Biology 11A Lab #3
Osmosis and Diffusion
Pre-Lab Exercise- Answer the questions below. Please use complete sentences.
Do not use the internet as it will give you definitions that are out of context with
what we are studying. It is best to use your text.
1. Let’s say you are making sugar syrup to feed your honeybees hive. You get a
cup filled with water and add 1 TBS sugar and set it aside.
In your mixtures, tell me which is your
a. Solute
b. Solvent
c. Solution
Define:
d. Concentration:
e. Concentration gradient:
f. Hypertonic solution:
g. Isotonic solution:
h. Hypotonic solution:
i. Crenation
2. How does diffusion differ from osmosis?
3. What conditions must be present for diffusion to occur? Explain.
4. What conditions must be present for diffusion to stop occurring? Explain.
5. Does movement of molecules ever completely stop occurring? Explain.
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Lab #3: Molecular Motion
Work in groups of three to four
This lab is designed to illustrate some principles of molecular motion: Diffusion, and
osmosis.
Today’s Objectives
1. Observe diffusion in a liquid.
2. Observe osmosis using a decalcified egg as a model "cell".
Activity 1 : Diffusion
Diffusion is the movement of solutes from an area of high concentration to an area of
low concentration.
Solutes are molecules that are dissolved in water. For example, each ion of a
dissociated salt is a solute. A single glucose molecule is another example of a solute
dissolved in water.
The movement of solutes along a concentration gradient is the driving force behind
many biological processes that occur in our bodies. For example, diffusion is important
to the maintenance and generation of nerve impulses, gas exchange in our lungs during
respiration, and the removal of waste products from our blood during the production of
urine.
In this demonstration you will make observations of diffusion through a semi permeable
membrane and be able to compare these observations to the functions of a cell
membrane. Iodine and starch are used in this demo because iodine is a known
indicator for starch. Iodine will change color from yellow to black IF in the presence of
starch.
If you are allergic to iodine, ask for gloves to do this experiment.
Materials needed:
• 6” piece of dialysis tubing
• String to tie tubing closed at both ends
• 20-25 drops of Iodine
• 100 ml of Starch Solution
• 1 large 250 ml beaker
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Methods:
1. Soak your piece of dialysis tubing for about 5 min. to unstick the sides. You
need to turn what looks like a strip of cellophane into a tube. The sides tend to
be stuck together.
2. Tie one end STRONGLY, with 2 separate knots so that you can fill the tube with
starch solution.
3. Fill the tube with starch solution allowing space at the top to tie the other end,
again with 2 knots. You should have at least two inches of filled tubing.
4. Rinse the whole filled tube to remove any starch that may have spilled to the
outside of the bag.
5. Fill a beaker halfway with distilled water and add 10-15 drops of iodine.
6. What do you think will happen when you put the dialysis bag in the iodine water
mixture?
7. Write down your predictions.
8. Place the tube in the beaker so that the cornstarch mixture is submerged in the
iodine water mixture
9. Wait for 1 hour. Proceed on to the osmosis experiment and come back to look
at your result later. Record and sketch results for before and after the hour.
Remember to be accurate and label the contents and materials.
10. What happened? How do you explain the phenomenon?
11. What would happen if you did an investigation in which the iodine solution was
placed in the tube and the starch solution were placed in the beaker? Explain.
12. How does this demonstration compare to the function of a cell membrane?
Draw a picture and provide examples in your explanation.
13. In this experiment, identify the
semipermeable membrane
solute that moves (why does it move?)
solute that does not move (why does it not move?)
solvent
Clean-up: The iodine may NOT GO DOWN THE DRAIN. Please dispose of in the
container provided. The filled tubes may go in the trash.
Activity 2. Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane. A
selectively permeable membrane allows water, but not most solutes, to freely pass.
The plasma membrane of a cell is a good example of a selectively permeable
membrane. When solutes cannot freely move in the presence of a concentration
gradient across a membrane, water will move instead in an attempt to equalize the
concentration of solutes on both sides of the membrane. Another way of thinking
about osmosis is the diffusion of water from a high concentration of water (what we
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usually think of as a dilute solution) to a low concentration of water (what we think of
as a more concentrated solution).
For example, if a red blood cell (RBC) is placed in a solution that contains a higher
concentration of solutes than what is normally found inside the cell (a hypertonic
solution), water will move from inside the cell in an attempt to dilute the solutes outside
the cell. The RBC will shrivel up as a result of the loss of water. The name for this is
crenation.
Conversely, if a RBC is placed in a solution containing a lower concentration of solutes
(a hypotonic solution), water will move from the outside to the inside of the cell. This
will result in the cell bursting from the excess of water moving in. This is called lysis.
It is important to use a solution that contains the same solute concentration as blood
(an isotonic solution) when administering fluids intravenously to prevent bursting
(hemolysis) or shriveling (crenation) the cells.
In the following experiment, you will be using decalcified eggs as model cells. The eggs
have been treated with vinegar to remove the calcium from the shell, leaving behind a
membrane that is permeable to water (solvent), but not to other molecules (solutes).
Materials:
• 4 decalcified eggs
• Beakers containing solutions A, B, & C
• Beaker containing solution X (unknown)
1 weigh boat for the eggs
scale
Paper towel
Procedure 2A:
1. Obtain four decalcified eggs. Gently dry and weigh each egg before immersing it in
Solution A, B, or C. Dry the egg by gently rolling it on a paper towel. Do not dry
the egg for too long because the paper towel will begin to draw out water from
inside the egg and will change the weight of the egg. Record the weight of the egg.
2. Let the three eggs soak in solutions A, B, and C for 20 minutes. Go on to Procedure
2B while you are waiting.
3. After 20 minutes, dry and weigh each egg and record your results.
4. The change in weight reflects the movement of water into or out of the egg. Based
on the movement of water, determine if the Solutions A, B, and C are hypo-, iso-, or
hypertonic.
Procedure 2B:
1. Gently dry and weigh your last egg. Record the results.
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2. Immerse the egg in solution X. Dry and reweigh this egg every two minutes for
about a half hour.
3. Plot your results on a graph to show the rate of water movement into or out of the
egg.
Data Sheet: Include in lab report. Copy them or cut them out and paste them (with
glue, not staples or electrical tape!) in your write-up. Make it look professional.
Results 2A: Determining the Tonicity of Solution A, B, & C
Weight
Before
Soaking
Weight After
Soaking
Difference in Wt.
Tonicity of Solution
(hyper-, hypo-, or iso-)
Egg in Solution A
Egg in Solution B
Egg in Solution C
Results 2B: Egg X
Time
(minutes)
0
2
4
6
8
10
12
14
Weight
(grams)
Change in
Weight
(grams)
Time
(minutes)
Weight
(grams)
Change in
Weight
(grams)
16
18
20
22
24
26
28
30
5
Diameter
(mm)
Time (minutes)
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Write-up Directions and Questions for Osmosis and Diffusion Lab
Due next week.
Diffusion Section (Part I)
Give a brief description of the methods you used for the diffusion experiment with the
dialysis tubing. If drawings make it clearer, you can do that. Do not copy the handout.
Summarize what you did instead. This should be one paragraph with 4-5 sentences. Next,
answer the following questions relating to the diffusion study.
1. What made the iodine move into the dialysis tube? Why didn’t the starch move too?
2. Give an estimate of the volume of iodine that moved in 15 minutes.
3. Would all the iodine move in if you left it soaking overnight? Explain.
Don’t forget to use complete sentences.
Osmosis (Part II)
For this experiment you will need to type up the results. You can glue or tape the charts
that you completed from page 5 (or copy them over), and the graph from page 6. Give
the tables and the graph a caption that briefly describes them. Give an explanation of
what the results mean.
Next, answer the following questions relating to the eggs experiment.
Please use complete sentences. These are explanations, they need to be clear.
4. Based on the change in the weight of the eggs, what is the tonicity of each solution?
5. What conditions would cause the water to move into the egg?
6. What conditions would cause the water to move out of the egg?
7. What was the source of energy allowing for the movement described in question 6? Is
this an example of active transport or passive transport?
8. Why is the egg considered a “model” for cells and not a true animal cell? (Is it not a
chicken egg, meaning one cell?) Is the soft membrane of the egg the same as the lipid
bilayer?
9. What are the consequences (to a red blood cell) of excessive movement of solvent into
it? What about excessive movement out of a red blood cell?
10. What would happen to a patient if instead of receiving saline solution in the vein after
surgery, the patient received tap water instead? Explain.
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