Adapted from “An Australian Biology
Perspective, Prac Manual”, p147.
DIFFUSION AND OSMOSIS EXPERIMENT
AIM: To determine the permeability of the ‘cell membrane’ to different chemicals.
THEORY: The cell membrane determines what substances can diffuse into a cell. This characteristic of a
cell membrane is called permeability. Many cells are selectively permeable - some substances can pass
through the membrane but others cannot. Dialysis tubing is an artificial semi-permeable membrane with
similar properties to the cell membrane.
MATERIALS:
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Dialysis tubing
Cotton
3 measuring cylinders (100mL)
3 measuring cylinders (10mL)
Marking pen
Cling wrap
Distilled water
METHOD:
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Glucose solution
Sucrose solution
Starch solution
‘Clinistiks’
Iodine
Sodium hydroxide
Hydrochloric acid
Rubberbands (very small)
1. Cut 3 strips of dialysis tubing, each approximately 15cm long. Tightly tie one end of the tubes with
a rubber band. Open one tube at the untied end by wetting your fingers and gently rubbing the end
of the tube.
2. Pour 50mL of distilled water into each measuring cylinder. Label the cylinders A, B, and C.
3. Carefully pour 10mL glucose solution into one dialysis tube, making sure not to spill any glucose on
the outside of the tube. Use a rubber band to tie the end of the tube, and tie the cotton on it.
Ensure that there is no air in the tube, so that the tube is firm. Rinse the outside of the tube and
then place this ‘cell’ into cylinder A, as pictured below. Cover cylinder with cling wrap.
4. Fill the second tube with Sucrose, using the same method as step 3. Place into cylinder B.
5. Fill the third tube with Starch, using the same method as step 3. Add 10mL of iodine to 40mL
distilled water in cylinder C. Place tube into cylinder C.
6. Set the three cylinders aside for 24 hours.
10ml
Starch
10ml
Sucrose
10ml
Glucose
40ml
Water
+ 10ml
iodine
50ml
Water
50ml
Water
Cylinder A
Cylinder B
Cylinder C
HOMEWORK: Membranes can block molecules based on their size. Research and then
compare the relative sizes of glucose, sucrose and starch molecules (include diagrams). What
is the main function of each molecule? From this, make predictions of what you think will
happen in each cylinder (PTO for response).
GLUCOSE
SUCROSE
STARCH
Diagram
Function
Prediction
After 24 hours:
1. Measure and record the amount of fluid in each ‘cell’ (tube).
2. Measure and record the amount of water in each measuring cylinder.
3. For Cylinder A, test the water for glucose using a ‘clinistik’ strip.
4. For Cylinder B, test for sucrose by removing a small amount of the water, then add 5mL of HCl then
5mL of NaOH. What happens? Also test for glucose using a ‘clinistik’ strip.
5. For Cylinder C, record the presence or absence of starch by identifying the colour of iodine in the
water. Did iodine enter the ‘cell’? Also test for glucose using a ‘clinistik’ strip.
6. Complete the following table:
RESULTS:
Cylinder
Substance
in tube
Volume of
liquid in
cylinder
Before After
Volume of
liquid in tube
Before After
Movement of
molecules
from cell to
water?
Y or N
Movement of
molecules
from water to
cell?
Y or N
Glucose in
cylinder
water?
Y or N
General
observations
A
B
C
DISCUSSION:
What has happened in each of the three cylinders (refer to the substance in the ‘cell’, and the water in the
cylinder)? Use scientific terminology in your response and try to explain WHY this has occurred.
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TEACHER NOTES:
THIS IS WHAT SHOULD HAVE HAPPENED…
Cylinder A: Glucose
An increase in the volume of liquid in the tubing and a corresponding decrease in
the volume of liquid in the cylinder (or volumes may have remained the same due to
movement of both water and glucose across the ‘membrane’).
We should also have observed glucose in the cylinder water as it was small enough
to diffuse through the dialysis tubing.
So, glucose should have diffused out of the tube through the cell membrane; and
water should have diffused by osmosis into the tube through the cell membrane.
Cylinder B: Sucrose
An increase in the volume of liquid in the tube and a corresponding decrease in the
volume of water in the cylinder.
The water moved across the ‘membrane’ (down the concentration gradient) by
osmosis to try to dilute the sucrose.
There should not have been any sucrose outside the ‘cell’ because the sucrose
molecules were too large to pass through the holes in the dialysis tubing.
Cylinder C: Starch
Same as what happened in Cylinder B, but substitute sucrose with starch.