Name________________________ Period ___
Egg Experiment: Introduction to Osmosis
I. What is happening to these eggs?
Most cells are tiny – much too small to see without the help of a microscope. In contrast, an
unfertilized chicken egg is a giant cell. You will use a chicken egg to investigate movement of water
across the cell membrane that surrounds each cell.
1. Why is it important for each cell to be surrounded by a cell membrane that can prevent large
molecules from leaving the cell?
2. The cell membrane allows some small molecules like oxygen and carbon dioxide to cross. Why
is it important for oxygen and carbon dioxide to be able to cross the cell membrane?
In this investigation you will see that water can cross the cell membrane surrounding an egg. You
will investigate which way water moves across the membrane, depending on the type of liquid
surrounding the egg. When water moves across the cell membrane, the egg changes in size and
appearance.
Your group will be given two eggs. To begin, record the weight and circumference of these eggs in
the day 1 row of the table. (Measure the circumference around the widest part, not lengthwise.)
Caution: Because these are raw eggs, they may carry salmonella, so you should use gloves when
handling the eggs.
Day
1
2
Weight (grams)
Egg 1
Circumference (cm)
(with shell)
Egg put into vinegar
(most of shell
removed)
Egg put into water
Egg 2
Weight (grams)
Circumference (cm)
(with shell)
Egg put into vinegar
(most of shell
removed)
Egg put into corn syrup
3
Put each egg in a container labeled Egg 1 or Egg 2 with enough vinegar to cover the egg. Cover the
container. Do you see bubbles forming around the egg? These are bubbles of CO2 which result from
the chemical reaction between the acetic acid in the vinegar and the calcium carbonate in the
eggshell. This reaction will dissolve most of the eggshell by day 2.
Day 2
Name____________________Period___
DIRECTIONS: Observe your eggs. Notice that most of the shell has been dissolved by the acetic acid
in the vinegar. Although most of the shell is gone, each egg is still surrounded by a shell membrane
outside the cell membrane. The shell membrane has protein fibers that give it much greater
strength than the cell membrane. However, the egg without its shell is still fragile, so you will need
to handle your eggs very gently and carefully!
Dry each egg and measure the weight and/or circumference of each egg. Record your results
for day 2 in the table on page 1.
3. Did the eggs become heavier/larger or lighter/smaller? What do you think happened to cause
this change in weight/size?
DIRECTIONS: Empty the vinegar from the container for egg 1 and replace it with water to cover the
egg. Empty the vinegar from the container for egg 2 and replace it with corn syrup to cover the egg.
As you pour the corn syrup, notice that it is viscous (thick, sticky).
4. What do you think causes the corn syrup to be so viscous?
Day 3
Name____________________Period___
5. Compare and contrast the appearance of the egg that has been in water vs. the egg that has
been in corn syrup.
6. You should be able to observe a layer of water on top of the corn syrup. Where do you think
this water came from?
DIRECTIONS: Rinse the corn syrup off of egg 2. Dry each egg and measure and record the weight
and/or circumference for day 3 in the table on page 1.
7. What happened to cause the change in weight/volume of the egg placed in corn syrup?
8. Why did the egg placed in water get bigger and heavier? Where did the additional
weight/volume come from?
II. Understanding Osmosis
The cell membrane that surrounds each cell is a selectively permeable membrane. A selectively
permeable membrane allows some types of molecules and ions to cross the membrane and
prevents other types of molecules and ions from crossing the membrane.
Osmosis is the diffusion of water across a selectively permeable membrane.
1a. During diffusion, more molecules will move
a. from regions of higher concentration to
regions of lower concentration
b. from regions of lower concentration to
regions of higher concentration
1b. The reason is that:
a. Crowded molecules want to move to an area with more room
b. Molecules tend to keep moving until they are uniformly distributed and then they stop moving.
c. The random motion of molecules results in their uniform distribution in the available space.
Explanation of Osmosis
Na+, Cl-, and the water molecules that are bound to
these ions cannot cross the selectively permeable
membrane (called a semipermeable membrane in this
figure). Only free water molecules (water molecules that
are not bound to ions or other solutes) can cross the
selectively permeable membrane.
During osmosis, diffusion results in movement of free
water molecules in both directions across the selectively
permeable membrane, but more free water molecules
move from the region of higher concentration of free
water molecules to the region of lower concentration.
2. Why is the concentration of free water molecules
lower in water with dissolved salt and higher in pure
water?
3. Why does osmosis result in a net flow of water from the side of the tube that has pure water to the
side of the tube that has water with dissolved salt? Include in your explanation the relative
concentrations of free water molecules on the two sides of the tube.
This figure shows the effects
of osmosis on animal and
plant cells put in three
different types of
surrounding fluid.
A
B
C
4a. Which animal cell looks
like the egg in corn syrup? ___
4b. Which animal cell looks
like what could have
happened to the egg in water,
if the egg didn't have a strong
shell membrane around it? __
Inside the cell membrane, each cell contains cytosol, a watery substance with a high concentration
of dissolved molecules and ions.
5. Which has a higher concentration of free water molecules –
the cytosol inside a cell ___ or pure water___ ?
If a cell is surrounded by pure water, will more water diffuse into the cell ___ or out of the cell __?
Explain your reasoning.
Does this situation match A __ or B __ or C __ in the figure?
6. If very salty water has a higher concentration of dissolved substances than cytosol, which has a
higher concentration of free water molecules
– the cytosol inside a cell ___ or the very salty water ___ ?
If a cell is surrounded by very salty water,
will more water diffuse into the cell ___ or out of the cell ___?
Explain your reasoning.
Does this situation match A __ or B __ or C __ in the figure?
7. Most animal cells are surrounded by a layer of water with dissolved substances. For animal cells
to function normally, there should be equal amounts of water diffusing into and out of the cell, as
shown in figure B on the previous page. Which type of surrounding fluid would result in equal
amounts of water diffusing into and out of a cell?
__ water with a higher concentration of dissolved substances than the cytosol
__ water with a lower concentration of dissolved substances than the cytosol
__ water with the same concentration of dissolved substances as the cytosol
8. If a person drinks a very large amount of water in a short time, this may result in confusion,
seizures, coma, or even death, due to abnormal functioning of nerve cells in the brain. Explain how
these problems could result from drinking too much water too rapidly.
9. Explain why the surrounding fluid has a different effect on animal cells vs. plant cells in figure C
on the previous page.
10. Suppose that an animal's cells had cell walls. What problems would this cause for the animal?
Challenge Questions
What do you think is the reason that a person who is stranded at sea should not drink ocean water?
How could drinking salty water harm a person's cells?
Some archaea (single cell organisms) live in extremely salty water such as the Great Salt Lake or the
Dead Sea. Most types of cells would shrivel and die in this very salty water. How do you think these
archaea prevent water loss while living in very salty water?