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History of Fluid Mosaic Model

History of Fluid Mosaic Model. For more on this history, see: http://www1.umn.edu/ships/9-2/membrane.htm. Phospholipids—Lipid Bilayar. Note that proteins can also be anchored to cytoskeleton. Membrane Fluidity.

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History of Fluid Mosaic Model

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  1. History of Fluid Mosaic Model For more on this history, see: http://www1.umn.edu/ships/9-2/membrane.htm

  2. Phospholipids—Lipid Bilayar

  3. Note that proteins can also be anchored to cytoskeleton Membrane Fluidity Lipid bilayers are fluid, at normal body (organismal) temperatures, but typically only in two dimensions!

  4. Membrane Proteins

  5. Membrane Protein Functions

  6. Membrane Assymetry This asymmetry allows the cell to automatically differ its intracellular environment from that existing extracellularly

  7. Membrane Glycoproteins

  8. Fluid Mosaic Model

  9. Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

  10. Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

  11. Primer on Diffusion • Diffusion is movement from area of high concentration to low • Diffusion is Passive Transport: no energy is required

  12. Simple Diffusion Across Bilayers • Passive diffusion • - unaided by transport proteins • - no metabolic energy expended • - movement is down chemical concentration gradient • - diffusion rate is proportional to concentration gradient and hydrophobicity • - rate limiting step is movement across hydrophobic portion of membrane • - the greater the hydrophobicity of a water-soluble molecule, the faster it diffuses across phospholipid bilayer.

  13. Simple Diffusion Across Bilayers

  14. Diffusion Across Membranes

  15. Osmosis: H20 Down Conc. Gradient

  16. Equilibrium Osmosis: H20 Down Conc. Gradient

  17. Tonicity: H20 Down Conc. Gradient

  18. Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

  19. Protein-Mediated Diffusion

  20. Proteins Involved in Membrane Transport • Transport of molecules and ions across the cell membrane is mediated by transport proteins • - integral (intrinsic) membrane proteins (transmembrane; contain multiple transmembrane a helices). • - Peripheral (extrinsic) membrane proteins (examples: spectrin and actin in RBC; protein Kinase C) It is Integral Membrane Proteins that are involved in Membrane Transport

  21. Membrane Transport Proteins • All are integral membrane proteins • All exhibit high degree of specificity for substance transported • Three major classes: • ATP powered pumps- ATPases that use energy of ATP hydrolysis to move ions or small molecules across membrane against a chemical concentration gradient or electrical potential – active transport • Channel proteins – form water filled pores across the bilayer through which ions move down their concentration or electrical potential gradients at rapid rates • Transporters– bind only specific # substrate molecule at a time (binding of substrate induces conformational change) • -if transported molecule crosses membrane passively then “downhill” process called passive transport or facilitated diffusion

  22. Uniporters and Cotransporters We can divide Transporters into two distinct types: • Uniporters • – move one molecule at a time down a concentration gradient • Symporter or Antiporter (cotransporters) • – couple movement of one or more ions down their concentration gradient with movement of another ion or molecule against its concentration gradient • – former is passive transport; latter is active transport

  23. Uniporter: Catalized Transport • Uniporters enable amino acids, nucleosides, and sugars to enter and leave cells down their concentration gradients • Accelerate a reaction that is already thermodynamically favored • Facilitated transport (or facilitated diffusion) • - accelerated rate • - like enzyme-mediated catalysis, transport is stereospecific (ie. D-glucose) • - transport occurs via limited number of uniporter molecules

  24. Note failure to separate from hydration shell Movement towards lower concentration: Separation from hydration shell Uniporter: Voltage-Gated Channel

  25. Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

  26. Active Transport Active transport requires energy to move substances from areas of lower concentration to areas of higher concentration

  27. Sodium-Potassium Pump In addition to their intrinsic relevance, studying Membrane-Transport Proteins allows us to appreciate Mechanisms of Protein-Mediated Catalysis without getting bogged down in the details of Chemical Reactions!

  28. Electrochemical Gradient • An Electrochemical Gradient is a Concentration Gradient with Ions: • - These ions want to move down their concentration gradient • - These ions (particularly) also want to move towards the opposite charge found on the other side of the membrane • - This attraction for the other side of membranes (membrane potential) can be harnessed to do work • - Electrochemical gradients essentially are batteries, i.e., means of physically storing electrical energy

  29. Membrane Potential

  30. Proton Pump Proton pumps are used by plants, bacteria, and fungi to create electrochemical gradients (sodium-potassium pumps are employed by animals for the same purpose)

  31. Cotranport as Active Transport

  32. Cotransport Catalized Transport Net equation for: two Na+ / one-glucose symporter 2Na+out + glucose out 2Na+in + glucose in 14 membrane-spanning  helices

  33. Movement towards lower concentration: Uniporter: Facilitated Diffusion Note that phosphorylation of glucose inside of cell drives this transport, making it an example Group Translocation

  34. Movement Across Membranes Movement Across Membranes Passive Transport Simple diffusion (passive diffusion) Facilitated diffusion Transporters Uniporters Cotransporters Channel proteins Active Transport Pumps Group Translocation Electrochemical Gradients

  35. Movement Across Membranes Movement Across Membranes Endocytosis Phagocytosis Pinocytosis Receptor mediated Exocytosis These are mechanisms that involve movement into and out of the lumen of the endomembrane system Not movement directly across membranes

  36. Endocytosis Note that the substance enters the Endomembrane System but not the Cytoplasm

  37. Phagocytosis

  38. Pinocytosis

  39. Endocytosis: Receptor Mediated

  40. Endocytosis: Receptor Mediated

  41. Exocytosis

  42. Link to Next Presentation

  43. Acknowledgements www.uiowa.edu/~c156201/PDFLecs/Schmidt/Schmidt01.ppt www.avs.uidaho.edu/Rod%20Class%20Notes/AVS221-Biol212%20-%20Topic%203-2.ppt bio.winona.edu/wilson/cell%20biology/924.ppt biology.ucf.edu/courses/bsc2010/08-2010C-02.PPT www.floyd.edu/ddaugherty/1010/membrane.ppt www.biosci.ohiou.edu/courses/200203/ fall/bios/103schutte/chap03n.ppt www.aw.com/bc/ppt/marieb_ap/chap03c.ppt

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