Chapter 5

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Chapter 5
2010-09-14 19:48:56

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  1. Membrane Structure
    • The fluid mosaic model of membrane structure contends that membranes consist of:
    • -phospholipids arranged in a bilayer
    • -globular proteins inserted in the lipid bilayer
  2. Cellular membranes have 4 components:
    • 1. phospholipid bilayer
    • 2. transmembrane proteins
    • 3. interior protein network
    • 4. cell surface markers
  3. Membrane structure is visible
    using an electron microscope.
  4. Transmission electron microscopes (TEM)
    can show the 2 layers of a membrane.
  5. Freeze-fracturing techniques
    separate the layers and reveal membrane proteins.
  6. Phospholipids
    • Phospholipid structure consists of
    • -glycerol
    • – a 3-carbon polyalcohol acting as a backbone for the phospholipid
    • -2 fatty acids attached to the glycerol
    • -phosphate group attached to the glycerol
  7. fatty acids
  8. are nonpolar chains of carbon and hydrogen.
  9. -Their nonpolar nature makes them
    hydrophobic ("water-fearing").
  10. The phosphate group is
    • polar and hydrophilic ("water-loving").
    • The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer:
    • -fatty acids are on the inside
    • -phosphate groups are on both surfaces of the bilayer
  11. Phospholipid bilayers are fluid.
    • -hydrogen bonding of water holds the 2 layers together
    • -individual phospholipids and unanchored proteins can move through the membrane
    • -saturated fatty acids make the membrane less fluid than unsaturated fatty acids
    • -warm temperatures make the membrane more fluid than cold temperatures
    • Phospholipids
  12. Membrane proteins have various functions:
    • 1. transporters
    • 2. enzymes
    • 3. cell surface receptors
    • 4. cell surface identity markers
    • 5. cell-to-cell adhesion proteins
    • 6. attachments to the cytoskeleton
  13. Peripheral membrane proteins
    • -anchored to a phospholipid in one layer of the membrane
    • -possess nonpolar regions that are inserted in the lipid bilayer
    • -are free to move throughout one layer of the bilayer
  14. Integral membrane proteins
    • -span the lipid bilayer (transmembrane proteins)
    • -nonpolar regions of the protein are embedded in the interior of the bilayer
    • -polar regions of the protein protrude from both sides of the bilayer
  15. Integral proteins possess at least one transmembrane domain
    • -region of the protein containing hydrophobic amino acids
    • -spans the lipid bilayer
  16. Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane.
    • -b sheets in the protein secondary structure form a cylinder called a b-barrel
    • -b-barrel interior is polar and allows water and small polar molecules to pass through the membrane
  17. Passive Transport
    • Passive transport is movement of molecules through the membrane in which
    • -no energy is required
    • -molecules move in response to a concentration gradient
  18. Diffusion
    is movement of molecules from high concentration to low concentration
  19. Selective permeability:
    integral membrane proteins allow the cell to be selective about what passes through the membrane.
  20. Channel proteins
    have a polar interior allowing polar molecules to pass through.
  21. Carrier proteins
    bind to a specific molecule to facilitate its passage.
  22. Channel proteins include:
    • -ion channels allow the passage of ions (charged atoms or molecules) which are associated with water
    • -gated channels are opened or closed in response to a stimulus
    • -the stimulus may be chemical or electrical
  23. Carrier proteins
    bind to the molecule that they transport across the membrane.
  24. Facilitated diffusion
    • is movement of a molecule from high to low concentration with the help of a carrier protein.
    • -is specific
    • -is passive
    • -saturates when all carriers are occupied
  25. In an aqueous solution
    • -water is the solvent
    • -dissolved substances are the solutes
  26. Osmosis
    • is the movement of water from an area of high to low concentration of water
    • -movement of water toward an area of high solute concentration
  27. When 2 solutions have different osmotic concentrations
    • -the hypertonic solution has a higher solute concentration
    • -the hypotonic solution has a lower solute concentration
  28. Osmosis moves water through aquaporins toward
    the hypertonic solution.
  29. Organisms can maintain osmotic balance in different ways.
    • 1. Some cells use extrusion in which water is ejected through contractile vacuoles.
    • 2. Isosmotic regulation involves keeping cells isotonic with their environment.
    • 3. Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid.
  30. Active Transport
    • -requires energy – ATP is used directly or indirectly to fuel active transport
    • -moves substances from low to high concentration
    • -requires the use of carrier proteins
  31. Carrier proteins used in active transport include:
    • -uniporters – move one molecule at a time
    • -symporters – move two molecules in the same direction
    • -antiporters – move two molecules in opposite directions
  32. Sodium-potassium (Na+-K+) pump
    • -an active transport mechanism
    • -uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell
    • -ATP energy is used to change the conformation of the carrier protein
    • -the affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane
  33. Coupled transport
    • -uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule
    • -a symporter is used
    • -glucose-Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient
  34. Bulk Transport
    • Bulk transport of substances is accomplished by
    • 1. endocytosis – movement of substances into the cell
    • 2. exocytosis – movement of materials out of the cell
  35. Endocytosis
    • occurs when the plasma membrane envelops food particles and liquids.
    • 1. phagocytosis – the cell takes in particulate matter
    • 2. pinocytosis – the cell takes in only fluid
    • 3. receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor
  36. Exocytosis
    • occurs when material is discharged from the cell.
    • -vesicles in the cytoplasm fuse with the cell membrane and release their contents to the exterior of the cell
    • -used in plants to export cell wall material
    • -used in animals to secrete hormones, neurotransmitters, digestive enzymes