Exchanging substances: Exchange surface

• 1. multicellular organisms have smaller surface area:volume compared to single-celled organisms - not enough substances can diffuse from their outside surface to supply their entire volume
• 2. they need some sort of exchange surface for efficient diffusion. The exchange surface structures have to allow enough of the necessary substances to pass through
• Exchange surfaces are adapted to maxmise effectiveness:

  • Thin membrane - substances have short distance to diffuse
  • large surface area - lots of a substance can diffuse at once
  • animals:lots of blood vessels - to get stuff in and out of blood quickly
  • animals (alveoli): often ventilated - air moves in and out