Cell models
It's straightforward to model cells using cubes.
We can investigate the effect of increasing size on surface area to volume ratios using models based on cubes:
So, as the volume increases, the surface area does not increase at the same rate.
If a graph is drawn:
Question
What is the surface area to volume ratio of the highlighted mark?
This cube will have a surface area:volume ratio of 1.
The volume = 6 × 6 × 6 = 216
The surface area = 6 × (6 × 6) = 216
A stacked bar chart can be drawn to illustrate the proportions of surface area and volume.
In the below table scientists have estimated the surface area:volume ratios of various organisms.
| Organism | Surface area in square metres | Volume in cube metres | Surface area:volume |
| Bacterium | 6 × 10-12 | 1 × 10-18 | 6 000 000:1 |
| Blow fly | 6 × 10-4 | 1 × 10-6 | 600:1 |
| Whale | 6 × 104 | 1 × 106 | 0.06:1 |
| Organism | Bacterium |
|---|---|
| Surface area in square metres | 6 × 10-12 |
| Volume in cube metres | 1 × 10-18 |
| Surface area:volume | 6 000 000:1 |
| Organism | Blow fly |
|---|---|
| Surface area in square metres | 6 × 10-4 |
| Volume in cube metres | 1 × 10-6 |
| Surface area:volume | 600:1 |
| Organism | Whale |
|---|---|
| Surface area in square metres | 6 × 104 |
| Volume in cube metres | 1 × 106 |
| Surface area:volume | 0.06:1 |
Large organisms have:
- mechanisms to increase surface area proportionately, such as additional absorbing areas or adaptations of shape
- transport systems and keep distances for diffusion to a minimum
Organisms living in harsh environmental conditions may reduce their surface area, eg cacti, to reduce loss of water.