SURFACE AREA TO VOLUME RATIO

• • Single celled organisms such as amoeba can rely on diffusion to transport molecules in and out of the cells
• • As organisms get larger, the surface area : volume ratio falls sharply
• How gills increase rate of transport of gases into and out of fish

  Oxygen rich water passes into the mouth, it then flows over gills where oxygen is transported into bloodstream

  Gills are covered in large number of fine filaments where gases pass in and out of the blood

  Deoxygenated blood passes into filament, oxygen diffuses from water into the blood, oxygenated blood returns to the body
• Adaptations of filament (making diffusion as efficient as possible)

  Filaments give gills a massive surface area

  Filaments also have thin membrane to provide short diffusion pathway

  Filaments have efficient blood supply which ensures concentration gradient is always high