Membrane pt.1


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Membrane pt.1

  1. 1. Membranes Structure & Function ~Transport & Signaling
  2. 2. The Cell Membrane <ul><li>Sometimes called the plasma membrane </li></ul><ul><li>Outer boundary of lipids and proteins </li></ul><ul><ul><li>Formed of phospholipids </li></ul></ul><ul><ul><li>Proteins are embedded in the lipids </li></ul></ul><ul><li>The membrane is approximately 5 nm thick </li></ul><ul><li>Holds the cell together - gives it shape </li></ul><ul><li>Regulates what enters and leaves the cell </li></ul>
  3. 3. The Phospholipid Bilayer <ul><li>Biological membranes are a bilayer formed from phospholipids . </li></ul><ul><li>Phospholipids create a spherical, three dimensional shell around the cell. </li></ul><ul><li>They are often represented 2-dimensionally as: </li></ul>
  4. 4. Orientation of Phospholipids <ul><li>Each phospholipid has a negatively charged phosphate head </li></ul><ul><ul><li>These are hydrophilic </li></ul></ul><ul><li>The heads point out : toward the environment and the interior of the cell </li></ul><ul><li>Each phospholipid has two tails that are highly hydrophobic hydrocarbon chains </li></ul><ul><li>The tails orient in , towards each other and away from the watery exterior of the cell </li></ul><ul><ul><li>Create a hydrophobic interior part of the membrane </li></ul></ul>
  5. 6. Membrane Polarity
  6. 7. The Fluid Mosaic <ul><li>Lipid bilayers are fluid </li></ul><ul><ul><li>Individual phospholipids diffuse throughout the 2-dimensional surface </li></ul></ul><ul><li>Mosaic property of the membrane </li></ul><ul><ul><li>proteins, cholesterol and other molecules are embedded in the phospholipids </li></ul></ul><ul><li>Membrane proteins diffuse throughout the membrane </li></ul>
  7. 9. Membrane Fluidity <ul><li>Several factors influence membrane fluidity: </li></ul><ul><li>Cholesterol </li></ul><ul><ul><li>A necessary component of biological membranes </li></ul></ul><ul><ul><li>Breaks up Van der Waals forces and the close packing of phospholipid tails </li></ul></ul><ul><ul><li>Makes membrane more fluid </li></ul></ul><ul><ul><li>Regulating cholesterol regulates membrane fluidity </li></ul></ul><ul><li>Ratio of saturated to unsaturated hydrocarbon chains in phospholipids </li></ul><ul><ul><li>Impacts fluidity </li></ul></ul><ul><ul><li>Phospholipids with saturated hydrocarbon chains pack close together & form numerous Van der Waals bonds </li></ul></ul>
  8. 11. The Membrane is Semi-permeable <ul><li>The arrangement of phopholipids and embedded proteins makes the membrane semipermeable </li></ul><ul><li>Some molecules are allowed to pass freely (diffuse) through the membrane. </li></ul><ul><li>Other molecules cannot enter the cell </li></ul><ul><ul><li>Virtually impermeable to large molecules </li></ul></ul><ul><ul><li>Molecules as small as charged ions need a special process to enter the cell </li></ul></ul><ul><ul><li>Quite permeable to lipid soluble low molecular weight molecules (like CO 2 & O 2 ). </li></ul></ul>
  9. 12. Homeostasis <ul><li>The process of maintaining a stable internal environment despite changing external conditions </li></ul><ul><li>The cells' environment is surrounded by fluids </li></ul><ul><li>The cytoplasm is mostly water </li></ul><ul><li>Most transport involves molecules in liquid state, in solution </li></ul><ul><ul><li>solvent - water in most cell processes </li></ul></ul><ul><ul><li>solute </li></ul></ul>
  10. 13. Diffusion <ul><li>Molecules move from areas of higher concentration to areas of lower concentration </li></ul><ul><li>This process continues until the material is evenly distributed throughout the substance </li></ul><ul><li>A major means of molecular transport in the cell </li></ul><ul><li>Diffusion through membrane doesn't require energy </li></ul><ul><li>Increased temperature increases diffusion rate </li></ul><ul><li>Increased pressure increases diffusion rate </li></ul><ul><li>Limited by the diffusion rate of the molecule </li></ul><ul><li>Effective for some substances: e.g. water </li></ul>
  11. 14. The Concentration Gradient <ul><li>Diffusion occurs down a concentration gradient </li></ul><ul><li>Concentration gradient </li></ul><ul><ul><li>Difference in concentration of molecules of a substance from the highest to the lowest concentration </li></ul></ul><ul><li>Moving from an area of high concentration to an area of low concentration is moving with the concentration gradient </li></ul><ul><li>The steeper the concentration gradient (greater difference between concentrations) the more rapid the diffusion. </li></ul>
  12. 16. Osmosis <ul><li>Diffusion of water molecules through a selectively permeable membrane from an area of greater concentration to an area of lesser concentration </li></ul><ul><li>Water flows back and forth across the cell membrane until the concentration of water molecules is = on each side </li></ul><ul><li>When concentration is = on both sides: equilibrium </li></ul><ul><li>Concentration of water on each side of membrane determined by concentration of solutes in water </li></ul><ul><li>Requires no energy </li></ul>
  13. 18. Solute Concentration <ul><li>The concentration of water on each side of membrane determined by concentration of solutes in water </li></ul><ul><li>Isotonic Solution </li></ul><ul><ul><li>concentration of solutes inside = outside </li></ul></ul><ul><ul><li>rate of osmosis equal both ways </li></ul></ul><ul><li>Hypotonic </li></ul><ul><ul><li>concentration of solutes outside is less than inside </li></ul></ul><ul><ul><li>water moves from solution into cell </li></ul></ul><ul><li>Hypertonic </li></ul><ul><ul><li>concentration outside is greater than concentration inside </li></ul></ul><ul><ul><li>water flows out of cells </li></ul></ul>
  14. 19. Water Balance
  15. 20. Cells in Hypotonic Solution <ul><li>Hypotonic </li></ul><ul><ul><li>concentration of solutes outside is less than inside </li></ul></ul><ul><ul><li>water moves from solution into cell </li></ul></ul><ul><li>Osmotic Pressure </li></ul><ul><ul><li>Pressure caused inside a cell or any sack by the passage of water in through osmosis </li></ul></ul><ul><li>Freshwater plants </li></ul><ul><li>Turgor = pressure in cells from water flowing in </li></ul><ul><ul><li>causes cell to be rigid </li></ul></ul><ul><ul><li>cell wall prevents bursting </li></ul></ul><ul><li>Animal cells can't reach equilibrium in hypotonic solution because they lack a cell wall </li></ul><ul><ul><li>Some cells have developed mechanisms to remove excess water before they burst </li></ul></ul>
  16. 21. Contractile Vacuole
  17. 22. Cells in Hypertonic Solution <ul><li>In hypertonic solution cells shrivel because water flows out of the cell </li></ul><ul><li>Drinking sea water </li></ul><ul><li>Road salt bad for plants </li></ul><ul><li>Some animals are specially adapted </li></ul><ul><ul><li>Salmon  </li></ul></ul>
  18. 23. Carrier Transport <ul><li>Most molecules cannot move across the membrane freely </li></ul><ul><ul><li>Require carriers </li></ul></ul><ul><li>Carrier molecules are proteins in the cell membrane </li></ul><ul><ul><li>transport large molecules or molecules that cannot dissolve in the lipids that make up the cell membrane </li></ul></ul><ul><li>  2 types: </li></ul><ul><ul><li>Facilitated Diffusion </li></ul></ul><ul><ul><li>Active Transport </li></ul></ul>
  19. 24. Facilitated Diffusion <ul><li>Like simple diffusion </li></ul><ul><li>Substances move with the concentration gradient </li></ul><ul><li>Carrier molecules speed up the movement of the diffusing substances </li></ul><ul><li>Protein channels </li></ul><ul><li>Allows charged molecules that couldn’t cross the membrane to diffuse freely in & out of the cell </li></ul><ul><li>Greatest use is small ions: Na + , K + , Cl - </li></ul><ul><li>Speed is limited by the number of protein channels, not the concentration gradient </li></ul>
  20. 25. Picturing Facilitated Diffusion
  21. 26. Active Transport <ul><li>Also uses carrier molecules </li></ul><ul><li>Involves movement of materials against the concentration gradient </li></ul><ul><ul><li>e.g. liver cells store glucose - have higher concentration of glucose than surrounding tissue, so active transport needed to move glucose in. </li></ul></ul><ul><li>Requires energy </li></ul><ul><li>Two categories: </li></ul><ul><ul><li>Primary & secondary </li></ul></ul>
  22. 27. Primary Active Transport <ul><li>Uses energy at the membrane protein itself to cause a conformational change </li></ul><ul><ul><li>Energy from ATP hydrolysis </li></ul></ul><ul><li>Shape change results in transport of molecule through the protein </li></ul><ul><li>Best known is Na + /K + pump </li></ul><ul><li>Antiport </li></ul><ul><ul><li>Transports K + in and Na + out at the same time </li></ul></ul>
  23. 28. The Na + /K + Pump – An Antiport
  24. 29. Secondary Active Transport <ul><li>Uses energy to establish a gradient across the cell membrane </li></ul><ul><li>Utilizes the gradient to transport the desired molecule up its concentration gradient </li></ul>
  25. 30. Establishing a Gradient
  26. 31. The E.coli Lactose Symport <ul><li>Symport </li></ul><ul><ul><li>coupled transport in the same direction across a cell membrane </li></ul></ul><ul><li>E. coli establishes a proton (H+) gradient across the cell membrane </li></ul><ul><li>Uses energy to pump protons out of the cell </li></ul><ul><li>Those protons are coupled to lactose at the lactose permease transmembrane protein </li></ul><ul><li>Lactose permease uses the energy of the protons moving down their concentration gradient to transport lactose into the cell </li></ul>
  27. 32. A Model Symport
  28. 33. The Lactose Symport
  29. 34. The Na + -Glucose Symport <ul><li>Another secondary active transport system </li></ul><ul><li>Uses the Na + -K + pump as its first step </li></ul><ul><li>Establishes a strong Na + gradient across the cell membrane </li></ul><ul><li>Glucose-Na + symport protein uses the Na + gradient to transport glucose into the cell </li></ul>
  30. 35. The Na + -Glucose Symport
  31. 36. Na + -Glucose Symport in Gut Epithelium <ul><li>Used in human gut epithelial cells </li></ul><ul><li>Cells take in glucose and Na + from the intestines </li></ul><ul><li>Transport them through the blood stream using Na + -glucose symports, glucose permease (a glucose facilitated diffusion protein) & the Na+/K+ pump </li></ul><ul><li>Epithelial cells are joined together by tight junctions </li></ul><ul><ul><li>Prevent anything from leaking through from the intestines to the blood stream without being filtered by the epithelial cells </li></ul></ul>
  32. 37. Na + -Glucose Symport in Gut Epithelium
  33. 38. Bulk Transport <ul><li>Allows movement across the cell boundary without passing through the membrane </li></ul><ul><li>Materials that cannot pass through the membrane need to be transported into or out of the cell </li></ul><ul><ul><li>e.g. droplets of fluid, particles of food, etc. </li></ul></ul><ul><li>  Endocytosis </li></ul><ul><ul><li>Bulk transport into the cell </li></ul></ul><ul><ul><li>Cell membrane encloses particle forming a pouch </li></ul></ul><ul><ul><li>Phagocytosis = feeding this way (amoebas, packman) </li></ul></ul><ul><ul><li>Pinocytosis = movement of liquids with solutes this way </li></ul></ul><ul><li>  Exocytosis </li></ul><ul><ul><li>the reverse - bulk transport out of cell </li></ul></ul><ul><li>Require energy </li></ul>
  34. 39. Endocytosis