NAME:___________________PERIOD:_____DATE:_______
Lab: Diffusion and Osmosis
Introduction:
Membranes surround cells and cellular organelles. These membranes act as
“gateways” between the outside environment and a cell’s cytoplasm. Membranes are
very important to the function of the cell, and allow only certain molecules to enter and
exit the cytoplasm freely. Because the cell’s membrane is more permeable to some
substances than others, it is said to be selectively permeable.
Diffusion is the movement of molecules from an area of high concentration to an
area of lower concentration. Since molecules are in constant motion, they will bounce
off each other and flow toward an area of fewer molecules (low concentration. The
difference between molecules of high concentration and low concentration is called a
concentration gradient. When a concentration gradient exists, diffusion will take place
and molecules will move until equilibrium of molecules exists across the gradient.
Osmosis is a process by which water moves through a selectively permeable
membrane. The osmotic process is a special case of diffusion involving the diffusion of
a solvent, such as water, rather than the diffusion of substances (solutes) dissolved in
the solvent. Osmosis takes place whenever there is an unequal concentration of water
on either side of a selectively permeable membrane. Water will continue to move
across the concentration gradient until equilibrium occurs.
Pre-Lab Discussion: (1pt each)
1. How are osmosis and diffusion similar and different?
2. Explain how the smell of a candle moves from one side of the room to the other.
3. Why is osmosis and diffusion important?
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Objectives:
 Demonstrate diffusion of molecules through a selectively permeable membrane.
 Determine which molecules are able to pass through a membrane and which
molecules are not able to pass.
 Demonstrate osmosis through in potato cells.
Experiment A: Diffusion
Materials:
 1 Beaker
 1 12” section of dialysis tubing
 1 pipette
 Graduated cylinder
 Glucose/Starch solution
 Iodine Potassium Iodine (IKI)
Procedure:
1. Pour 15ml of prepared glucose/starch solution into a graduated cylinder.
2. Obtain a piece of dialysis tubing from the beaker of water. Tie a tight knot in one
end of the tubing, or use a piece of string to tie off the end.
3. Open the tubing by rubbing the untied end between your fingers. Pour 15ml of
the glucose/starch solution into the tubing.
4. Note the color of the solution in the bag. Record the color in table 1.
5. Carefully tie a knot in the open end to seal the bag. Be sure to leave enough
space in the bag for expansion.
6. Rinse the dialysis tubing off in the sink. Be gentle.
7. Fill a beaker approximately 2/3 full with distilled water. Add 1ml of IKI to the
water in the beaker.
8. Note the color of the solution. Record the color in table 1.
9. Completely immerse the dialysis bag in the solution in the beaker.
10. Wait 30 minutes.
11. Remove the dialysis bag from the beaker. Record the final color of the solutions
in the bag and the beaker in Table 1.
Experiment B: Osmosis
Materials:
 2 potato slices
 Cookie cutter
 2 small cups
 2 ziploc baggies
 Sugar
 Water
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Procedure:
1. Label one cup “water” and the other cup “sugar + water”.
2. Label one baggie” water” and the other baggie “sugar + water”.
3. Put 200ml of water in each cup.
4. Add 4tbs of sugar to the cup labeled “sugar + water”.
5. Use a cookie cutter to cut a shape out of the center of each potato slice. Make sure
you keep the each outer section of the potato slices you just cut out with them.
6. Put one cut out shape into cup labeled “water” and its outer portion into the baggie
labeled “water”.
7. Put the other cut out shape into the cup label “sugar + water” and its outer portion
into the baggie labeled “sugar + water”.
8. Let them sit undisturbed over night.
9. The next day, remove the potato piece from each up and try to insert the cut out
shape back into the correct outer section.
10. Observe and record your results in table 2.
Data and Observations:
Table 1 (4pts)
Color
Dialysis Bag
Beaker
Time
Start
30 Minutes
Table 2 (4pts)
Solution
Observation
H2O
sugar + H2O
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Type of Solution
(hypertonic, hypotonic,
isotonic)
Analysis: (1pt each)
Experiment A: Diffusion
1. IKI when mixed with starch reacts to cause a color change from clear to a
blue/black? Explain.
2. Which, if any, substance(s) do you believe did not diffuse through the membrane?
Why?
3. Was there any evidence that osmosis occurred through the dialysis tubing? Explain.
4. Molecules of a similar substance are about the same size, whereas molecules of
different substances are known to have different sizes. By looking at your results,
can you determine the relative sizes of molecules that did or did not diffuse across
the dialysis membrane? Explain.
5. Can it be said that the dialysis membrane is similar to a plasma membrane?
6. Predict what would happen if you allowed the dialysis tubing to sit in the beaker
overnight?
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7. Below is a diagram of a chamber separated by a semi-permeable membrane. One
side contains a mixture of molecules in water and the other side contains only water.
In the space next to the picture, draw what you think the chamber would look like
after an extended period of time.
Experiment B: Osmosis
1. What happened to the potato in each of the solutions? Why?
H2O –
sugar + H2O –
2. If you were to do the experiment again and add a third cup, what would have to do to
make that third solution isotonic to the potato cells?
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