Gas exchange in single celled organisms and insects

• Gas exchange surface

  • Large surface area
  • Thin (1 layer of epithelial cells)
  • Short diffusion pathway across the gas exchange surface
• Gas exchange in single cell organisms

  Absorb and release gas by diffusion through the cell surface membrane
• Gas exchange in single cell organisms


  • Large surface area
  • Thin surface
  • Short diffusion pathway
  • No need for a specialised gas exchange system
• Insects

  • Developed a network of tubes called tracheae for gas exchange
  • Tracheae are supported by strengthened rings to prevent them from collapsing
• Gas exchange in insects

  1. Atmospheric air containing oxygen is brought directly to respiring tissues

  2. Short diffusion pathway from tracheoles to any body cells
• Tracheae


  Network of tubes for gas exchange in insects
• Tracheoles


  Smaller dead end tubes that tracheae divide into
• Tracheoles extend throughout all body tissues in the insect
• Respiratory gases move in and out tracheal system

  1. Mass transport

  2. Diffusion gradient

  3. Water movement
• Mass transport

  • contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out. Further speeding up exchange of respiratory gases.
• A long diffusion gradient

  • When cells are respiring, oxygen is used up so its concentration towards the end of the tracheoles falls. This creates a diffusion gradient causing gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cell. Carbon dioxide is produced by cells during respiration; this creates a diffusion gradient in the opposite direction causing gaseous carbon dioxide to diffuse along tracheoles and tracheae from the cells to the atmosphere. As diffusion in the air is more rapid than water so respiratory gases are exchanged quicker by this method.
• Ends of tracheoles are filled with water

  • during periods of major activity muscle cells respire via anaerobic respiration. This produces lactate which is soluble and lowers water potential of the muscle cells. Water moves into cells from the tracheoles via osmosis. Water in the end of the tracheoles decreases in volume and in doing so draws air further into them. This means the final diffusion pathway is in a gaseous state not a liquid so diffusion is more rapid.
• What are spiracles?

  Gases enter and leave tracheae through tiny pores called spiracles on the body surface. Spiracles are opened and closed by valves. When spiracles are open, water vapour can evaporate from the insect. So most of the time the spiracles are closed to prevent water loss but open occasionally for gas exchange.
• Limitations of the tracheal system:

  • Relies mostly on diffusion to exchange gases between environment and cells 
  • For diffusion to be effective the diffusion pathway must be short which is why insects are of a short size so length of a short pathway limits the size an insect can attain.