Exchange

FEATURES OF SPECIALISED EXCHANGE SURFACES:

• A large surface area relative to the volume of the organism which increases the rate of exchange.
• Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly.
• Selectively permeable to allow selected materials to cross.
• Movement of the envrionmental medium, for example, air, to maintain a diffusion gradient.
• A transport system to ensure the movement of the internal medium for example blood, in order to maintain a diffusion gradient.

GAS EXCHANGE IN INSECTS:

• Insects have evolved an internal network of tubes called tracheae.
• The tracheae are supported by strengthened rings to prevent them from collapsing.
• The tracheae are supported by strengthened rings to prevent them from collapsing.
• The tracheae divide into smaller dead-end tubs called tracheoles.
• The tracheoles extend throughout all the body tissues of the insect.
• In this way atmospheric air, with the oxygen it contains, is brought directly to the respiring tissues, as there is a short diffusion pathway from a tracheole to any body cell.
• Respiratory gases move in and out of the tracheal system in 3 ways:
  • Along a diffusion gradient:
    • When cells are respriring, oxygen is used up and so its concentration gradient towards the ends of the tracheoles falls.
    • This creates a diffusion graident that causes gaseous oxygen to the cells.
    • Carbon dioxide is produced by cells during respiration.
    • This creates a diffusion gradient in the opposite direction.
    • This causes gaseous carbon dioxide to diffuse along the tracheoles and tracheae from the cells to the atmosphere.
    • As diffusion in air is much more rapid than in water, respiratory gases are exchanged quickly by this method.
  • Mass transport:
    • The contraction of muscles in insects can squeeze the trachea enabling mass movemnts of air in and out.
    • This further speeds up the exchange of respiratory gases.
  • …