Biology: Diffusion and Osmosis Investigation
Diffusion across a Selectively Permeable Membrane
Cell Membrane
1. Introduction:
Each cell is surrounded by a selectively permeable cell
membrane which regulates what gets into and out of the cell.
A selectively permeable membrane allows some types of
molecules and ions to diffuse across the membrane and
prevents other types of molecules and ions from crossing the
membrane. For example, oxygen can cross the selectively
permeable cell membrane, but large molecules like proteins
and DNA cannot cross the cell membrane.
Nucleus
This investigation will use a synthetic selectively permeable membrane which provides a simplified model of the
selectively permeable cell membrane.
2. Hypothesis: Which molecules will cross the membrane?
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Which of the following molecules do you think will diffuse across the selectively permeable membrane?
Molecule or Ion
Will it cross the
Why or why not?
(Molecular Formula)
membrane?
Iodine (I3-)
Water (H2O)
Starch (polysaccharide made up of
many molecules of glucose)
3. Materials and Methods: You can add a solution of starch to a bag made of the synthetic membrane and
put the bag in a beaker of iodine solution. Allow time for the substances to diffuse across the membrane
and then test which of the substances have crossed the membrane.
If iodine or starch have crossed the synthetic membrane, you will see a color change. A solution of iodine is
tan and a solution of starch is clear or milky white; when iodine and starch are together in the same
solution, the solution is purple, dark blue or black. If water can cross the synthetic membrane, water could
diffuse into the bag or out of the bag. You will weigh the bag before and after to see if water has left or
entered the bag.
Initial State
Synthetic
membrane
bag
Beaker
W I
W S
Final State
Key
W–Water
I—Iodine
S—Starch
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Biology: Diffusion and Osmosis Investigation
Materials:
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Dialysis tubing
String
250 mL beaker
Transfer pipettes
Graduated cylinders 250 mL
Distilled water
Scale
Iodine and starch solutions
Stopwatch or timer (cell phone)
Procedure:
 Obtain a piece of pre-soaked dialysis tubing and two pieces of string. Fold the bottom of the piece of tubing 1
cm up and use a piece of string to tie the folded part tightly to create a bag.
 To open the other end of the tube, rub the end between your fingers until the edges separate. Use pipette to
add 4 mL of starch solution to the tube.
 Next, fold 1 cm of tubing at the top of the bag and tie it off tightly. Check to make sure there are no leaks. If
scissors are available, trim the strings short.
 Rinse the bag and strings thoroughly in fresh water. Dry the bag and strings thoroughly.
 In the table below record the weight of the bag as a measure you are using to evaluate the movement of water.
 Add 200 mL of distilled water to a 250 mL beaker. Add about 1 mL iodine to the water in the beaker.
Caution: Be careful to avoid staining your clothes or fingers with the iodine.
 Put the bag in the beaker.
 Collect observations at beginning and after 25 minutes.
4. Analysis: Use the tables below. You will neatly collect your observations on these.
For each substance, indicate how you will know whether it crossed the synthetic membrane. What observation will
be different, depending on whether or not each substance crossed the membrane?
Substance
Expected Observation – HYPOTHESIS (e.g. color of solution in the bag or in the beaker)
If this substance crossed the membrane
If this substance did not cross the membrane
Iodine
Starch
Water
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Biology: Diffusion and Osmosis Investigation
After 20-30 minutes, record your observations in the “Final State” row of the table.
Complete the following table.
Did this
Molecule or Ion
molecule or
How do you know?
(Molecular Formula)
ion cross the
membrane?
Iodine (I3-)
Water (H2O)
Starch
(polysaccharide made up of
many molecules of glucose)
6. Conclusion: Either support or reject your hypothesis and explain what happened by answering these questions:
Based on your results, what characteristic appears to determine which molecules and ions can cross the synthetic
selectively permeable membrane?
Did any of your results differ from your predictions? If so, what were the differences between your results and your
predictions?
What do you think is the explanation for any differences between your predictions and your observations?
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Biology: Diffusion and Osmosis Investigation
A model is a simplified representation of a complex biological structure or process. A model focuses on a few key
features in order to help us understand a biological structure. Because a model is simpler than the biological
structure it represents, a model does not demonstrate all the features of the actual biological structure. For
example, the synthetic membrane in your experiment is a model of the selectively permeable cell membrane, but it
shows a much simpler kind of selective permeability than an actual cell membrane.
This diagram provides a different model of the cell membrane. It gives some idea of the complexity of the selective
permeability of an actual cell membrane. The diagram shows that a cell membrane contains proteins that aid in the
transport of biologically important ions and molecules across the cell membrane.
Fluid outside
the cell
Cell membrane
has proteins and
a double layer of
phospholipids.
Watery cytosol
inside the cell
O2, Na+, and glucose each diffuse across the cell membrane from a region of higher concentration to a region of
lower concentration. Specific proteins facilitate the diffusion of Na+ and of glucose. A third type of membrane
protein pumps Na+ and K+ across the cell membrane, each from a region of lower concentration to a region of
higher concentration. This protein pump uses energy from ATP which is produced by the cell.
(http://hyperphysics.phy-astr.gsu.edu/hbase/biology/imgbio/lipbitran.gif)
This activity has presented two models of the selectively permeable cell membrane: this diagram and the synthetic
membrane in your experiment. Both models show that small molecules and ions can cross a selectively permeable
membrane. However, there are some differences in other features of these models.
Diffusion and Osmosis Questions – Do These!!!
10. During diffusion, more molecules move
a. from regions of higher concentration to regions of
lower concentration
b. from regions of lower concentration to regions of
higher concentration
11. The reason is that:
a. Crowded molecules want to move to an area with more room.
b. Molecules tend to keep moving until they are uniformly distributed and then they stop moving.
c. The random motion of molecules results in their uniform distribution in the available space.
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Biology: Diffusion and Osmosis Investigation
Osmosis is the diffusion of water across a selectively permeable membrane.
Na+, Cl-, and the water molecules that are bound to these ions
cannot cross the selectively permeable membrane (called a
semipermeable membrane in this figure). Only free water
molecules (water molecules that are not bound to ions or other
solutes) can cross the selectively permeable membrane.
During osmosis, diffusion results in movement of free water
molecules in both directions across the selectively permeable
membrane, but more free water molecules move from the region
of higher concentration of free water molecules to the region of
lower concentration.
12. Why is the concentration of free water molecules lower in
water with dissolved salt and higher in pure water?
13. Why does osmosis result in a net flow of water from the side of the tube that has pure water to the side of the
tube that has water with dissolved salt? Include in your explanation the relative concentrations of free water
molecules on the two sides of the tube.
14. In your experiment, the concentration of dissolved substances is higher in the starch solution in the bag than in
the iodine solution in the beaker. Which way would you expect more water to diffuse – into the bag or out of the
bag? Explain your reasoning. Include in your explanation the relative concentrations of free water molecules inside
and outside the bag.
W I
W S
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