I (an American) was discussing science education with my colleague Lesley (a Kiwi) at work (in Scotland) one day. I mentioned that the definition of osmosis had been so thouroughly drilled into me half a lifetime ago by my manic biology teacher that I could probably still recite it. She looked doubtful, so I started off:

"The movement of a substance from an area of greater concentration... (Lesley joined in)
... to an area of lesser concentration... (Alison, educated in South Africa, looked up from her desk and chimed in)
... through a semi-permeable membrane... (Sandy, passing by, added the benefit of his Scottish education)
... to reach a state of equilibrium!" We finished triumphantly, in unison.

Our educational experience spanned four continents, but only one rote memorized definition of osmosis. The existence of the global village may be debatable, but we clearly all went to the global village school.

"Osmosis is the diffusion of a solvent across a selectively permeable membrane from an area of lesser solute concentration to an area of greater solute concentration towards equilibrium."


Okay. You can see osmosis for yourself, and it looks pretty cool. All it requires are:

Here's what to do:

  1. Important: You are working with a RAW EGG here. Raw eggs can cause various diseases, so make sure to wash your hands often during this experiment. It's a good idea for kids to get parental permission and supervision for this experiment.
  2. Put the egg in the container.
  3. Fill the container with enough vinegar to submerge the egg.
  4. Wait overnight. The shell will dissolve.
  5. Dump the vinegar out of the container, but make sure you don't dump the egg out too. Be EXTREMELY CAREFUL with the egg from now on, because the shell will not be able to protect it.
  6. Observe the size of the egg.
  7. Add enough karo syrup to submerge the egg.
  8. Wait overnight again.
  9. Observe the size of the egg. It should now be significantly smaller than before.
  10. Dump out the karo syrup from the container.
  11. Fill the container again with enough water to submerge the egg. You may want to rinse out the container and CAREFULLY rinse off the egg with water first, to wash away the karo syrup.
  12. Wait overnight again. (Sorry about these waits, it just takes time)
  13. Observe the size of the egg. It should now be even bigger than when you started off. If you have clean enough water, you can sometimes even get the egg to rupture!.
  14. Dispose of the water and egg.
  15. Here's how it worked:

    You dissolved the shell, which protects the egg from (among other things) just the torture you were about to give to it!

    You put it in karo syrup. The karo syrup is very thick and has a lot of stuff (solutes) in it, dissolved in water, (a solvent). The egg has some solutes in it, but significantly less than the karo syrup. The water in the egg moved outside the egg's plasma membrane, which all cells have. That's right: an egg is really an oversized cell. The water moved outside the egg to balance out the karo syrup.

    You put it back in (somewhat) pure water. The water rushed into the cell to balance out the stuff in the cell, making it larger than before... even before the karo syrup. If enough water moves into the egg, the membrane, which is stretchy, like a balloon, will POP.

    Look at these diagrams to see what I mean:

    Cell in normal environment:
    \                     /
     | .:      :     .:  |
     |    ___________    |
     |   /  egg      \ . |
     |  | .      .    |  |
     |: |  . . :  .   |: |
     |   \___________/   |
     | .   :     .   .   |
    Cell in karo syrup
    \                     /
     | .:  ::::: ... .:  |
     |::: ___________   :|
     |   /  egg      \ . |
     |::| .      .    |  |
     |: |  . . :  .   |: |
     |:: \___________/   |
     | .  :::::::. : . ::|
    Cell then gets smaller due to
    water going out:
    Note how the egg gets more
    densely packed with stuff
    (solute), and the extra water
    spreads the solute in the
    karo syrup out
    \                     /
     | .:  :  :  ... .:  |
     | . ::    .:  :.    |
     |:   ___________   :|
     |   /  egg      \ . |
     |: | . :::  . .. |  |
     |:  \___________/   |
     | .  :: : : . : .  :|
    Cell placed in water:
    \                     /
     | .                 |
     |         .    .    |
     |    ___________    |
     |   /  egg      \ . |
     |  | . :::  . .. |  |
     |   \___________/   |
     | .             .   |
    Cell adjusts to water:
    cell grows, and solutes
    inside the cell space out.
    The container's solutes
    are pushed together by
    the growing egg.
    \                     /
     |  . ___________    |
     |   /  egg    . \ . |
     |  |      . .    |  |
     | .|  ..         |  |
     |  | .   .  .  . |  |
     |   \___________/   |
     | .          .  .   |

    Now back to the original definition, stuff in parentheses are added to explain:

    "Osmosis is the diffusion (free movement of something on a small scale) of a solvent (water) across a selectively permeable membrane (the egg's thin membrane) from an area of lesser solute concentration (less stuff dissolved in the liquid) to an area of greater solute concentration (more stuff dissolved in the liquid) towards equilibrium (so that the amount of stuff dissolved inside and outside the egg are about the same)."

    Now doesn't osmosis sound a lot easier (and more fun?)

Osmosis is the movement of a fluid across a semipermeable membrane from a solution with lower solute concentration to a solution with higher solute concentration. Osmosis continues until equilibrium is achieved.

The above statement, though scientifically precise and accurate, contains many technical terms that should be further elucidated. A membrane is a thin layer of tissue (in plants and animals) or synthetic material (in a machine, or a laboratory setup). A semipermeable membrane is one which allows molecules of certain substances to pass through it, but is an effective seal against any other substance. A basic definition of a solution, for someone without a background in chemistry, would be to describe it as a crystallized or powdered substance (the solute) dissolved in a liquid substance (the solvent). Equilibrium, in this case, is the condition where the solutions on both sides of the membrane have equal concentrations of solute.

Osmosis is a vital process in your body’s endless quest to maintain homeostasis, that is, a consistent and healthy biochemical state. The term osmosis is most commonly used in the special case where the fluid in question is water, and the semipermeable membrane is that of a cell. The cells expend water in the fulfillment of their natural functions, thus increasing the solute concentration inside of them. If you follow a healthy diet, you drink at least eight glasses of water in a day; this water decreases the solute concentration in the cells’ surroundings. Thus, water moves into the cells and allows them to continue fulfilling their duties. This is also why drinking salt water (if you are shipwrecked, etc.) is a very bad idea. The fluid entering your body will have a solute concentration higher or equal to that of your depleted cells, so no water will move into the cells. Dehydrated, they will not function properly, and you will die.

Os*mo"sis (?), n. [NL.]



© Webster 1913.

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