Activity 2.3 The effect of size on uptake by diffusion
• To investigate the effect of surface area to volume
ratio on uptake by diffusion.
• To show why a large surface area in the lungs,
combined with a circulation system, is required to
meet the body’s demand for oxygen and need to
eliminate carbon dioxide.
Diffusion limits size
• Block of agar jelly
• White tile
• Scalpel or sharp knife
• Paper towel or filter paper
• Beaker (100 cm3)
• Potassium manganate (VII) solution
• (0.02 M) or hydrochloric acid (0.1 M)
• Rubber or plastic gloves
• Graph paper
Organisms that rely completely on diffusion for
the absorption of substances and their movement
around the body rarely grow to be more than a few
millimetres thick. The surface area to volume ratio limits the size of the organism. You can
investigate the effect of increasing size on uptake by diffusion using agar jelly ‘animals’.
Wear eye protection.
Avoid skin contact with potassium manganate.
If potassium manganate solution is spilt do not
clean it up yourself – tell the teacher/lecturer.
1 Cut the agar jelly to give three cubes with heights of 0.5 cm, 1 cm and 2 cm. Putting graph
paper under the dish of agar jelly is helpful when cutting the blocks.
Place the cubes in the beaker and cover with the potassium manganate solution. If your
jelly is green due to universal indicator then use weak acid rather than the potassium
manganate (VII) solution. Leave the cubes for 5 minutes.
While you wait, calculate the surface area, volume and surface area to volume ratio
(surface area divided by the volume) for each of the cubes. Work in cm. See page 58 of
the textbook for some help.
Pour off the solution and blot the surfaces of each cube dry with a paper towel.
Cut each of the cubes in half and measure the distance from the edge that has changed