Transport Across the Membrane

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Transport Across the Membrane
2013-01-21 11:39:58

Advanced Higher Biology - Unit One - Transport Across the Membrane
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  1. The cell membrane is said to be _____ permeable?
  2. How are large molecules/cells taken into the cell?
    • Endocytosis - large molecules
    • Phagocytosis - another cell/big structure
    • Pinocytosis - liquid
  3. How are large molecules/particles removed from the cell?
    Exocytosis - eg waste products
  4. How can small molecules pass over the membrane?
    • Passive diffusion
    • Facilitated diffusion
    • Active transport
  5. What is passive diffusion?
    When lipid soluble molecules move across the membrane from a high to a low concentration down a concentration gradient (using no energy)
  6. What is facilitated diffusion?
    When lipid insoluble molecules move across the membrane using transport proteins - either carrier proteins or channel proteins.  This is a specific and passive process (still no energy is used).
  7. What is active transport?
    When molecules move across the membrane against a concentration gradient.  This requires energy and specific transport molecules
  8. What is an example of a specific case of active transport across the cell membrane?
    The sodium-potassium pump
  9. How many sodium ions does the sodium-potassium pump pump out of the cell?
  10. How many potassium ions does the sodium-potassium pump pump into the cell?
  11. Describe how the sodium-potassium pump works
    • Inside a cell there is a low concentration of sodium ions.  Outside a cell there is a low concentration of potassium ions.
    • Three sodium ions inside the cell cytosol binds to sites in the sodium-potassium pump
    • This binding enables the proteins ATPase function to hydrolyse ATP to ADP + Pi phosphorylating the protein (the phosphate adheres to the protein)
    • The binding and protein phosphorylation causes a conformational change (the ion pump opens up to the outside of the cell).  The phosphorylation of protein reduces the affinity for sodium)
    • The reduced affinity of sodium sites for Na+ means the sodium is released to the outside of the cell
    • When the sodium has been released, the potassium binding sites can then be occupied by two K+ ions from outside the cell
    • The binding of potassium causes the phosphate to be released
    • The dephophorylation restores the proteins original form.  The channel faces inside the cell and potassium is released into the cell.  Affinity for Na+ is high again.