Surface area to volume ratio

• Surface area to volume ratio

  Ratio of the surface area to the volume of an organism
• Single-celled organisms like amoeba have a huge surface area for their volume
• Single-celled organisms can rely on diffusion to transport molecules in and out of their cell
• Calculating surface area to volume ratio

  1. Calculate surface area

  2. Calculate volume

  3. Divide surface area by volume
• Organisms with different side lengths

  • 1 mm sides
  • 2 mm sides
  • 3 mm sides
• As organisms get larger


  The surface area to volume ratio falls sharply
• Multicellular organisms have a surface area that is not large enough for their volume
• How animals have solved the problem of insufficient surface area

  • Special structures for gas exchange with high surface area (e.g. lungs)
  • Transport system to carry gases around the body
• Gills


  Where gases pass in and out of the blood in fish
• Adaptations of fish gills

  • Massive surface area from many fine filaments
  • Thin membrane for short diffusion pathway
  • Efficient blood supply to maintain concentration gradient
• These adaptations make diffusion as efficient as possible in fish gills