Supplying the cell
For an organism to function, substances must move into and out of cells. Three processes contribute to this movement - diffusion, osmosis and active transport.
Cell models
It's straightforward to model the cells of organisms using cubes. By doing this we can easily see how the surface area to volume ratio changes as organisms increase in size.
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 |
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, for example cacti have adapted to have less surface area to lose water through.