Gas exchange in single celled organisms
SIngle celled organisms = small so have large surface area to volume ratio. Oxygen is absorbed by diffusion across their body surface (which is their cell surface membrane).
Carbon dioxide from respiration diffuses across their body surface as well.
The cell walls are completely permeable.
Gas exchange in insects
Most insects are terrestrial (live on land) so water easily evaporates from surface of their bodies = they become dehydrated so need to conserve water.
Efficient gas exchange needs thin permeable surface with a large area but this conflicts the need to conserve water.
Terrestrial organisms therefore usually exhibit two features:
- waterproof coverings - in insects this is a rigid outer skeleton with a waterproof cuticle.
- small surface area to volume ratio (minimises area over which water can be lost).