Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
Explain the need for exchange surfaces and a transport system in…
Full specification wording
Topic 8 – Exchange and transport in animals: 8.2 Explain the need for exchange surfaces and a transport system in multicellular organisms including the calculation of surface area : volume ratio
Multicellular organisms are large, so diffusion across the outer surface is too slow to supply every cell.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 2 of 10
Study notes
From the checked teaching notes for this specification point.
The idea
Multicellular organisms are large, so diffusion across the outer surface is too slow to supply every cell. They therefore need specialised exchange surfaces and a transport system. An exchange surface has a large surface area, a thin membrane for a short diffusion distance, and in animals a good blood supply to maintain a steep concentration gradient. A transport system such as blood and a heart, or xylem and phloem in plants, carries substances to and from all cells. As an organism gets bigger, its surface area : volume ratio falls, so diffusion alone becomes less adequate. To calculate the ratio, find surface area and volume, then divide surface area by volume. For a cube of side 2 cm: surface area = 6 × 2² = 24 cm², volume = 2³ = 8 cm³, so the ratio is 24 : 8, which simplifies to 3 : 1.
What earns the marks
- Multicellular organisms have a small surface area : volume ratio, so diffusion across the body surface is too slow.
- Exchange surfaces increase surface area and are thin to shorten the diffusion distance.
- A good blood supply maintains a steep concentration gradient across an exchange surface.
- A transport system carries substances to and from all cells of a multicellular organism.
- Surface area : volume ratio is calculated by dividing total surface area by volume.
- For a cube of side 2 cm, surface area is 24 cm² and volume is 8 cm³, giving a ratio of 3 : 1.
Common mistakes and how to put them right
Adding surface area and volume instead of dividing surface area by volume.
Put it right by dividing surface area by volume to find the ratio.
Forgetting to square the side when finding the area of one face of a cube.
Put it right by using side² for each face and multiplying by six.
Claiming a large organism has a large surface area.
Put it right by stating the ratio decreases as size increases.
Exam tips
- Show the surface area and volume calculations separately before forming the ratio.
- Simplify the ratio to its simplest whole-number form where possible.
- Link each feature of an exchange surface to its effect on diffusion rate when explaining the need.
Try the lesson
The start of the Topic 8 – Exchange and transport in animals: 8.2 Explain the need for exchange surfaces and a transport system in multicellular organisms including the calculation of surface area : volume ratio lesson, made from checked teaching notes for this specification point.
Have a go first
A cube-shaped organism has sides of 2 cm. What is its surface area : volume ratio, written in its simplest form?
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