Investigating the effect of surface area to volume ratios on osmosis.
Introduction
Surface area to volume ratios are a way of measuring the size of an object. The surface area of the outermembrane is numerically compared to the volume. For example, in Figure 1 there are three cubes that have
different sizes but are the same shape. The smallest cube has the largest surface area to volume ratio and the
largest cube has the smallest.
Figure 1. The surface area to volume ratio of three objects
Dimensions:
1cm x 1cm x 1cm
Surface area =
1+1+1+1+1+1cm2
=6cm2
Volume = 1cm3
SA:Vol = 6:1
Dimensions:
2cm x 2cm x 2cm
Surface area =
4+4+4+4+4+4cm2 =
24cm2
Volume = 8cm3
SA:Vol = 24:8 = 3:1
Dimensions:
3cm x 3cm x 3cm
Surface area =
9+9+9+9+9+9cm2 =54cm2
Volume = 27cm3
SA:Vol = 54:27 = 2:1
Osmosis is the movement of water across a semi-permeable membrane from areas of high concentration to
low concentration.
Aim
To determine the effect of surface area to volume ratios on the movement of water through cubes of potato.
Variables
Variable
Description
Independent
Dependent
Controlled
Hypothesis
Use third person, present tense. [If the size of an object is larger, then …………….. .
This will happen because……. .]
Materials




Potato
Water
2 x 200ml beakers
White tile



Scalpel
Ruler
50ml Measuring
cylinder



Pipette
Paper towel
Electronic balance
Method
1. Pour 200ml of water into the 300ml beaker (use a measuring cylinder)
2. Carefully cut 2 cubes of potato (approximately 2x2x2cm, use a ruler to measure)
3. Carefully cut 1 of the cubes of potato into four cubes (each approximately 1x1x1cm, use a ruler to
measure)
4. Weigh the 2cm3 cubes using a balance, record the mass in a table
5. Weigh the combined mass of the four 1cm3 cubes, record mass in a table
6. Place the cubes into the beaker, ensure the solution completely covers the cubes
7. Leave for 15 minutes
8. Remove the cubes from the beakers and remove excess water from cubes with paper towel
9. Reweigh the chips and record results in table
10. Collect class results
11. Calculate the average change in mass and then the percentage average change in mass of each set of
cubes.
Results
Individual Results:
Dimensions of cube
(cm)
Mass at time = 0
(g)
Mass after 15mins
(g)
Change in mass
(g)
% mass change
1x1x1
2x2x2
3x3x3
Class Results:
Dimensions of cube (cm)
CLASS INDIVIDUAL
RESULTS
Average % mass change
1x1x1
2x2x2
3x3x3
Discussion
1. The Independent variable was the different sizes of the potato cubes. What relationship was found
between this variable and the % mass change?
2. What was happening for the mass of the potato cubes to change? (hint: refer to osmosis)
3. Rank the potato cube sizes in order of the largest surface area to volume ratio (SA:volume) to the
smallest?
4. Link and explain how the SA:volume affects the mass change of the potato cubes.
5. How does this investigation help us understand why cells are small?