Single celled organisms

  • exchange gases, nutrients and waste products across outer surfaces because they have a larger SA:V (surface area to volume ratio).
  • low metabolic activity—diffusion via outer surface meets demands.

Multicellular organisms

  • smaller SA:V
  • higher metabolic demand
  • outer surface not large enough for diffusion to meet demands
  • substances have to travel further to reach the centre of the organism
  • require specialized exchange surfaces
  • specialized exchange surfaces: surfaces over which materials are exchanged for another material.

Features of good exchange surfaces

  • increased SA—overcomes small SA:V of organism
  • thin layers reduce distance for diffusion
  • good blood supply creates a steep concentration gradient
  • ventilation—maintains steep concentration gradient for movement of gases