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:
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 x 10-12 | 1 x 10-18 | 6 000 000 |
| Blow fly | 6 x 10-4 | 1 x 10-6 | 600 |
| Whale | 6 x 104 | 1 x 106 | 0.06 |
| Organism | Bacterium |
|---|---|
| Surface area in square metres | 6 x 10-12 |
| Volume in cube metres | 1 x 10-18 |
| Surface area : volume | 6 000 000 |
| Organism | Blow fly |
|---|---|
| Surface area in square metres | 6 x 10-4 |
| Volume in cube metres | 1 x 10-6 |
| Surface area : volume | 600 |
| Organism | Whale |
|---|---|
| Surface area in square metres | 6 x 104 |
| Volume in cube metres | 1 x 106 |
| Surface area : volume | 0.06 |
Unicellular organisms, such as bacteria, have a very high surface area:volume ratio. Substances can diffuse in and out at a high rate and easily reach all parts of the cell.
Because of their smaller surface area: volume ratio, larger organisms need transport systems to move substances, such as oxygen, around the body to where they are needed. In many animals, this is the bloodstream. They also need specialised exchange surfaces where substances can enter and leave the transport system by diffusion. An example is the lungs in mammals. Exchange surfaces are adapted to increase their surface area to maximise the rate of diffusion.
Organisms living in harsh environmental conditions may reduce their surface area, eg cacti, to reduce loss of substances such as water.
