6 Exchange in single-celled organisms & insects

  • Created by: lee8444
  • Created on: 05-03-20 09:11

Gas exchange in single-celled organisms

  • Very small
  • Large SA:V ratio
  • Oxygen is absorbed by diffusion
  • Only has to diffuse through the membrane
  • Carbon dioxide diffuses out
  • When there is a cell wall, there is no added resistance to diffusion
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Gas exchange in insects

  • Increasing their surface area for better exchange of gases can cause insects to lose water quickly as water will evaporate
  • Insects conserving water is topic 6.5 (limiting water loss)
  • For gas exchange, insects have tracheae
  • Tracheae are supported by rings to prevent them from collapsing
  • Tracheae divide into dead-end tubes called tracheoles
  • Tracheoles extend through the tissues
  • Atmospheric air is brought directly to the respiring tissues due to the short diffusion pathway
  • Gas enters and leaves through small pores called spiracles
  • Spiracles can be opened and closed by valves
  • When open, water vapour can evaporate out of the insect which is bad
  • Therefore, spiracles are mainly kept shut to prevent water loss
  • The tracheal system is an extremely efficient method of gas exchange
  • However, it relies heavily on diffusion and so the size of insects is limited as the bigger the insect, the larger the diffusion distance
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Gases move in and out in 3 ways

  • Along a diffusion gradient
    • When cells respire, oxygen is used up decreasing the concentration of oxygen at respiring tissues creating a diffusion gradient
    • Oxygen diffuses down the tracheae and tracheoles to the respiring tissues along this concentration gradient
    • Carbon dioxide is produced during respiration so respiring tissues cause an increase in carbon dioxide concentration making a concentration gradient of carbon dioxide
    • Carbon dioxide therefore diffuses along the diffusion pathway out of the insects
    • This causes rapid transport of gases
  • Mass Transport
    • Contraction of muscles in the insect can squeeze the tracheae
    • This creates the mass movement of gas in and out of the insect
    • This increases the speed of exchange of respiratory gases
  • Water filled tracheoles
    • Anaerobic respiration produces lactate which decreases the water potential of muscle cells
    • Water moves into the cells by osmosis from the tracheoles bringing oxygen with it
    • The final diffusion pathway is in a gas not liquid state making it faster
    • However, this method increases the rate of evaporation increasing water loss
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