Explain how gas exchange occurs in an insect, referring to the structures involved and the process by which gases move.

AQA A-Level Biology (7402) — 3.3.2 Gas Exchange · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Insects possess small openings called spiracles on the surface of their body. Air enters through these spiracles and travels into a network of tubes called tracheae. The tracheae branch into finer tubes called tracheoles, which extend to individual cells and tissues. Oxygen diffuses from the tracheoles into the cells, while carbon dioxide diffuses out of the cells into the tracheoles. The large surface area, thin walls and moist lining of the tracheoles maintain a steep concentration gradient, allowing efficient diffusion of gases.

Examiner tips

  • Use the exact terms spiracles, tracheae, tracheoles; mention diffusion of O₂ and CO₂; explain the role of surface area and moist lining.
  • Structure the answer in a logical sequence: spiracles → tracheae → tracheoles → diffusion.
  • Show the concentration gradient explanation to earn the extra mark.

Common mistakes

  • Confusing tracheae with blood vessels; not mentioning spiracles; omitting the diffusion of CO₂ or the role of surface area/moist lining.

Mark scheme (5 marks)

  1. Insects have small openings called spiracles on the surface of their body
  2. Air moves through spiracles into a network of tubes called tracheae (tracheal tubes)
  3. Tracheae branch into smaller tubes called tracheoles which reach individual cells/tissues
  4. Oxygen diffuses from the tracheoles into the cells (and carbon dioxide diffuses out of cells into tracheoles)
  5. A large surface area / thin walls of tracheoles / moist lining maintains a steep concentration gradient / allows efficient diffusion

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