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Cell membranes

 
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Last revised: Tuesday, October 19, 1999
Reading: Ch. 8 in text
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 1999. Thomas M. Terry

Models Of Membrane Structure

Diffusion And Size Limits

Osmosis And Water Balance

Membranes separate compartments of different concentration

Ion Extracellular Intracellular Difference
Na+ 140 mM 10 mM 14x
K+ 4 mM 140 mM 35x
Ca++ 2.5 mM 0.1 microM 25,000x
Cl- 100 mM 4 mM 25x

Movement Of Small Molecules Across Membranes can involve simple diffusion or protein-mediated transport

Some protein transporters require energy; others do not

Active transport can involve ATP pumps, symport, or antiport

  1. ATP pumps.
    • ATP-powered pumps (ATPases) couple the splitting, or hydrolysis, of ATP with the movement of ions across a membrane against a concentration gradient.
    • ATP is hydrolyzed directly to ADP and inorganic phosphate, and the energy released is used to move one or more ions across the cell membrane.
    • As much as 25% of a cell's ATP reserves may be spent in such ion transport.
    • Examples include:
      1. The Na+-K+ ATPase pumps Na+ out of the cell while it pumps K+ in. Because the pump moves three Na+ to the outside for every two K+ that are moved to the inside, it creates an overall charge separation known as polarization. This electrical potential is required for nervous system activity, and supplies energy needed for other types of transport such as symport and antiport.
        View animation of ATP pump
      2. Ca++ ATPases are responsible for keeping intracellular Ca++ at low levels, a necessary precondition for muscle contraction.
  2. Symport.
    • To transport some substances against a concentration gradient, cells use energy already stored in ion gradients, such as proton (H+) or sodium (Na+) gradients, to power membrane proteins called transporters.
    • When the transported molecule and the co-transported ion move in the same direction, the process is known as symport.
    • Example: transport of amino acids across the intestinal lining in the human gut.
    • View animation of symport
  3. Antiport
      Cell uses movement of an ion across a membrane and down its concentration gradient to power the transport of a second substance "uphill" against its gradient.
    • In this process, the two substances move across the membrane in opposite directions.
    • Example: transport of Ca2+ ions out of cardiac muscle cells. Muscle cells are triggered to contract by a rise in intracellular Ca2+ concentration, so it is imperative that Ca2+ be removed from the cytoplasm so that the muscle can relax before contracting again. This antiport system is so effective that it can maintain the cellular concentration of Ca2+ at levels 10,000 times lower than the external concentration.
    • View animation of antiport

Movement Of Large Molecules: Endocytosis, Exocytosis, Phagocytosis, Carrier-Mediated Endocytosis


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