The need for exchange surfaces

Organisms must take in food,oxygen and water, and other essential substances, from the environment. Plants also need carbon dioxide for photosynthesis. Organisms also need to remove waste substances.

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.

Modelling cells

If we represent the cell of an organism by a cube:

It’s straightforward to model cells using cubes.

This is what happens when the cube increases in size:

A table showing the different dimensions of a cube

As the volume increases, surface area does not increase at the same rate.

As multicellular organisms increase in size, they face two problems:

ProblemSolution to the problem
Their surface area does not increase as fast as the volumeInsufficient surface area to meet their needsBody systems that add additional absorbing area to exchange surfaces
Their volume increasesDiffusion is not quick enough to move substances to where they’re needed in the organism’s bodyA transport system