Name
Class
Date
Guided Inquiry • Skills Lab
Chapter 7 Lab
Detecting Diffusion
Big Idea
Problem
How can you determine whether solutes are diffusing across a
membrane?
Introduction
A cell membrane is a selectively permeable barrier. In other words,
some particles can pass through the cell membrane while other
particles are held back. Solutes that can move across the membrane
generally do so by diffusion. When solutes diffuse, they move from
an area of high concentration to an area of lower concentration.
In this lab, you will use a small plastic bag to represent the cell
membrane. The bag has small openings, or pores, that allow the
passage of relatively small molecules. You will use an indicator to
determine which molecules diffuse through the pores.
The cell membrane regulates
what enters and leaves the cell.
Skills Objectives
Students will be able to
• detect diffusion across a
membrane.
• predict how the size of a
molecule and concentration
affect diffusion.
Preparation Time
30 minutes
Class Time
20 minutes
Group Size
Small groups
Materials
Because elemental iodine is
relatively insoluble in water,
you need to use Lugol’s
solution as your indicator.
Skills Focus
Use Models, Analyze Data, Infer
Advance Preparation
Materials
• 400-mL beaker
• 25-mL graduated cylinder
• 1% starch solution
• plastic sandwich bag
• twist tie
• iodine solution
• forceps
Build Vocabulary
Term
To prepare the 1 percent
starch solution, dissolve 2 g of
cornstarch in 198 mL of water.
Bring the mixture to a boil to
fully dissolve the starch. Allow
time for the solution to slowly
cool before class.
Build Vocabulary
Students will have a more
successful lab experience if
they understand these terms.
Definition
permeable
Letting liquids or gases pass through, or permeate,
a barrier such as a membrane
selective
Able to choose, or select, some options and reject
others
solute
A substance that dissolves in another substance,
such as water, to form a solution
indicator
Something that is used to show, or indicate, the
presence of a solute in a solution
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Safety
Safety
Read the MSDS on iodine.
If you prepare the iodine
solution, be sure to wear
goggles, plastic gloves, and an
apron. Ask if any student has a
known sensitivity to iodine.
Iodine solution can irritate the eyes and skin and can stain
clothing. Wear safety goggles, gloves, and a laboratory apron while
handling any solution that contains iodine. Rinse off any solution
that spills on your skin or clothing. Wash your hands thoroughly
with soap and warm water before leaving the lab.
Pre-Lab Questions
1. Draw Conclusions How will you know whether starch has
diffused across the membrane?
If the solution outside the plastic bag turns blue-black, then starch
diffused out of the bag.
2. Draw Conclusions How will you know whether iodine has
diffused across the membrane?
If the solution inside the plastic bag turns blue-black, then iodine
diffused into the bag.
3. Use Analogies How is a window screen similar to a cell
membrane?
Sample answer: Like a cell membrane, a window screen prevents the
passage of some things, such as insects, while allowing other things,
such as air, to flow in and out of a house.
Procedure
Teaching Tip
Demonstrate the reaction
of iodine with starch so
that your students know
what to look for.
You will use iodine as an indicator to determine whether the
plastic bag is permeable to starch. Iodine will turn blue-black when
it comes into contact with starch.
1. Put on your goggles, apron, and gloves.
2. Add about 200 mL of water to a 400-mL beaker.
3. Pour 25 mL of starch solution into the plastic sandwich bag.
4. Use a twist tie to tightly seal the bag.
5. Use tap water to thoroughly rinse the outside of the bag in case
any starch solution spilled onto the outside of the bag. Be sure
to rinse the twist tie as well.
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6. Place the plastic bag in the beaker so that the bag is completely
covered with water.
7. Add 8 drops of iodine solution to the water in the beaker.
Record your initial observations in the data table. Wait
10 minutes, and then record your final observations.
8. Use a forceps to remove and dispose of the plastic bag as
instructed by your teacher.
Sample Data
Data Table
Inside Bag
Outside Bag
Color
Is starch
present?
Is iodine
present?
Color
Is starch
present?
Is iodine
present?
Initial
Clear
Yes
No
Brown
No
Yes
Final
Black
Yes
Yes
Brown
No
Yes
Analyze and Conclude
1. Infer After you placed the plastic bag in the beaker, what happened to the
iodine? What happened to the starch?
The iodine diffused into the bag, but the starch did not diffuse out.
2. Apply Concepts Use what you know about the structure of starch
molecules to explain your results.
The pores in a plastic bag are relatively small. Starch, which is a polymer of glucose,
is too large to pass through the pores.
3. Infer Did water move into the bag or out of the bag? Why?
Sample answer: Water moved into the bag. The concentration of water was higher in
the beaker than it was in the starch solution inside the bag.
4. Perform Error Analysis When Alyssa did the experiment, she observed the
solution outside the plastic bag turn black. What might have happened?
Sample answer: There may have been starch solution on the outside of her bag. The
bag might not have been tightly sealed, or there may have been a tear in the bag.
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5. Use Models Describe two functions of a cell membrane that cannot be
modeled with a plastic bag.
Sample answer: A cell membrane uses facilitated diffusion to control the movement of
some solutes and active transport to move some solutes against their concentration
gradients.
6. Predict Red blood cells are placed in water that has been distilled so that
there are no dissolved substances in the water. Are the red blood cells
likely to swell up or shrink? Why?
Sample answer: Water will diffuse into the cells because the initial concentration of
water outside the cell is higher than the concentration of water in the cytoplasm.
This movement of water will cause the cells to swell up, or even burst.
Build Science Skills
Is a sandwich bag permeable to glucose? Describe how you could modify the
procedure in this experiment to answer this question. What could you use to
test for the presence of glucose?
Students would need to substitute a glucose solution for the starch solution and use
glucose test strips instead of iodine.
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