AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Adaptations of gas exchange surfaces, shown by gas exchange: across the…
Full specification wording
Gas exchange: Adaptations of gas exchange surfaces, shown by gas exchange: across the body surface of a single-celled organism in the tracheal system of an insect (tracheae, tracheoles and spiracles) across the gills of fish (gill lamellae and filaments including the counter-current principle) by the leaves of dicotyledonous plants (mesophyll and stomata).
Gas exchange surfaces share key features: large surface area, short diffusion pathway, and a maintained concentration gradient.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 1 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Gas exchange surfaces share key features: large surface area, short diffusion pathway, and a maintained concentration gradient. Single-celled organisms have a large surface area to volume ratio, allowing direct diffusion across the cell-surface membrane. Insects draw air through spiracles into tracheae, then highly branched tracheoles with thin walls, providing a short pathway directly to tissues. Fish gills use filaments covered in lamellae to increase surface area. Blood and water flow in opposite directions (counter-current principle), maintaining a diffusion gradient along the entire lamella. Dicotyledonous leaves use spongy mesophyll for a large internal surface area, with stomata pores allowing gases to diffuse in and out.
What earns the marks
- Link specific adaptations to their consequences, such as a thin surface (one cell thick) providing a short diffusion pathway for rapid diffusion.
- Identify structures that produce a large surface area, such as branching tracheoles, folded mesophyll, or numerous gill lamellae.
- Describe counter-current flow in fish as water and blood flowing in opposite directions across the gill lamellae.
- Explain that counter-current flow maintains a concentration gradient along the whole length of the lamella, ensuring continuous oxygen diffusion into the blood.
- Detail how stomata open to allow carbon dioxide to diffuse into the leaf for photosynthesis, while oxygen diffuses out.
Common mistakes and how to put them right
Naming an adaptation without explaining its consequence on the rate of diffusion.
Stating that counter-current flow means all the oxygen is absorbed, instead of explaining that it maintains a concentration gradient across the entire lamella.
Confusing gill filaments with gill lamellae, or tracheae with tracheoles.
Stating that gas exchange happens in the tracheae, rather than at the thin-walled tracheoles.
Exam tips
- Write every adaptation as 'feature, so consequence': e.g., walls one cell thick, so a short diffusion pathway, so rapid diffusion.
- For counter-current questions, explicitly state that blood and water flow in opposite directions before explaining the maintained gradient.
- Use the specific organism named in the stem; generic answers about 'exchange surfaces' often miss application requirements.
Try the lesson
The start of the Gas exchange: Adaptations of gas exchange surfaces, shown by gas exchange: across the body surface of a single-celled organism in the tracheal system of an insect (tracheae, tracheoles and spiracles) across the gills of fish (gill lamellae and filaments including the counter-current principle) by the leaves of dicotyledonous plants (mesophyll and stomata). lesson, made from checked teaching notes for this specification point.
Have a go first
A single-celled organism such as Amoeba is small and has a large surface area to volume ratio. Which statement best explains why this allows oxygen to reach all of its cytoplasm by diffusion alone?
Listen
Gas exchange surfaces share key features: large surface area, short diffusion pathway, and a maintained concentration gradient. Single-celled organisms have a large surface area to volume ratio, allowing direct diffusion across the cell-surface membrane. Insects draw air through spiracles into tracheae, then highly branched tracheoles with thin walls, providing a short pathway directly to tissues. Fish gills use filaments covered in lamellae to increase surface area. Blood and water flow in opposite directions (counter-current principle), maintaining a diffusion gradient along the entire lamella. Dicotyledonous leaves use spongy mesophyll for a large internal surface area, with stomata pores allowing gases to diffuse in and out. A strong answer earns marks for these points. Link specific adaptations to their consequences, such as a thin surface (one cell thick) providing a short diffusion pathway for rapid diffusion. Identify structures that produce a large surface area, such as branching tracheoles, folded mesophyll, or numerous gill lamellae. Describe counter-current flow in fish as water and blood flowing in opposite directions across the gill lamellae. Explain that counter-current flow maintains a concentration gradient along the whole length of the lamella, ensuring continuous oxygen diffusion into the blood. Exam tip: Write every adaptation as 'feature, so consequence': e.g., walls one cell thick, so a short diffusion pathway, so rapid diffusion.
Key words
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large surface area
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short diffusion pathway
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counter-current flow
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mesophyll and stomata
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See every definition
- large surface area
- A feature of gas exchange surfaces that provides more space for diffusion, shown by branching tracheoles, folded mesophyll or numerous gill lamellae.
- short diffusion pathway
- A feature of gas exchange surfaces such as a thin surface one cell thick, giving rapid diffusion.
- counter-current flow
- Water and blood flowing in opposite directions across the gill lamellae, maintaining a concentration gradient along the whole length of the lamella.
- mesophyll and stomata
- Spongy mesophyll gives a large internal surface area, while stomata are pores allowing gases to diffuse in and out of the leaf.
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