Small organisms exchange these essential and waste substances between themselves and the environment. They do this over their body surface. Simple chemical substances can diffuse in and out of their bodies.
Inside their bodies, in small organisms, substances don’t have to move far.
The size of their surface, or surface area, defines how quickly they can absorb substances. The size of their volume defines how much of these substances they need.
If we represent the cell of an organism by a cube:
This is what happens when the cube increases in size:
As the volume increases, surface area does not increase at the same rate.
As multicellular organisms increase in size, they face two problems:
|Problem||Solution to the problem|
|Their surface area does not increase as fast as the volume||Insufficient surface area to meet their needs||Body systems that add additional absorbing area to exchange surfaces|
|Their volume increases||Diffusion is not quick enough to move substances to where they’re needed in the organism’s body||A transport system|