membrane is the boundary Can it be an impenetrable boundary? IN food carbohydrates sugars, proteins amino acids lipids salts, O2,H2O OUT waste ammonia salts CO2 H2O products OUT IN cell needs materials in & products or waste out

6. Building a membrane • With what do you build a barrier that keeps the watery contents of the cell separate from the watery environment? Your choices carbohydrates? proteins? nucleic acids? lipids? LIPIDS oil & water don’t mix!!

7. I hope you remembered your phospholipds! inside cell outside cell Lipids of cell membrane • Membrane is made of phospholipids • phospholipidbilayer phosphate hydrophilic hydrophobic fatty acid tails

8. Here are those layers again! Phospholipids

9. Semi-permeable membrane • Need to allow passage through the membrane for a lot of stuff! • But it needs to control what gets in or out • membrane needs to be semi-permeable sugar aa lipid H2O salt NH3

10. [low]  [high] Simple diffusion across membrane Which way will the lipids move? (net movement) lipid lipid lipid inside cell lipid lipid lipid lipid outside cell lipid lipid lipid lipid lipid lipid lipid

11. inside cell outside cell Phospholipid bilayer • What molecules can get through directly? Fats and other non-polar (hydrophoblic) molecules can slip directly through the phospholipidbilayer membrane, but… …what about all the other stuff? lipid NH3 salt sugar aa H2O

12. Permeable cell membrane • Need to allow other material through • membrane needs to be permeable to… • all materials a cell needs to bring in • all wastes a cell needs excrete out • all products a cell needs to export out inside cell sugar aa H2O lipid “holes” or channels in cell membrane allow polar (hydrophilic) materials in & out salt NH3 outside cell

13. [low]  [high] Diffusion through a channel Which way will the sugars move? (net movement) • Movement from high to low sugar sugar sugar inside cell sugar sugar outside cell sugar sugar sugar sugar sugar sugar sugar

14. Semi-permeable cell membrane • But the cell still needs control • membrane needs to be semi-permeable • specific channels allow specific material in & out inside cell H2O aa sugar salt outside cell NH3

15. So… how do you build a selectively permeable cell membrane? • What molecule will sit “comfortably” in a phospholipidbilayer forming channels? bi-lipid membrane protein channelsin bi-lipid membrane PROTEINS!

16. Why proteins? • Proteins are mixedmolecules • some hydrophobicamino acids • stick in the lipid membrane • anchors the protein in membrane • some hydrophilicamino acids • stick out in the watery fluid in & around cell • specialized “receptor” for specific molecules

17. Membrane Proteins • Proteins determine most of membrane’s specific functions • cell membrane & organelle membranes each have unique collections of proteins • Membrane proteins: • peripheral proteins = loosely bound to surface of membrane • integral proteins = penetrate into lipid bilayer, often completely spanning the membrane • a.k.a. transmembraneproteins

18. Facilitated Diffusion • Movement from HIGH to LOW concentration through a protein channel • passive transport • no energy needed • facilitated = with help

19. Facilitated Diffusion • Globular proteins act as doors in membrane • channels to move specific molecules through cell membrane open channel = fast transport [high] [low] “The Bouncer”

20. Osmosis is diffusion of water • Water is very important, so we talk about water separately—but same idea! • Diffusion of water from high concentrationof waterto low concentrationof water • across a semi-permeable membrane

21. hypotonic hypertonic Concentration of water • Direction of osmosis is determined by comparing totalsoluteconcentrations! • hypertonic- more solute, less water • hypotonic- less solute, more water • isotonic- equal solute, equal water molecule of solute Which way will the water move? (net movement) water net movement of water

22. Managing water balance • Cell survival depends on balancing water uptake & loss freshwater balanced saltwater

23. Managing water balance • Isotonic • animal cell immersed in isotonic solution • blood cells in blood • no net movement of water across plasma membrane • water flows across membrane, at same rate in both directions • volume of cell is stable

24. Managing water balance • Hypotonic • animal cell in hypotonic solution will gain water, swell & burst • Paramecium vs. pond water • Paramecium is hypertonic • H2O continually enters cell • to solve problem, specialized organelle, contractile vacuole • pumps H2O out of cell using ATP • plant cell • turgid

25. Remember the red onion lab from freshman year? Managing water balance • Hypertonic • animal cell in hypertonic solution will lose water, shrivel & probably die • salt water organisms are hypotonic compared to their environment • they have to take up water & pump out salt • plant cells • plasmolysis= wilt

26. 1991 | 2003 Aquaporins • Water moves rapidly into & out of cells • evidence that there were water channels Peter Agre John Hopkins Roderick MacKinnon Rockefeller

27. .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 cell

28. What aboutedema & histamines? Osmosis in Action • Interstitial fluid flows back into capillaries due to osmosis • plasma proteins  osmotic pressure in capillary Lymphatic capillary Fluid & solutes flows out of capillaries to tissues due to blood pressure • “bulk flow” BP > OP BP < OP Interstitial fluid 85% fluid returns to capillaries Blood flow Capillary 15% fluid returns via lymph Arteriole Venule

29. Active Transport • Cells may need molecules to move against concentration situation • need to pump against concentration • protein pump • requires energy • ATP Na+/K+ pump in nerve cell membranes

30. Active Transport • Globular proteins act as ferry for specific molecules • shape change transports solute from one side of membrane to other  protein “pump” • “costs” energy (ATP) conformational change

31. Active Transport • Many models & mechanisms • uniports, symports, antiports • primary vs. secondary active transport using ATP using ATP

32. nutrients are valuable…grab all you can get! Absorption of Nutrients • Passive transport • fructose • Active (protein pumps) transport • pump amino acids, vitamins & glucose • against concentration gradients across intestinal cell membranes • allows intestine to absorb much higher proportion of nutrients in the intestine than would be possible with passive diffusion • worth the cost of ATP!

33. Ooooh… Structure-Functiontheme! Absorption by Small Intestines • Absorption through villi & microvilli • finger-like projections • increase surface area for absorption

34. Gated channels • Some channel proteins open only in presence of stimulus (signal) • stimulus usually different from transported molecule • ex:ion-gated channelswhen neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell • ex:voltage-gated channelschange in electrical charge across nerve cell membrane opens Na+ & K+ channels

35. Getting through cell membrane • Passivetransport • diffusion of hydrophobic (lipids) molecules • high  low concentration gradient • Facilitatedtransport • diffusion of hydrophilic molecules • through a protein channel • high  low concentration gradient • Active transport • diffusionagainst concentration gradient • low  high • uses a protein pump • requires energy – often ATP

36. Transport summary

37. How about large molecules? • Moving large molecules into & out of cell • through vesicles & vacuoles • endocytosis • phagocytosis = “cellular eating” • pinocytosis = “cellular drinking” • receptor-mediated endocytosis • exocytosis

38. Endocytosis fuse with lysosome for digestion phagocytosis non-specificprocess pinocytosis triggered byligand signal receptor-mediated endocytosis

39. Receptor-Mediated Endocytosis

40. An important step in the evolution of multicellular life! But more on that later… Cell Junctions

41. Cells  Tissues • binding can be homotypic (between molecules of the same protein) or heterotypic (between different but complementary proteins) Cell Recognition and Adhesion

42. Plant cell wall • Structure • cellulose • primary cell wall • secondary cell wall • middle lamella = sticky polysaccharides

43. Intercellular junctions • Plant cells • plasmodesmata • channels allowing cytosolto pass between cells plasmodesmata

44. Animal cell surface • Extracellular matrix • collagen fibers in network of glycoproteins • support • adhesion • movement • regulation

45. Intercellular junctions in animals

46. Intercellular junctions • Animal cells • tight junctions • membranes of adjacent cells fused forming barrier between cells (epithelial) • forces material through cell membrane • gap junctions • communicating junctions • allow cytoplasmic movement between adjacent cells • desmosomes • anchoring junctions • fasten cells together in strong sheets (smooth muscles)

47. More than just a barrier… • Expanding our view of cell membrane beyond just a phospholipidbilayer barrier • phospholipids plus…

48. …proteins. Plus… • In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer

49. Membrane Carbohydrates • Play a key role in cell-cell recognition • called glycoproteins • ability of a cell to distinguish neighboring cells from another • important in organ & tissue development • basis for rejection of foreign cells by immune system (ABO blood system)

50. Membranes provide a variety of cell functions