Single-celled organisms have relative large surface area to volume ratios. Larger multicellular organisms have smaller surface area to volume ratios. So, they have evolved exchange surfaces to exchange molecules with their surroundings.
The effectiveness of exchange surfaces in plants and animals is increased by having:
A large surface area:
A short distance required fordiffusion:
Animals have additional adaptations for effective exchange surfaces.
An efficient blood supply to transport molecules to and from the exchange surface increases effective exchange. Examples of this include:
The process of breathing, or ventilation, brings air to, and removes air from the exchange surface – the alveoli.
The moving blood and ventilated surfaces mean that a steep concentration gradient can be maintained. This increases effective exchange.