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Membranes

Membranes. Fluid compartments in our bodies are separated by membranes. Greater number of osmotically active particles. Composition of body fluids. Where are membranes located?. Plasma membrane Membrane-bound organelles. What is the plasma membrane made of?.

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Membranes

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  1. Membranes

  2. Fluid compartments in our bodies are separated by membranes

  3. Greater number of osmotically active particles Composition of body fluids

  4. Where are membranes located? • Plasma membrane • Membrane-bound organelles

  5. What is the plasma membrane made of? Our cells inhabit an aqueous environment • …but they must let ‘stuff’ (nutrients, ions, molecules,waste products) in and out • …..and they need to communicate with each other… How is this achieved?....

  6. Incredible features of biological membranes • They form a ‘fluid mosaic’ pattern: - flexible, mobile, 8 ηm • They areselectively permeable • They are self-sealing

  7. Let’s meet the components of the Cell membrane • Phospholipid bilayer • Cholesterol • Glycoproteins • Integral proteins • Peripheral proteins • Let's get to know them.....

  8. Glycoprotein Glycolipid Transmembrane proteins Peripheral protein Filaments ofcytoskeleton A collage of proteins & other molecules are embedded in the fluid matrix of the phospholipid bilayer Extracellular fluid Phospholipids Cholesterol Cytoplasm

  9. Phospholipids • Fatty acid tails • hydrophobic • Phosphate group head • hydrophilic • Arranged as a bilayer Phosphate Fatty acid

  10. Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads

  11. Membrane lipid 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

  12. More than lipids…the fluid mosaic model In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer: You can link to the 1972 original paper here

  13. Membrane Proteins • Proteins determine a 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 • Embedded in lipid bilayer • transmembrane proteins • transport proteins • channels, permeases (pumps)

  14. Why areproteins the perfect molecule to build structures in the cell membrane?

  15. Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic

  16. Classes of amino acids What do these amino acids have in common? polar & hydrophilic

  17. Domains anchor protein molecule Polar areas of protein Within membrane • nonpolar amino acids • hydrophobic • anchors protein into membrane On outer surfaces of membrane • polar amino acids • hydrophilic • extend into extracellular fluid & into cytosol Nonpolar areas of protein

  18. Porin monomer H+ Retinal chromophore b-pleated sheets NH2 Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH Cytoplasm H+ Examples water channel in bacteria proton pump channel in photosynthetic bacteria function through conformational change (shape change)

  19. Many Functions of Membrane Proteins Outside Plasma membrane Inside channels for passive transport Immobilised Enzyme Cell surfacereceptor (pumps) Cell adhesion Cell surface identity marker Attachment to thecytoskeleton

  20. Membrane glycoproteins 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

  21. Blood group is determined by glycoproteins on the surface of red blood cells

  22. Let’s Review… • John Kyrk Cell membrane

  23. Movement across the Cell Membrane

  24. How are substances transported into and out of the cell? • Simple Diffusion(passive) • Facilitated diffusion (Passive) • Osmosis • Active transport • Endocytosis • Exocytosis

  25. Simple Diffusion 2nd Law of Thermodynamicsgoverns biological systems • universe tends towards disorder (entropy) • Diffusion • movement from highlow concentration

  26. Diffusion Movement is from HIGH to LOW concentration • “passive transport” • no energy needed movement of water diffusion osmosis

  27. Diffusion across cell membrane • Cell membrane is the boundary between inside & outside… • separates cell from its environment NO! Can it be an impenetrable boundary? OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2,H2O OUT IN cell needs materials in & products or waste out

  28. inside cell outside cell Diffusion through phospholipid bilayer What molecules can get through directly? • fats & other lipids • What molecules can NOT get through directly? • polar molecules • H2O • ions • salts, ammonia • large molecules • starches, proteins lipid salt NH3 sugar aa H2O

  29. Let’s watch some animations • Diffusion animation 1 • Diffusion animation 2

  30. high low Facilitated Diffusion Diffusion through protein channels • channels move specific molecules across cell membrane • no energy needed facilitated = with help open channel = fast transport “The Bouncer”

  31. Channels through cell membrane Membrane becomes semi-permeable with protein channels • specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell NH3

  32. Which substances are transported by facilitated diffusion? • Glucose • Urea • Amino acids • Animation

  33. low high Active Transport • Cells may need to move molecules against concentration gradient • shape change transports solute from one side of membrane to other • protein “pump” • “costs” energy = ATP conformationalchange ATP “The Doorman”

  34. Active transport • Many models & mechanisms ATP ATP antiport symport

  35. 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

  36. Transport summary simplediffusion facilitateddiffusion ATP activetransport

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

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

  39. The Special Case of WaterMovement of water across the cell membrane

  40. 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 membrane

  41. hypotonic hypertonic 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 net movement of water

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

  43. 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 balanced

  44. 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 • plant cells • turgid ATP freshwater

  45. Water regulation • Contractile vacuole in Paramecium ATP

  46. 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 saltwater

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

  48. 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 cell

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