Osmosis
is the process by which water enters our tissues. All of our cells
are surrounded by a membrane that selectively allows in anything
the cell needs but prevents unwanted molecules from entering.
This works because the membrane contains lots of tiny holes that
will let anything smaller through but will stop big molecules
from entering the cell. Water is small enough to get through and
enters by a process called diffusion - this means that it will
travel from an area with a high water concentration to an area
with a lower concentration until it is balanced out with both
areas having the same concentration (known as isotonic).
An egg also has a membrane surrounding it so we can use it to
represent a cell and see how osmosis works.
The first stage of the experiment was to soak the egg in vinegar.
You will have seen that the shell completely disappears. Vinegar
in fact contains acetic acid and this reacts with the calcium
carbonate making up the shell of the egg. This reaction gives
off carbon dioxide so if you were paying attention you will have
seen bubbles coming off the egg as soon as it was added to the
vinegar.
The membrane around the egg also becomes very rubbery during
this soaking in vinegar. This occurs because the acid denatures
the protein making up the egg white (albumin). Denaturation of
proteins can occur through exposure to acids, bases or high temperature.
This is what happens when you cook an egg- the clear egg white
turns opaque and white as the protein chains become tangled due
to the heat.
If you soak a cooked egg in vinegar it also becomes very rubbery
- so much so that it will bounce when dropped from a low height!
It's not a good idea to try this will an uncooked egg as inside
the rubbery membrane the yolk and white are still runny and it
will make a big mess if split!
So after soaking in vinegar you should have also noticed that
the egg increases a little in size. This is because the water
in the vinegar can enter the egg through the membrane, moving
from the higher water concentration in vinegar to the lower concentration
in the egg.
If you now put the egg in water, it will grow in size a lot more
due to the much larger water concentration gradient across the
membrane. If you add food coloring to the water you can see the
process of osmosis in action as the coloured water passes into
the egg. Water is known as hypotonic, ie. very dilute and contains
more water than the egg.
If instead you place the vinegar soaked egg in corn syrup you
will see the opposite. There is a much higher water concentration
in the egg than in the syrup so water will pass in the opposite
direction. This means that the egg will shrink in size. The corn
syrup is a hypertonic liquid, ie. very concentrated with not much
water compared to the egg.
