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Megan Sechrist
Diffusion Lab
Problem: What can go in and out of our solution in the bag?
Hypothesis: If starch and sugar solution are added in a bag together then it
will diffuse.
Molecule
Molecular Formula
Starch
polysaccharide made up of many
glucose molecules
Glucose
C6H12O6
Iodine
I3-
Water
H2O
Will it cross selectively
permeable membrane?
No
Why or why not?
Molecules too big
Yes
Molecules are smaller
Yes
Easy liquid
Yes
Easy liquid
Materials: Beaker
Distilled water
String
Cylinder
Cellophane paper
Sugar
Starch
Iodine
Benedict solution
Procedure:
1. Obtain one 15 cm piece of cellophane wrapping paper and two 6 inch pieces
of string.
2. Make a purse out of the cellophane wrapping paper by tying a piece of string
tightly to create a bag.
3. Using a pipet add 3 mL of glucose solution and 3 mL of starch solution to the
bag.
4. Tie off the top of the bag with string after each bag is filled.
5. Add 50 mL of water to a 200 mL beaker. Make sure you don’t add extra
water.
6. Add about 0.2 mL iodine to the water in the beaker. In the presence of
starch, iodine changes color from brown to dark blue.
7. Record your observations of the solution in the bag, the solution in the
beaker, and the measure you are using to evaluate movement of water in the
“Initial State” row in the table below.
In the tube
Color
Glucose?
In the beaker
Color
Glucose?
Initial State (Before
placing tube into
beaker)
cloudy
yes
Measure to evaluate
movement of water
No
clearer
Yes
Final State
8. Place the bag into the beaker.
9. After 10 minutes extract 5 ml of water from the beaker. Add 10 drops of
Benedicts solution to the test water.
10. Record your observations in the “Final State” row in the table.
Data/Observation:
1. Did starch move out of the bag? How do you know?
No it didn’t change color outside of the bag.
2. Did iodine move into the bag? How do you know?
Yes it changed blue.
3. Did glucose move out of the bag? How do you know?
No it didn’t changr color while we tested it
4. Did water move into or out of the bag? How do you know?
Moved out because when we measured it, it only had 8 ML
Conclusion:
1. Using your observations fill in the chart describing which molecules passed
through the membrane.
Molecule
Molecular Formula
Did it cross selectively permeable
membrane?
Starch
polysaccharide made up of many
glucose molecules
No
Glucose
C6H12O6
Iodine
I3-
No
Yes
10 ML
Water
H2O
Yes
2. Based on the molecular structure of starch, give a possible explanation as to
why starch did or did not pass through the membrane.
It’s a massive sugar. It was too big to pass through the pores of the paper.
3. Explain how your observations support the conclusion that cellophane
wrapping paper is a selectively permeable membrane.
Iodine went through the cellophane paper turning the starch blue.
4. There was water on both sides of the membrane, in the tube and in the
beaker. Why did water move in the direction that it did?
The water had to go from a high concentration to a low concentration
because it was so dense.
Each living cell is surrounded by a selectively permeable cell membrane
which allows water to move into or out of the cell by diffusion. The diffusion of
water across a selectively permeable membrane plays such an important role
in biology that this process has been given a special name, osmosis. What
will determine whether water moves into or out of the cell by osmosis?
Going from high to low concentration

Small molecules that do not have an electrical charge can easily
diffuse across the selectively permeable cell membrane, but larger
molecules or charged atoms or molecules (ions) cannot.
Sometimes a cell needs to transport molecules that are too big or
have too much charge to diffuse through the cell membrane.
Special proteins embedded in the cell membrane allow certain ions
and molecules to diffuse across the cell membrane. This is called
facilitated diffusion.
Sometimes a cell needs to move molecules from a region of lower
concentration to a region of higher concentration. For example, cells need to
move nutrients such as amino acids into the cell. Explain why diffusion can
not be used to move amino acids from a region of lower concentration (in the
fluid surrounding a cell) to a region of higher concentration (in the cytoplasm
inside a cell).
Special proteins embedded in the cell membrane can act as pumps to move
molecules from a region of lower concentration through the cell membrane to
a region of higher concentration. This type of active transport requires
energy.
In this investigation you used a synthetic selectively permeable membrane
that is incapable of producing energy. What type of molecular transport was
shown in this investigation—diffusion or active transport? Explain how you
know this.
Diffusion because cellophane can’t create energy.
5. Based on the results of this investigation, describe one important issue
scientists must consider when developing a new medicine that acts on an
enzyme inside cells.
It must be small enough to get across the membrane.