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. Surface area = 6 x side length^2

  2. Volume = side length^3

  3. Ratio = surface area / volume
• As organisms get larger, the surface area to volume ratio falls sharply
• Problem for multicellular organisms

  • Surface area is not large enough for their volume
  • Cells in the center cannot get enough oxygen by diffusion
• How animals solve the problem

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


  Where gas exchange occurs in fish
• Adaptations of gills to increase rate of gas diffusion

  • Massive surface area from many fine filaments
  • Thin membrane for short diffusion pathway
  • Efficient blood supply to maintain concentration gradient
• Gills increase the rate of transport of gases into and out of fish