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Why do larger organisms need exchange surfaces, and how are the alveoli adapted for gas exchange?

Explain the need for exchange surfaces and a transport system using surface area to volume ratio, how the alveoli are adapted for gas exchange, and the factors affecting the rate of diffusion including Fick's law.

A focused answer to Edexcel GCSE Biology 8.1 to 8.5B, covering the need for exchange surfaces and transport, surface area to volume ratio, how the alveoli are adapted for gas exchange, and Fick's law.

Generated by Claude Opus 4.810 min answer

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  1. What this dot point is asking
  2. The need for exchange and transport
  3. Surface area to volume ratio
  4. How the alveoli are adapted
  5. Fick's law (Biology only)
  6. Try this

What this dot point is asking

Edexcel statements 8.1 to 8.5B want you to explain the need to transport substances, why large organisms need exchange surfaces and a transport system (using surface area to volume ratio), how the alveoli are adapted for gas exchange, and the factors affecting the rate of diffusion including Fick's law (8.4B and 8.5B are Biology only).

The need for exchange and transport

Surface area to volume ratio

As an organism gets bigger, its volume (the number of cells needing supply) grows faster than its surface area (where exchange happens). So large organisms have a small surface area to volume ratio: their outer surface is too small, and inner cells are too far from the surface, for diffusion to supply everything.

That is why large multicellular animals need:

  • Exchange surfaces with a large surface area (such as the alveoli in the lungs and the villi in the gut), and
  • a transport system (the blood and circulation) to carry substances quickly between the exchange surfaces and every cell.

A single-celled organism, by contrast, has a large SA:V ratio and short diffusion distances, so it exchanges everything across its surface without special structures.

How the alveoli are adapted

Fick's law (Biology only)

Fick's law summarises what speeds up diffusion: a larger surface area, a bigger concentration difference, and a shorter diffusion distance all increase the rate. This is exactly why the alveoli (large area, steep gradient, thin walls) are such efficient exchange surfaces.

Try this

Q1. State why a large animal needs a transport system but a single-celled organism does not. [2 marks]

  • Cue. A large animal has a small surface area to volume ratio and long diffusion distances, so diffusion alone is too slow; a single cell has a large ratio and short distances, so diffusion is enough.

Q2. Give two adaptations of the alveoli for gas exchange. [2 marks]

  • Cue. Any two of: large surface area, thin (one-cell) walls, good blood supply, moist surface.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20194 marksExplain why large multicellular animals need a specialised exchange surface and a transport system, but a single-celled organism does not.
Show worked answer →

A 4-mark explain question rewards the surface area to volume argument.

A single-celled organism has a large surface area to volume ratio, so diffusion across its surface is fast enough to supply all its needs and remove waste over the short distances involved.

A large animal has a small surface area to volume ratio, so its outer surface is too small, and the diffusion distances to inner cells are too great, for diffusion alone to supply enough oxygen and nutrients. It therefore needs exchange surfaces (such as alveoli) with a large surface area, and a transport system (blood) to carry substances to and from every cell.

Markers reward the contrast in surface area to volume ratio and the link to diffusion distance. Saying big animals just need more oxygen, without the ratio argument, scores little.

Edexcel 20214 marksDescribe three ways the alveoli in the lungs are adapted for efficient gas exchange.
Show worked answer →

A 4-mark question rewards adaptations each linked to faster diffusion.

  1. There are millions of alveoli, giving a very large total surface area for gas exchange.
  2. The alveoli have thin walls (one cell thick), giving a short diffusion distance for gases.
  3. They have a good blood supply from many capillaries, which maintains a steep concentration gradient.
  4. They are moist, so gases dissolve before diffusing.

Markers reward at least three adaptations, each with the link to the rate of diffusion (large surface area, short distance, steep gradient, moist surface). Listing features with no link to gas exchange caps the marks.

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