1.7 MANDATORY PRACTICAL.
Overview: The phenolphthalein in the agar cubes reacts with the acid, changing the colour of the cube
from pink to clear. After the cubes are exposed to the acid you will see how far the acid diffused
based on the change in colour which it caused. This will allow you to determine the relationship
between diffusion and the surface area and volume of the cubes.
Procedure
1. Cut 8 cubes 2 of each size.
2. Calculate the surface area and volume of each agar cube and record these values in the table
below.
3. Carefully fill 3 beakers with acid so that the cubes will be completely submerged when placed in
the beaker.
4. Place the 2 agar cubes of the same size into each beaker. After 10 minutes, remove one cube of
each size them and place them on a paper towel; keep timing the other cubes.
5. Using the paper towel, blot the cubes dry.
6. Use the scalpel to carefully cut each cube in half.
7. Measure the distance travelled into each cube by the acid. Then calculate the volume of the
coloured proportion of the cube, and the uncoloured portion.
8. Finally calculate the percentage of uncoloured cube.
9. Whilst you are doing the above calculations watch the remaining cubes and time how long each
one takes to go completely clear. Record each time in the table below.
Results
Cube Size (mm)
10
20
30
40
Surface Area (A)
Volume (Y)
Surface area / Volume Ratio (A/Y)
Dimensions of Coloured Cube
Volume of Coloured Cube (X)
Volume of Uncoloured Portioned (Y-X= Z)
Percentage of Block Uncoloured (Z/Yx100)
Time Taken for all of the blocks to go
completely clear (s)
Questions
1. Explain why parts of the cube became clear
2. If the length of the side of a cube is decreased, what happens to the surface area to volume
ratio of the cube?
3. Which cube has the least volume remaining not reached by the acid (the least pink colour)?
4. In which size cube would substances be able to reach the middle fastest? Why is this?
5. Imagine that ,on a smaller scale, the gelatine block represents a single celled organism
a) What substances need to penetrate to all parts of the cell
b) What substances need to diffuse out of the cell in the opposite direction
6. Most single celled animals are less than 1mm across and the largest are only about 2mm.By
reference to your results in this experiment suggest one reason why single celled organisms
are no larger than this?
7. On the basis of the results of this experiment the penetration of substances into a large
cellular organism such a fish should take a long time. What aspects of structure and
organisation in a fish allow it to survive and be active despite such a slow rate of diffusion?
8. Suggest a shape for a volume of one cm3 of gelatine, which would increase the rate of
penetration of substances.