First 236 words of the document:
Single Celled Organism
> Large surface area to volume ratio
> Gas exchange by diffusion
> Short diffusion pathway, distance from edge of cell to centre is small increases rate of diffusion.
> Can have flagella for movement maintain concentration gradient.
> Rigid outside skeleton, coated in waxy, waterproof substance to minimise water loss
prevents insect from drying out. Waxy layer is impossible for gases to diffuse though.
> Tracheal system Spiracles are holes in the side of the body of the insect that can be
opened or closed. The air enters trachea through spiracles then into tracheoles which
penetrate between cells into muscle fibres where gaseous exchange takes place.
> In very small insects this system can provide sufficient oxygen by simple diffusion.
> Larger animals close spiracles then muscles pull skeletal plates of abdominal segments
together, pumping air more quickly further into tracheoles.
> Larger animals, at rest, also fill the very ends of the tracheoles with water. When the
muscles respire (partly anaerobically) lactate is produced, which is soluble. Therefore the
build up of lactate in the muscle lowers the water potential so the water from the
tracheoles moves into the muscles cells, drawing air closer to muscle cells for short
diffusion pathway for when oxygen is needed most.