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 to volume ratio
• 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 multicellular organisms solve the problem of low surface area to volume ratio

  • Have special structures for gas exchange with high surface area (e.g. lungs)
  • Have a transport system to carry gases around the body
• Gills

  Where gas exchange occurs in fish
• Adaptations of fish 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
• the small intestine and lungs in mammals, gills in fish, and the roots and leaves in plants, are adapted for exchanging materials.
• In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism's

  needs.
• The effectiveness of an exchange surface is increased by:

  • having a large surface area
  • a membrane that is thin, to provide a short diffusion path
  • (in animals) having an efficient blood supply
  • (in animals, for gaseous exchange) being ventilated.