AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Students should be able to explain the need for exchange surfaces and a…
Full specification wording
Diffusion: Students should be able to explain the need for exchange surfaces and a transport system in multicellular organisms in terms of surface area to volume ratio.
Multicellular organisms are large, so their surface area to volume ratio is small.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 9 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Multicellular organisms are large, so their surface area to volume ratio is small. Diffusion across the outer surface alone would not supply enough oxygen and nutrients to every cell, nor remove enough carbon dioxide and other waste products, because the diffusion distance to the centre is too long and the surface area available is too small relative to the volume. Specialised exchange surfaces, such as the alveoli in lungs or villi in the small intestine, increase the surface area and shorten the diffusion path. A transport system, such as the circulatory system in mammals or the xylem and phloem in plants, moves substances between exchange surfaces and all body cells. Together these adaptations allow sufficient molecules to enter and leave cells to meet the organism's needs.
What earns the marks
- State that multicellular organisms have a small surface area to volume ratio.
- Explain that diffusion over the outer surface alone would be too slow or insufficient to supply all cells.
- Describe how exchange surfaces increase surface area and provide a short diffusion path.
- Explain that a transport system carries substances between exchange surfaces and body cells.
- Link these features to meeting the metabolic demands of a large organism.
Common mistakes and how to put them right
Saying large organisms have a large surface area to volume ratio.
Put it right by stating that the ratio decreases as size increases.
Confusing exchange surfaces with transport systems.
Put it right by describing exchange surfaces as sites of diffusion and transport systems as the means of moving substances around the body.
Claiming diffusion stops in large organisms.
Put it right by explaining that diffusion still occurs but is too slow or limited to meet the organism's needs without additional adaptations.
Exam tips
- Use the phrase surface area to volume ratio explicitly in your answer.
- Link each adaptation to a consequence, such as a short diffusion path increasing the rate of diffusion.
- Refer to named examples, such as alveoli or villi, to support your explanation.
Try the lesson
The start of the Diffusion: Students should be able to explain the need for exchange surfaces and a transport system in multicellular organisms in terms of surface area to volume ratio. lesson, made from checked teaching notes for this specification point.
Have a go first
Why does a large multicellular organism need a transport system, while a small single-celled organism does not?
Listen
Multicellular organisms are large, so their surface area to volume ratio is small. Diffusion across the outer surface alone would not supply enough oxygen and nutrients to every cell, nor remove enough carbon dioxide and other waste products, because the diffusion distance to the centre is too long and the surface area available is too small relative to the volume. Specialised exchange surfaces, such as the alveoli in lungs or villi in the small intestine, increase the surface area and shorten the diffusion path. A transport system, such as the circulatory system in mammals or the xylem and phloem in plants, moves substances between exchange surfaces and all body cells. Together these adaptations allow sufficient molecules to enter and leave cells to meet the organism's needs. A strong answer earns marks for these points. State that multicellular organisms have a small surface area to volume ratio. Explain that diffusion over the outer surface alone would be too slow or insufficient to supply all cells. Describe how exchange surfaces increase surface area and provide a short diffusion path. Explain that a transport system carries substances between exchange surfaces and body cells. A common slip: Saying large organisms have a large surface area to volume ratio. Instead, stating that the ratio decreases as size increases. Exam tip: Use the phrase surface area to volume ratio explicitly in your answer.
Key words
Say what each word means before you turn the card.
Surface area to volume ratio
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Exchange surfaces
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Transport systems
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Diffusion and organism size
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See every definition
- Surface area to volume ratio
- In multicellular organisms this is small because they are large, so the surface area available is too small relative to the volume.
- Exchange surfaces
- Specialised surfaces, such as the alveoli in lungs or villi in the small intestine, that increase the surface area and shorten the diffusion path.
- Transport systems
- Systems, such as the circulatory system in mammals or the xylem and phloem in plants, that move substances between exchange surfaces and all body cells.
- Diffusion and organism size
- Diffusion across the outer surface alone would not supply enough oxygen and nutrients to every cell, nor remove enough carbon dioxide and other waste products, because the diffusion distance to the centre is too long.
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