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
• Organisms with different side lengths

  • 1 mm side length: surface area 6 mm^2, volume 1 mm^3, ratio 6:1
  • 2 mm side length: surface area 24 mm^2, volume 8 mm^3, ratio 3:1
  • 3 mm side length: surface area 54 mm^2, volume 27 mm^3, ratio 2:1
• As organisms get larger, the surface area to volume ratio falls sharply
• Multicellular organisms


  • Their surface area is not large enough for their volume
  • Cells on the surface can get enough oxygen by diffusion, but not enough oxygen can diffuse into the cells in the center
• How animals solve the problem of low surface area to volume ratio

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


  • Covered in a large number of fine filaments
  • Filaments have a thin membrane for short diffusion pathway
  • Filaments have an efficient blood supply to take oxygenated blood away
• The adaptations of gill filaments make diffusion as efficient as possible
• You'll find plenty of questions on surface area to volume ratio in the revision workbook