Ch 13 Text 5

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  1. ·         __ (cell-to-cell channels) are passageways between the interiors of contiguous cells and are clustered in discrete regions of the plasma membranes of apposed cells
    A __, the lumen of the channel, is prominent in each gap junction, spanning the __, between apposed cells
    • Gap junctions
    • 20-angstrom central hole
    • intervening space, or gap
  2. o   __ and __pass through
    §  All polar molecules with a mass less than 1 kd can pass; thus, __ and __ can flow between the interiors of cells joined by __
    o   __, __, and __ cannot pass
    • Small hydrophilic molecules and ions 
    • inorganic ions and most metabolites
    • gap junctions
    • Protiens, nucleic acids, and polysaccharides
  3. ·         Gap junctions are important for __
    o   Cells are coupled by the __ through the junctions, __them
    ·         Gap junctions are also important for __, as well as __ and __
    • intercellular communication
    • rapid flow of ions
    • synchronizing 
    • nourishment of cells that are distant from blood vessels
    • development and differentiation
  4. Structure of gap junctions
    • ·         12 molecules of connexin, which appears to have four-membrane spanning helices
    • o   Six connexin molecules are hexagonally arrayed to form a half-channel (connexon or hemichannel)
    • §  Two connexons are joined end to end to form a functional channel
  5. ·         Cell-to-cell channels differ from other membrane cannels in three respects:

    o They traverse two membranes, not one

    o They connect cytoplasm to cytoplasm, rather than to the extracellular space or the lumen of an organelle

    o They form a channel that is synthesized by different cells
  6. ·         They are closed by __ and __. The closing of gap junctions by Ca2+ and H+ serves to __. They can also be controlled by __ and __
    • high concentrations of calcium ion and by low pH
    • seal normal cells from injured or dying neighbors
    • membrane potential and by hormone-induced phosphorylation
  7. ·         __increase the rate at which water flows through membranes. In certain tissues, water must be rapidly transported
  8. Structure of aquaporins: __
    six membrane-spanning alpha helices. Two loops containing hydrophilic residues line the actual channel
  9. · Water passes through __; and, specific __ toward the center of the channel prevent the transport of protons through aquaporins
    • single file
    • positively charged residues
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Ch 13 Text 5
Test Three
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