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AQA A-level Biology — Exchange and Transport flashcards

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AQA A-level Biology topic 3: surface area to volume ratio, gas exchange in insects, fish, leaves and lungs, digestion and absorption, haemoglobin, the heart and blood vessels, tissue fluid, xylem and phloem (required practical 5).

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Learn deck · part of AQA A-level Biology · written against: AQA AS and A-level Biology (7401/7402) specification, version 1.6, July 2026 (eight topics). Updated 2026-10-10.

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  1. How does the surface area to volume ratio change as an organism gets larger?
    It decreases: volume grows faster (with the cube of length) than surface area (with the square).

    So a large organism has relatively less surface through which to exchange substances.
    Exchange of Substances › Surface Area to Volume Ratio
  2. Why can't a large multicellular animal rely on diffusion across its body surface alone?
    Its surface area to volume ratio is small and most cells are too far from the surface, so diffusion would be too slow to meet demand.

    Large animals therefore have specialised exchange surfaces linked to mass transport systems.
    Exchange of Substances › Surface Area to Volume Ratio
  3. What is the surface area to volume ratio of a cube with sides of 3 cm?
    2 : 1

    Surface area = 6 × 3² = 54 cm²; volume = 3³ = 27 cm³; 54 ÷ 27 = 2.
    Exchange of Substances › Surface Area to Volume Ratio
  4. Why do small mammals have a higher metabolic rate per gram than large mammals?
    Their large surface area to volume ratio means they lose heat faster, so they must respire faster to replace it.

    This is why shrews eat almost constantly.
    Exchange of Substances › Surface Area to Volume Ratio
  5. How does a flatworm's body shape make up for having no gas exchange system?
    Its flattened shape gives a large surface area to volume ratio and keeps every cell close to the surface.

    The diffusion distance stays short, so diffusion alone can supply its cells.
    Exchange of Substances › Surface Area to Volume Ratio
  6. Agar cubes of different sizes containing indicator are placed in acid. Which changes colour all the way through first, and why?
    The smallest cube, because it has the largest surface area to volume ratio and the shortest distance to its centre.

    Acid diffuses in at a similar rate per unit area, but has less volume to fill and less distance to travel.
    Exchange of Substances › Surface Area to Volume Ratio
  7. How is a single-celled organism adapted for gas exchange?
    Gases diffuse across its body surface, which is enough because it has a large surface area to volume ratio and a short diffusion distance.

    No specialised gas exchange system is needed.
    Exchange of Substances › Gas Exchange
  8. Trace the path of oxygen from the air to a flight muscle cell of an insect.
    Spiracles → tracheae → tracheoles → muscle cells: the tracheoles reach right into the tissues.

    Oxygen diffuses down a gradient maintained by the muscle cells using it in respiration.
    Exchange of Substances › Gas Exchange
  9. How can an active insect ventilate its tracheal system?
    Abdominal pumping: muscles contract and relax, squeezing the tracheae so air moves in and out by mass transport.

    This keeps a steep diffusion gradient, faster than diffusion alone could.
    Exchange of Substances › Gas Exchange
  10. How do terrestrial insects limit water loss while still exchanging gases?
    They close their spiracles with valves when possible, and have a waterproof waxy cuticle; some have hairs round the spiracles.

    This is a compromise: spiracles must open for gas exchange, and water vapour escapes when they do.
    Exchange of Substances › Gas Exchange

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About these cards

Review status: Second-reader reviewed, with corrections applied.

Checked against:

  • Topic outline follows the AQA AS and A-level Biology (7401/7402) specification, version 1.6 (July 2026) (outline only; no specification text reproduced).
  • OpenStax Biology 2e and OpenStax Anatomy and Physiology 2e, consulted as references; no text copied.
  • Also checked against the AQA AS and A-level Biology required practical handbook, version 2.3 (January 2026) (reference only; no text copied).

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