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
• 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 alone
• 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 fish gills

  • Massive surface area from many fine filaments
  • Thin membrane for short diffusion pathway
  • Efficient blood supply to maintain concentration gradient