Practical
Diffusion and Osmosis (1hr)
Introduction
Diffusion is the passage of molecules from a region of high concentration through a semi permeable
membrane to a region of low concentration. Osmosis is the passage of water from a region of high
water concentration to a region of low water concentration. Semi permeable membranes are very
thin layers of material which allow small molecules such as water, potassium iodide and glucose to
pass through them, but prevent larger starch molecules from passing
Equipment List
through. In a hypertonic solution, the solute concentration is higher than
Thistle funnels
in the cells. In a hypotonic solution, the solute concentration is below
Dialysis tubing cut
that in cells. In an isotonic solution, the solute concentration is equal
into 30 cm strips
inside and outside the cell.
Rubber bands
In this experiment, dialysis tubing is used to demonstrate the action of a
Iodine solution in
differentially permeable membrane. When plant cells are placed in
droppers
hypertonic solution, they lose water and become flaccid. When plant
20% Glucose
cells are placed in hypotonic solution, they gain water and become
solution
turgid. In an isotonic solution, cells are unchanged because the amount
Soluble starch
of water leaving the cell is equal to the amount of water entering the
solution
cell.
Glass markers
The aim of this practical are to:
Tes tape
 Investigate how starch, glucose and water molecules diffuse
through a dialysis tubing which is acting as a differentially
permeable membrane.
Instructions
1. Moisten the dialysis tubing with warm water and then open it by rubbing it between your
thumb and forefinger.
2. Seal the middle of the tubing by tying a knot 15 cm from the end. Take care not to damage
the tubing and make sure that the knot is leak proof and in the middle.
3. Slide the thistle funnel into the open end of the dialysis tubing. Fix the bag and the loose
opposite end tightly to the thistle funnel by winding the rubber band around it securely. One
side of the dialysis tubing is to be filled and the other is empty.
4. Pour starch solution into the thistle funnel until the dialysis tubing is filled and the starch
solution rises 3 cm up the stem of the funnel. Make sure there are no air bubbles.
5. Rinse the outside of the thistle funnel and dialysis tubing with cold water to remove starch.
6. Suspend the bag in the gas jar and fix the apparatus in place with a retort stand and clamp as
shown in Figure 1.
7. Add water to the gas jar until the level is 1 to 2 cm above the level of the bag and measure.
Record the height in the table.
8. Pour enough iodine solution into the water to give a yellowish colour.
9. Label the jar “A” with a waterproof marking pen and mark the level of the water in the gas
jar and the level of the starch solution in the thistle funnel.
10. Leave the apparatus undisturbed for one hour. Note any colour changes in the gas jar and
the dialysis tubing.
11. Record any changes in the water level in the gas jar and the stem of the thistle funnel.
12. Set up a second apparatus in the same way, but add glucose instead of starch in the dialysis
tubing. Do not add iodine to the water in the gas jar.
13. Label the second apparatus “B” and mark the level of glucose solution in the thistle funnel
and measure and record it.
14. Dip a piece of Tes-tape into the water in the jar and record colour change. Leave the
apparatus undisturbed for an hour.
15. Observe the level of glucose solution in the tube and the water level in jar “B”. Measure and
record any changes. Dip a piece of tes-tape into the water in the jar and record any colour
change. Note: Remember that iodine turns starch dark blue and tes tape turns green in the
presence of glucose.
Results
Table 1: Starch test
Experimental Colour of Colour
Level of
Interpretation
Set up
water in
of starch water in
jar
in tubing thistle funnel
“A” before
diffusion
“A” after
diffusion
Table 2: Glucose Test
Experimental Colour of
set up
Tes tape in
gas jar
“B” before
diffusion
“B” after
diffusion
Level of
water in gas
jar
Interpretation
Analysis of Results
1. What caused the colour change in the starch solution when the dialysis tubing was left in the
beaker containing iodine solution?
2. Did starch molecules diffuse through the dialysis tubing in jar “A”? Explain.
3. Did glucose molecules diffuse through the dialysis tubing in jar “B”? What was the evidence
that it was diffusing?
4. Was there evidence of water molecules moving across the membrane in gas jar “A” and
“B”?
Conclusion
1. Write a summary paragraph of what your experiment was about and what your results
were?
2. With reference to the aim, write a statement of conclusion about the ability of starch,
glucose, water and iodine to move through dialysis tubing.