Surface area to volume ratioSpec 4.1.3.1
A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.
As an organism gets bigger, its volume increases faster than its surface area, so its surface area to volume ratio becomes smaller. A multicellular organism cannot get enough molecules in and out of every cell by diffusion across its outer surface alone.
| Side length | Surface area (6 × side²) | Volume (side³) | Surface area : volume |
|---|---|---|---|
| 1 cm | 6 cm² | 1 cm³ | 6 : 1 |
| 2 cm | 24 cm² | 8 cm³ | 3 : 1 |
| 3 cm | 54 cm² | 27 cm³ | 2 : 1 |
Comparing surface area to volume ratios
Calculate the surface area to volume ratio of a cube with sides of 4 cm. Is it larger or smaller than for a 2 cm cube?
- Surface area = 6 × 4 × 4 = 96 cm².
- Volume = 4 × 4 × 4 = 64 cm³.
- Ratio = 96 : 64 = 1.5 : 1. The 2 cm cube has 3 : 1.
Answer: 1.5 : 1, which is smaller than the ratio of 3 : 1 for the 2 cm cube.
You should be able to explain the need for exchange surfaces and a transport system in multicellular organisms in terms of surface area to volume ratio. Their surface area to volume ratio is small, so they cannot rely on diffusion across the outside of the body to supply all of their cells. They need specialised exchange surfaces and a transport system to move substances to and from cells deep inside the body.