|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!
|Question Date: 2010-11-09|
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
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.
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.
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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