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Egg Osmometers

Nancy Iversen, Cooperstown, New York


Aims: to examine the dynamics of concentration gradients and diffusion with a simple egg osmometer.

Materials:
  • fresh egg
  • drinking straws
  • hot glue gun with glue
  • distilled water
  • 50 ml beaker
Procedure:
  1. Prepare a fresh hen's egg by gently tapping on the shell at one end with a blunt instrument to crack the shell without damaging the underlying shell membranes.
  2. Remove the shell fragments, but do not remove or tear the membrane. Allow the contents of the egg to remain undisturbed.
  3. At the other end of the egg, remove a small portion of the shell and puncture a tiny hole in the membrane, as well. Allow the contents of the egg to stay inside the egg!
  4. Insert a plastic drinking straw into the top of the egg through the hole in the membrane.
  5. Seal the straw into position with a hot glue gun, being certain the seal does not leak.
  6. Set the membrane end of the egg in a 50 ml beaker of distilled water. Add more water through the beaker's spout from time to time as needed. Be sure the water covers the membrane at all times.

Observations:
  1. Check your osmometer frequently over the next twenty-four hour period. Each time you check it, record the time and measure the height of any fluid in the straw. Add more water as needed.
  2. Make a graph of fluid rise over time.

Discussion:
  1. Explain your observations in detail in terms of concentration gradient, diffusion, osmosis, osmotic pressure, passive transport, and active transport.
  2. How might we be able to use our simple egg osmometers to estimate the amount of pressure exerted on the egg membrane at any one time during the twenty-four hour observation period?

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