Explain how the tracheal system of insects achieves efficient delivery of oxygen directly to respiring cells, without relying on the circulatory system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Tracheae are air‑filled tubes that branch repeatedly into smaller tracheoles, bringing oxygen close to every cell.
Oxygen diffuses directly from the tracheole tips to respiring cells along a short or steep concentration gradient, so a blood transport system is unnecessary.
Spiracles on the body surface can open and close to allow air entry whilst limiting water loss.
Active ventilation (e.g. body‑wall muscle contractions) or fluid movement at tracheole tips during intense activity reduces the effective diffusion distance and increases the rate of oxygen delivery.
Oxygen diffuses directly from the tracheole tips to respiring cells along a short or steep concentration gradient, so a blood transport system is unnecessary.
Spiracles on the body surface can open and close to allow air entry whilst limiting water loss.
Active ventilation (e.g. body‑wall muscle contractions) or fluid movement at tracheole tips during intense activity reduces the effective diffusion distance and increases the rate of oxygen delivery.
Examiner tips
- Use the exact terms ‘tracheae’, ‘tracheoles’, ‘spiracles’, ‘diffusion’, ‘ventilation’.
- Show the sequence: tracheae → tracheoles → cells, emphasising the short diffusion distance.
- Mention the role of spiracles in water‑loss control.
- Include the idea of active ventilation to reduce diffusion distance.
Common mistakes
- Confusing the circulatory system with the tracheal system; students may mention haemolymph transport.
- Omitting the function of spiracles or the importance of water‑loss control.
- Failing to explain how active ventilation reduces diffusion distance.
Mark scheme (4 marks)
- Tracheae are air-filled tubes that branch repeatedly into smaller tracheoles, bringing oxygen close to every cell.
- Oxygen diffuses directly from the tracheole tips to respiring cells along a short / steep concentration gradient, so a blood transport system is unnecessary.
- Spiracles on the body surface can open and close to allow air entry whilst limiting water loss.
- Active ventilation (e.g. body-wall muscle contractions) or fluid movement at tracheole tips during intense activity reduces the effective diffusion distance and increases the rate of oxygen delivery.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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