Diffusion-Osmosis Lab

Name________________________________Period_______Date___________Code______
Diffusion and Osmosis
In Living Systems
Introduction
Substances that are dissolved in liquids produce solutions. These dissolved substances move into and out of
the cells of living things. This movement is called diffusion; it is one of the main methods used by living
things to maintain a steady state. Diffusion throughout a cell involves the movement of substances from a
region of greater concentration to a region of lesser concentration. It is by this process that cells obtain the
chemicals they need to carry on their work and to rid themselves of waste products.
Objectives
• Apply the principles of diffusion to explain observations made in basic diffusion experiments.
• Explain the cause and effect of water gain or loss in a living cell.
• Apply the principles learned in these experiments to other similar types of diffusion problems.
Materials
dialysis tubing (12 cm lengths) distilled water iodine (I-KI) solution (diluted)
test tubes a large beaker 1% starch solution
250 ml beakers slide 3% NaCl solution
string coverslip 80% glucose solution
hot plate living Elodea plants Benedict’s solution
dropper
Procedure
Diffusion Through a Membrane
In this experiment, the dialysis membrane
represents the cell membrane. Though it is
nonliving, it has several properties in common
with a living membrane.
1. Preliminary tests:
Add a drop of iodine to 2 ml of starch
solution in a test tube. Describe the
reaction.
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Add 10 drops of Benedict’s solution to 5 ml of
glucose solution in a test tube. Place the tube in
boiling water for 3 minutes. Describe the
reaction.
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The above tests will be used to determine the
presence of starch and glucose in the diffusion
experiment.
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7. Complete the chart indicating which
materials have diffused and a brief
supporting reason. Suggest a reason for
the failure of some of the molecules to
diffuse through the membrane.
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2. Take a piece of dialysis tubing that has
been soaking in water and rub it
between the thumb and index finger
until it opens. Using string, tie a knot
very tightly at one end of the bag as
shown in the picture.
3. Fill the tube ½ full of starch solution
and then add approximately 6
dropperfuls of 80% glucose solution to
it. Tie the open end of the bag and rinse
the outside of the bag under a faucet.
4. Place the bag in a 250 ml beaker and
add only enough water to cover bag.
Add enough iodine to the beaker to turn
the contents light orange.
5. Make observations of the bag and
beaker contents for approximately 15
minutes. The results of the preliminary
tests should help you understand what is
happening.
6. After 15 minutes, place 5 ml of beaker
fluid in a test tube. Add enough
Benedict’s solution to turn the contents
blue. Heat the test tube in boiling water
for 3 minutes. Observe the reaction.
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A membrane that allows some molecules to
pass but not others is called:
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Can materials diffuse through a membrane
in both directions at the same time?
Explain.
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In what ways is the dialysis membrane like
the cell membrane?
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In each case, the molecules moved from
__________ concentration to __________
concentration.
Diffusion through
membrane?
Experimental Evidence
iodine
starch
glucose
water

The Effect of Osmosis
On Living Cells
1. This experiment is about the diffusion
or osmosis of water. Prepare two wet
mounts of elodea - one in pond water
and the other in a 3% NaCl solution.
Remember that 3% NaCl means 97%
water. A typical elodea cell will
contain about 1% salt.
2. Examine the elodea in pond water under
a microscope. Sketch and label some
typical cells in the space provided in
this activity. Describe the position of
the chloroplasts in the cell.
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3. Observe the salt water preparation and
sketch the cells. What has happened to
the location of the cell membrane and
chloroplasts?
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Has water moved in or out of the cell?
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Explain the water movement in terms of
concentration difference.
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4. Now, remove the coverslip and
thoroughly rinse the elodea leaf under
tap water. Prepare a new slide of the
same leaf using distilled water. Look
for changes in the cells.
Sketch these cells below. Why did the cells
change appearance?
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Elodea Cells
Pond Water 3% NaCl Solution After adding distilled water

Analysis
1. Why do molecules diffuse?_________________________________________________________
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2. Why would molecules of a gas diffuse more rapidly than those of a liquid?___________________
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3. The two circles A and B, represent cells and the dots are water molecules.
Which sketch represents a fresh water cell that has just been placed in salt water?____________
Which cell will expand?_______________
Do any molecules move out of cell B? Explain.
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4. In the third circle C, draw the distribution of water molecules of a cell in osmotic balance.
5. The shrinkage of cytoplasm within a cell from diffusion is known as _______________________.
How can this process be a disadvantage to a cell?___________________________________________
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How would taking in too much water be a disadvantage to a cell?______________________________
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6. A cell contains 3% salt and 97% water. How would this cell respond if placed in a distilled water
solution?
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In a 10% salt solution?________________________________________________________________
7. From evidence obtained in this experiment, list three factors that determine whether a molecule
will diffuse into a cell.
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