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The Cell Membrane

The cell membrane is a thin, 8nm thick barrier that separates the living cell from its nonliving surroundings. It controls the traffic of substances in and out of the cell, allowing some to cross more easily than others. Made up of phospholipids, proteins, and other macromolecules, the cell membrane is hydrophobic on the inside and hydrophilic on the outside. This overview provides an understanding of the cell membrane's structure, composition, and functions.

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The Cell Membrane

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  1. The Cell Membrane

  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

  3. Phospholipids Phosphate • Fatty acid tails • hydrophobic • Phosphate group head • hydrophilic • Arranged as a bilayer Fatty acid Aaaah, one of thosestructure–function examples

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

  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!

  6. The Fluid Mosaic Model “A sea of phospholipids with floating icebergs of protein” • Membrane composed of different components • Asymmetrical b/c of different proteins on either side of membrane • Lateral protein movement (Frye & Edidin Heterocaryon Expt) http://course1.winona.edu/sberg/ANIMTNS/frey.htm • proteins do NOT flip-flop • membrane is always changing its look

  7. The Fluid Mosaic Model • Freeze-Fracture Technique

  8. Glycoprotein Glycolipid Transmembrane proteins Peripheral protein Filaments ofcytoskeleton Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Extracellular fluid Phospholipids Cholesterol Cytoplasm

  9. 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

  10. 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 • transport proteins • channels, permeases (pumps)

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

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

  13. Classes of amino acids What do these amino acids have in common? I like thepolar onesthe best! polar & hydrophilic

  14. Proteins domains anchor 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

  15. 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

  16. Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Enzymeactivity Cell surfacereceptor Cell adhesion Cell surface identity marker Attachment to thecytoskeleton

  17. 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. Any Questions??

  19. Movement across the Cell Membrane

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

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

  22. 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

  23. Diffusion through phospholipid bilayer • What molecules can get through directly? • fats & other lipids • water & small polar molecules • What molecules can NOT get through directly? • Large polar molecules • glucose, amino acids • ions • salts • large molecules • starches, proteins lipid H2O inside cell salt NH3 sugar aa outside cell

  24. 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 H+

  25. 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”

  26. 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”

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

  28. 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

  29. Transport summary simplediffusion facilitateddiffusion ATP activetransport

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

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

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

  33. 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

  34. 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

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

  36. 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

  37. 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

  38. Water regulation • Contractile vacuole in Paramecium ATP

  39. 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

  40. 1991 | 2003 Aquaporins • Water moves rapidly into & out of cells • evidence that there were water channels Peter Agre John Hopkins Roderick MacKinnon Rockefeller

  41. 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

  42. Any Questions??

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