exchanging substances

• Surface Area to Volume Ratio

  The ratio between the surface area and volume of an object
• Single-celled organism


  • Large surface area to volume ratio
  • Needs of organism can be met by exchange across outer surface
• Multicellular organism


  • Small surface area to volume ratio
  • Many cells too far away from outer surface to get substances in and out this way
  • Exchange surfaces and transport systems are needed so needs of every cell can be met
• Exchanging Substances


  Enough substances can pass across outer surface to meet the needs of organism
• Exchange Surfaces


  • Large surface area (so lots can diffuse at once)
  • Thin membrane
  • Efficient blood supply (in animals)
  • Ventilation (in gas exchange in animals)
• Leaves


  • Flat shape
  • Stomata let gases in and out
• Small intestine


  • Single layer of surface cells
  • Covered in villi
• Gills


  • Lots of gill filaments
  • Lamellae have lots of capillaries
  • Lamellae have a thin surface layer of cells
• Alveoli in lungs


  • There are millions of alveoli
  • Air moves in and out
  • Alveoli have thin walls with capillary networks