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Hello- We are doing an osmosis lab involving a decalcified egg in water, salt water and corn syrup to show how concentration affects the movement of water across a membrane. The egg shrinks in salt water and corn syrup and swells in fresh water due to osmosis- pretty straightforward stuff. One of the students asked what would happen if we put the egg in oil as opposed to the other solutions being tested- great idea, so we hypothesized and tested. We supposed that since oil has no water in it, it represents an area of higher concentration, and therefore the water should move out of the egg and into the oil. This did not happen. The egg did not appear to lose any water, and with testing, it actually seems to have gained a bit of mass. What gives? Is this due to the nonpolar nature of the oil? I am stumped and looking for a decent explanation for my students. Thanks for your help!
Answer 1:

You're right with regard to oil being non-polar and its connection to observing what you did. As you know water and oil do not fix very well--in the best case scenario they make emulsions (a suspension of two immiscible liquids). This is due to the fact that oil is hydrophobic. It is hydrophobic because it is non-polar while water is a highly polar molecule.

So in effect what you did by surrounding the egg with oil was create a membrane (egg shell and oil layer) that was impenetrable to water. While the water content was higher in the egg compared to the surrounding environment, it had nowhere to go.

As far as the egg gaining mass, this is probably a real effect. A number of biological molecules (fats, lipid membranes, and even proteins) are non-polar in nature. What you observed was the oil penetrating the egg shell (via the same phenomenon of osmosis). While the water couldnt cross the membrane, the oil could and did because it moved from a high concentration of oil (outside the egg) to a lower concentration (inside the egg)?joining some of the other non-polar biomolecules on and in the shell.


Answer 2:

This is a pretty cool demonstration of osmosis.The problem with the egg in oil is the fact that the oil is hydrophobic. So, the interaction of water and oil is too unfavorable for osmosis to occur. Additionally, because the oil is nonpolar it can interact with the phospholipid bilayer that is the cell membrane, which could lead to the weight gain that you are seeing. Typically, polar molecules cannot diffuse through the cell membrane and cells actually must undergo active transport in order to get ions, etc. into or out of the cell. If interested, I think this website does a really great job explaining cell membranes and different forms of transport across the membrane.


Answer 3:

In order for diffusion to occur, the two media on either side of the membrane need to be miscible (oil and water are not). If they are not,then no net transport can occur. If you tried this with 100% ethanol (nowater) then you would see the egg dehydrate, as you would expect.

The possible weight gain could have been due to residual oil sticking to the exterior membrane of the egg (oils and the fatty acid side chains of membrane lipids are both hydrophobic).



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