AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Students should be able to explain how the small intestine and lungs in…
Full specification wording
Diffusion: Students should be able to explain how the small intestine and lungs in mammals, gills in fish, and the roots and leaves in plants, are adapted for exchanging materials.
Exchange surfaces are adapted to maximise diffusion.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 10 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Exchange surfaces are adapted to maximise diffusion. In mammals, the small intestine has villi and microvilli that increase surface area, a thin wall one cell thick giving a short diffusion path, and a rich blood supply that maintains a steep concentration gradient. The lungs contain alveoli with a large total surface area, thin walls, a dense capillary network and ventilation that refreshes air. Fish gills have many filaments and lamellae, a thin surface, a counter-current blood flow and ventilation by water passing over the gills. Plant roots have root hair cells that increase surface area for water and mineral ion uptake, while leaves have a large flat surface, thin lamina, stomata for gas exchange and internal air spaces. Each structure therefore increases surface area, shortens the diffusion path or maintains a concentration gradient.
What earns the marks
- Describe how villi and microvilli in the small intestine increase surface area and how a thin wall and blood supply aid absorption.
- Explain how alveoli, thin walls, capillaries and ventilation adapt lungs for gas exchange.
- Describe gill filaments and lamellae, thin surfaces and counter-current flow in fish gills.
- Explain how root hair cells increase surface area for uptake of water and mineral ions.
- Describe leaf adaptations such as a flat blade, thin lamina, stomata and air spaces for gas exchange.
Common mistakes and how to put them right
Describing villi as increasing volume rather than surface area.
Put it right by stating that villi increase the surface area for absorption.
Saying fish gills exchange gases with air.
Put it right by stating that gills exchange gases with water.
Forgetting that a concentration gradient must be maintained.
Put it right by linking blood flow or ventilation to keeping the gradient steep.
Exam tips
- Name the organism and organ, then state the adaptation and its benefit.
- Use comparative language such as increases surface area or shortens the diffusion path.
- Include at least one adaptation linked to maintaining a concentration gradient for each example.
Try the lesson
The start of the Diffusion: Students should be able to explain how the small intestine and lungs in mammals, gills in fish, and the roots and leaves in plants, are adapted for exchanging materials. lesson, made from checked teaching notes for this specification point.
Have a go first
Which feature of the small intestine increases the surface area available for absorbing digested food?
Listen
Exchange surfaces are adapted to maximise diffusion. In mammals, the small intestine has villi and microvilli that increase surface area, a thin wall one cell thick giving a short diffusion path, and a rich blood supply that maintains a steep concentration gradient. The lungs contain alveoli with a large total surface area, thin walls, a dense capillary network and ventilation that refreshes air. Fish gills have many filaments and lamellae, a thin surface, a counter-current blood flow and ventilation by water passing over the gills. Plant roots have root hair cells that increase surface area for water and mineral ion uptake, while leaves have a large flat surface, thin lamina, stomata for gas exchange and internal air spaces. Each structure therefore increases surface area, shortens the diffusion path or maintains a concentration gradient. A strong answer earns marks for these points. Describe how villi and microvilli in the small intestine increase surface area and how a thin wall and blood supply aid absorption. Explain how alveoli, thin walls, capillaries and ventilation adapt lungs for gas exchange. Describe gill filaments and lamellae, thin surfaces and counter-current flow in fish gills. Explain how root hair cells increase surface area for uptake of water and mineral ions. A common slip: Describing villi as increasing volume rather than surface area. Instead, stating that villi increase the surface area for absorption. Exam tip: Name the organism and organ, then state the adaptation and its benefit.
See it
Key words
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Surface area and diffusion
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Concentration gradients
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Exchange surfaces in animals
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Exchange surfaces in plants
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See every definition
- Surface area and diffusion
- Villi and microvilli in the small intestine, alveoli in the lungs, gill filaments and lamellae, root hair cells and a large flat leaf surface all increase surface area to maximise diffusion.
- Concentration gradients
- A rich blood supply, a dense capillary network, ventilation and counter-current blood flow all maintain a steep concentration gradient so diffusion continues.
- Exchange surfaces in animals
- The small intestine, lungs and fish gills are adapted with thin walls, a short diffusion path and a blood supply for exchanging materials.
- Exchange surfaces in plants
- Roots have root hair cells for uptake of water and mineral ions, while leaves have a thin lamina, stomata and internal air spaces for gas exchange.
Label the diagram
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