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Membranes also act as barriers.
Cell membranes can let some compounds pass through them, while restricting most others. Water can usually pass freely, and its molecules tend to distribute themselves ("diffuse").

In the diagram below, let us assume that we have two solutions in physical communication, but separated by a membrane that will only let water through. 

  • Solution on the left is relatively short on salts (hypotonic
  • Solution on the right has a surplus of salts (hypertonic
  • Only water can move through the membrane
  • The relative "surplus" water in the left chamber would move into the right-hand chamber, making it swell, as seen on the right

What would happen if you put cells in water? Remember, that the cell membrane lets water through and cells have lots of salts and other dissolved substances, whereas water would have little salt.

osmosis.gif (11163 bytes)

If we think of the hypertonic solution as being in a red blood cell placed in a  beaker of water, the cells would swell up and burst, because water keeps diffusing in until the membrane breaks because of the internal pressure.

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