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Chapter 7: Membrane Structure and Function

Chapter 7: Membrane Structure and Function. Important Point:. If you are having trouble understanding lecture material: Try reading your text before attending lectures. And take the time to read it well!.

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Chapter 7: Membrane Structure and Function

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  1. Chapter 7:MembraneStructureand Function

  2. Important Point: If you are having trouble understanding lecture material: Try reading your text before attending lectures. And take the time to read it well!

  3. The “ability of the cell to discriminate in its chemical exchanges with the environment is fundamental to life, and it is the plasma membrane that makes this selectivity possible.” Lipid Bilayer

  4. Fluid Mosaic Model

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

  6. Movement of Phospholipids

  7. Membrane Fluidity

  8. Cholesterol: Temp. Buffer “Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting regular packing of phospholipids.”Campbell & Reece (2005) p. 126

  9. Lateral Movement of Proteins Protein diffusion is not as fast as phospholipid diffusion

  10. Not all Proteins Move Proteins attached to cytoskeleton or ECM don’t move

  11. Transmembrane Protein Know definition of transmembrane protein Note distinct orientation

  12. Membrane Proteins

  13. Membrane Protein Functions Membrane asymmetry allows cells to automatically differ their intracellular environment from that existing extracellularly

  14. Membrane Protein Functions

  15. Note the equivalent orientation of endomembrane lumen and extracelullar environment Membrane Protein Trafficking Note the orientation of carbohydrate (always lumen or outside of cell)

  16. Transport Across Membranes Active transport is pumping against concentration gradients

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

  18. Passive Transport

  19. Passive Transport

  20. Simple Diffusion Across Bilayers • 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 Selective permeability

  21. Simple Diffusion Across Bilayers

  22. Osmosis More water Less water Down water’s concentration gradient

  23. Osmosis: H20 Down Conc. Gradient Equilibrium

  24. Tonicity More solute in than out

  25. Facilitated Diffusion

  26. Facilitated Diffusion The parallels between the properties of transport proteins and enzymes are fairly extensive, to the point where one may consider a transport protein simply as an enzyme-like protein that “catalyzes” the physical process of movement from one side of a membrane to another

  27. Group Translocation Note that ATP-mediated phosphorylation of glucose inside of cell drives this transport, making it an example Group Translocation

  28. Sodium-Potassium Pump

  29. The sodium-potassium pump is a specific (and important) example ATP-powered active transport • 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! • As we shall see, there are other means of powering active transport that don’t involve a direct hydrolysis of ATP • First, though, we will take another look at the sodium-potassium pump Sodium-Potassium Pump

  30. Sodium-Potassium Pump Why is the sodium-potassium pump considered to be an eletrongenic pump?

  31. 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 • Proton pumps are used by plants, bacteria, and fungi to create electrochemical gradients • Sodium-potassium pumps are employed by animals for the same purpose Electrochemical Gradient

  32. Movement Across Membranes

  33. Movement Down Conc. Gradient

  34. Electrogenic Pump

  35. Electron Transport & Proton Pumping

  36. Cotranport as Active Transport

  37. Endocytosis • Phagocytosis • Pinocytosis • Receptor mediated endocytosis • Exocytosis • These are mechanisms that involve movement into and out of the lumen of the endomembrane system • Not movement directly across membrane • That is, substances enter the Endomembrane System but not the Cytoplasm Movement “Across” Membranes

  38. Phagocytosis Allows digestion in confined space Taking up solids

  39. Phagocytosis: Examples

  40. Pinocytosis Taking up fluids Increases absorptive membrane area

  41. Receptor-Mediated Endocytosis A way of taking up specific substances

  42. Exocytosis Exocytosis

  43. Endocytosis & Membrane Orientation

  44. The End

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