Diffusion and Osmosis
  1. diffusion and osmosis are underlying physical processes of physiological function
    1. diffusion: movement of molecules from areas of high to low concentration
      1. solute: molecules in a solution
    2. membranes may be permiable to only some solutes (molecules in solution)
        3. if a membrane is permiable, the solute will move from the side of high to that of low concentration
      1. examples: small atoms such as sodium, chloride, small fat-soluble molecules such as small hormones
    3. nearly all membranes are water-permiable
      1. even if the solute cannot move, the water can
      2. water, like solutes, move down the gradient of concentration
    4. osmosis is the movement of water molecules across a membrane
      1. movement of water into a cell can make it full (turgid) or even burst (lysis)
      2. movement of water out of a cell can make it loose (flacid)
    5. if there is no net movement, then the cell is said to be isotonic to its environment
    6. Isotonic conditions may exist even if the solutes are not the same
      1. positive salt ions can "balance" each other (i.e., Na+, K+)
      2. organic "salts" (charged molecules) can balance inorganic salts
    7. solutes may be actively or passively transported across membranes
    8. to date no organisms are known that can actively transport water
  2. Scaling and diffusion
    1. physiological processes are functioning in a volume that is separated from ambient conditions by a surface
    2. diffusion and osmosis occur across the surface
      1. surface area scales to the square of the length of the cell or organism
      2. volume scales to the cube of the length of the cell or organism
        1. small organisms will have a high surface area:volume ratio
        2. large organisms will have low surface area:volume ratio
      3. there are advantages and disadvantages to large (or small) size
        1. small organisms can rely on diffusion to transport most substances
        2. small organisms will more rapidly change chemistry  in response to ambient changes
Study problems

1.  If a cell becomes turgid when bathed in a test solution, the test solution is, osmotically

  1. similar to sea water
  2. lower in solute concentration than the cell
  3. about the same solute concentration as the cell
  4. higher in solute concentration than the cell
2.  One of the major physiological advances in arthropod evolution was the development of a waxy outer coating on the exoskeleton (making the exoskeleton water-resistant).  Why might this be a great advantage to a very small organism living on dry land (hint:  evaporation can be considered as osmosis, with water moving from an area of high concentration to low concentration).

3.  Many "primative" invertebrates have open circulatory systems:  the internal organs are bathed in body fluids, and there may not even be a heart to circulate these fluids.  Would you expect these to be larger or smaller animals?

4.  You find a small animal in a puddle of salt water at the beach during low tide. You know that such puddles become increasingly concentrated on sunny days, as the water evaporates leaving behind the salts.  The creature seems to be quite alive and well.  What simple hypothesis could you form about this animals ability to regulate its osmotic conditions relative to the environment?  What data do you need to test your hypothesis?  (i.e., what data would lead you to reject your hypothesis?)

See also page 87 Content revies #4, 5, 6;  Conceptual review #2, 3

Answers