Uncommon Scents Episode Two: Activity Three | Teacher Pages
DIFFUSE IT… OR NOT
BACKGROUND
TEACHER PAGES
In this activity, students plan and conduct an experiment that illustrates the diffusion
of organic solvent molecules across the alveolar membrane.
In Uncommon Scents, players learn inhaled chemicals don’t go away when you
exhale. Instead, they are absorbed from the lungs into the bloodstream and then
distributed throughout the body.
> See PowerPoint “Uncommon Scents02_Activity03”
But how and why are chemicals like organic solvents so easily absorbed? When chemicals are inhaled, they travel
along the respiratory tract in the same way as non-contaminated air, ending up in the alveoli. The walls of the alveoli
are very thin and are richly supplied with tiny blood vessels. Chemicals like organic solvents can easily pass through the
semi-permeable membranes of the alveolar and blood vessel cells by diffusion.
Since diffusion depends upon random particle movement and is based on the difference in concentration of the
chemicals between the alveoli and the blood, it does not require input of energy (passive transport). Diffusion will
continue until equilibrium is reached, e.g., until the relative number of chemical molecules is equal in the two areas.
Equilibrium is maintained because molecules continue to move.
In this activity, students conduct an experiment that demonstrates diffusion of molecules (cornstarch or iodine) across
a semi-permeable membrane (the polyethylene membrane of a zip-lock bag).
LEARNING OBJECTIVES
The students will:
• Formulate a hypothesis
• Plan and conduct an experiment to demonstrate diffusion across a semi-permeable membrane
• Understand that diffusion is a form of passive transport of molecules across the alveolar membrane
STANDARDS
National Science Content Standards: 5-8
Standard A:
• Students should develop abilities necessary to do scientific inquiry.
• Students should develop understandings about scientific inquiry.
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Uncommon Scents | Teaching Materials
MATERIALS
Water
500 ml beakers or other large glass container (one per group)
Small zip-lock bags (one per group)
Graduated cylinder or measuring cup
Starch solution (100ml per group plus some for the starch test)
Iodine tincture (available at drug stores)
Student Activity Sheet: Directions (one reusable copy per group)
Student Activity Sheets: Day 1; Day 2 (one copy per group)
PREPARATION
TEACHER PAGES
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Prepare a starch solution by taking one quart of water, adding ¼ cup of cornstarch, stirring well, and cooking this
solution until it turns from white to semi-clear. This quantity will provide solution for 9 groups. Store the starch solution
in a container of your choice.
PROCEDURE
Day 1
1.Remind students of what they learned in the game regarding chemicals like organic solvents entering the body.
Emphasize the fact that chemicals are absorbed into the bloodstream by the lungs. Depending on your students’
prior knowledge of the respiratory system, you might need to explain the flow of air from the nose to the alveoli.
> Show PowerPoint “Uncommon Scents02-Activity03”
2.Ask students if they know how chemicals can cross the membrane between the alveoli and the blood vessel.
Introduce the following terms:
• Diffusion: The movement of particles from areas of higher concentration to areas of lower concentration.
• Semi-permeable membrane: A membrane that lets certain molecules pass through and prevents others
from crossing. Small molecules like organic solvents move back and forth freely. All cell membranes are
semi-permeable.
3.Explain that they are about to plan and conduct an experiment that illustrates the movement of molecules across
a cell membrane, and introduce the materials that are used in the experiment. Mention that the zip-lock bag is
made out of polyethylene. Tell them they can use water as needed in their experiment.
4.Ask students about the role of each material (zip-lock bag: semi-permeable membrane; cornstarch: molecules
that diffuse across the membrane).
5.Demonstrate the starch test by adding some drops of the iodine tincture to a small amount of the starch solution
(the solution will turn blue). Additionally, add iodine tincture to water to show that there is no reaction other than
tinting the water a pale orange color. Explain that iodine is an indicator solution for the presence of starch and a
positive test is the blue color.
6.Divide class into groups of 2-4 students.
7.Draw the following items on the board so students will know what items they have to use in their experiment:
water, graduated cylinder, iodine tincture, zip-lock bag, cornstarch solution. They may use a few or all of these
items as they design their experiment. The goal is to design an experiment that will show the movement of
molecules across a membrane. Have students complete the research question, hypothesis, and step-by-step
procedure on the student activity sheet. (Note that there are several ways to design the experimental step-bystep procedure. The example sheets below show some possible options.)
8.Have a few groups who are willing to share their experimental design procedure. Have the class analyze the
plans. Then, discuss the possible procedures from the experimental designs A and B tables with the whole class.
Uncommon Scents | Teaching Materials
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Have students improve their procedure, if necessary (you might need to hand out fresh copies of the student
activity sheets).
9.Have students set up the experiment and label them with their group names. Warn them to carefully seal the
zip-lock bag because leakage will invalidate their results.
EXTENSION ACTIVITY
TEACHER PAGES
Day 2
10. Have students finish steps 4-6 of their experiment. Explain that it doesn’t matter if their hypothesis is
proven or disproven, because both results are equally valuable for research.
Have students brainstorm factors that could affect the diffusion rate of chemicals into the bloodstream.
Examples can be:
• Concentration of the chemical in the alveoli. Explanation: The higher the concentration of a chemical
in the alveoli, the greater is the concentration difference between alveoli and lung capillaries, and the
higher is the diffusion rate.
• Rate of blood circulation. Explanation: When the blood circulation in the lung capillaries is high,
chemicals are transported faster throughout the body, while fresh blood streams into the capillaries.
Thus, the concentration gradient between the alveoli and the capillaries is maintained, resulting in a
higher diffusion rate. The rate of blood circulation depends on physical exercise.
• Pulmonary ventilation rate. Explanation: An average person breathes in and out about 12 times a
minute. Each of the 12 inhalations brings about 500 ml of air into the lungs. If the air is contaminated
with chemicals and pulmonary ventilation rate rises (for example after exercising), more chemicals get
into the lung.
• The solubility of the chemical in fat. Explanation: Some gases are highly soluble in water and insoluble
in fat (lipophobic), whereas other gases – like organic solvents – are insoluble in water and highly
soluble in fat (lipophilic). The more lipophilic a chemical is, the better it will diffuse across the lipid bilayer
of the cell membrane.
RESOURCES
• Canadian Center for Occupational Health and Safety – How workplace chemicals enter the body
www.ccohs.ca/oshanswers/chemicals/how_chem.html
• Absorption, Distribution, and Storage of Chemicals
www.visionlearning.com/library/module_viewer.php?mid=106&mcid=&l=
• Animated sequence about Passive Transport – Diffusion
www.wisc-online.com/objects/ViewObject.aspx?ID=AP1903
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Uncommon Scents | Teaching Materials
Student Activity Sheet: Day 1 (EXAMPLE)
Research question
Hypotheses
Experimental Design B
Will cornstarch diffuse through a
polyethylene membrane?
If a zip-lock bag filled with water is placed a
beaker filled with cornstarch solution, then
the cornstarch molecules will NOT diffuse
into the zip-lock bag.
Day 1
Day 1
1. Fill zip-lock bag with 100ml cornstarch 1. Fill zip-lock bag with 100ml water and
solution and seal the bag tightly.
seal the bag tightly.
2. Fill the beaker with water 500 ml water. 2. Fill the beaker with 400 ml water and
3. Place the bag with the cornstarch
100 ml cornstarch solution.
solution into the water
3. Place the bag with the water into the
beaker with the water-cornstarch
Day 2
mixture.
1. Take the bag out of the water.
Day 2
2. Add some drops of iodine tincture to
the water in the beaker and observe if
1. Take the bag out of the water.
the water changes color.
2. Add some drops of iodine tincture
to the water in the zip-lock bag and
observe if the water changes color.
Student Activity Sheet: Day 2 (EXAMPLE)
Observation
Conclusion
Our Hypothesis is
Experimental Design A
After the zip-lock bag was removed from
the water-filled beaker and some drops of
iodine tincture were added to the water in
the beaker, the water turned blue.
The starch test showed that cornstarch
is present in the beaker. The cornstarch
molecules must have diffused through the
polyethylene membrane of the zip-lock bag.
 Proven
 Disproven
Uncommon Scents | Teaching Materials
TEACHER PAGES
Step-by-step
procedure
Experimental Design A
Will cornstarch diffuse through a
polyethylene membrane?
If a zip-lock bag filled with cornstarch
is placed in water, then the cornstarch
molecules will diffuse into the water.
Experimental Design B
After the zip-lock bag was removed from
the cornstarch-filled beaker and some
drops of iodine tincture were added to the
water in the bag, the water turned blue.
The starch test showed that cornstarch is
present in the zip-lock bag. The cornstarch
molecules must have diffused through the
polyethylene membrane of the zip-lock bag.
 Proven
 Disproven
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DIFFUSE IT… OR NOT
In this activity, you will plan and conduct an experiment that illustrates the
diffusion of organic solvent molecules across the alveolar membrane.
STUDENT ACTIVITIES
Uncommon Scents Episode Two: Activity Three | Student Activity Sheet
When chemicals like organic solvents are inhaled, they don’t pass out of the body
when you exhale. Instead, they pass through the semi-permeable membranes
between the lung alveoli and the blood vessels by diffusion. After absorption into
the bloodstream, the chemicals are distributed throughout the body.
In this activity, you will research if molecules like cornstarch or iodine can diffuse
through the polyethylene membrane of a zip-lock bag.
MATERIALS
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Water
500 ml beaker or other large glass jar or container
Small zip-lock bag
100 ml cornstarch solution
Iodine tincture
Student Activity Sheets: Directions; Day1; Day2
PROCEDURE
Day 1
1.Discuss a research question, a hypothesis and a step-by-step procedure for the experiment with your group and
record it on the student activity sheet.
2.Revise your procedure, if necessary, after you have discussed it with your teacher.
3.Set up the experiment.
Day 2
4.Finish your experiment and record your observations.
5.Discuss your conclusion with your group. Using complete sentences, record your findings.
6.Determine whether you have proven or disproven your hypothesis.
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Uncommon Scents | Teaching Materials
DIFFUSE IT… OR NOT > DAY 1
Research question:
STUDENT ACTIVITIES
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Hypothesis:
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Step-by-step procedure (number the steps):
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Uncommon Scents | Teaching Materials
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DIFFUSE IT… OR NOT > DAY 2
Observation:
STUDENT ACTIVITIES
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Conclusion:
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Our hypothesis is:
 Proven Disproven
Analysis: How is this experiment similar to what happens when an organic solvent enters the body? Make sure you
use the terms diffusion, alveoli, and cell membrane in your explanation.
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Uncommon Scents | Teaching Materials