3.1.3Exchange and Transport
Adaptation to Facilitate Exchange — AQA A Level Biology 7402 Revision Notes & Topical Questions
5 min read 9 parts Lesson 6 of 48 1 quiz
Lesson summary
3.1.3 Adaptation to Facilitate Exchange — AQA A Level Biology 7402 notes: Explain how organisms of different sizes and in different environments are adapted to maximise exchange, relating adaptations to surface area, diffusion distance and concentration gradients.Because SA:V fal…
- Changes to body shape
- Specialised exchange organs
- Transport systems and ventilation
- Large surface area
- Thin barrier
- Good blood supply
Section 1 of 9Three strategies
Because SA:V falls as size increases, larger or more active organisms use one or more of three strategies:
Three strategies
Changes to body shape
Changes to body shape flat, thin or elongated shapes keep the SA:V ratio high, like a flatworm or a leaf.
Specialised exchange organs
Specialised exchange organs structures with enormous folded or branched surfaces: lungs, gills, tracheal systems, villi and root hairs.
Transport systems and ventilation
Transport systems and ventilation a circulatory system carries substances between the exchange surface and the cells, and ventilation or flow keeps the gradient steep.
Section 2 of 9Comparison of organisms
| Organism | Exchange surface | How the gradient is maintained |
|---|---|---|
| Unicellular (amoeba) | Cell-surface membrane | Oxygen used inside, CO2 diffuses out |
| Flatworm | Body surface; flat shape | Short diffusion distance, aquatic environment |
| Insect | Tracheoles | Ventilation by abdominal pumping; oxygen used up |
| Fish | Gill lamellae | Counter-current flow |
| Mammal | Alveoli | Breathing; blood circulation |
| Plant | Spongy mesophyll and stomata | Photosynthesis uses CO2; diffusion through air spaces |
Section 3 of 9Adaptations in detail
Adaptations in detail
Large surface area
Large surface area the human lungs have about 300 million alveoli giving a surface area of about 70 square metres; the small intestine has villi and microvilli; leaves are broad and flat with many air spaces in the mesophyll.
Thin barrier
Thin barrier alveolar walls and capillary walls are one cell thick, so the diffusion path is about 0.5 micrometres.
Good blood supply
Good blood supply capillary networks remove oxygen and bring carbon dioxide, maintaining the gradient.
Ventilation
Ventilation movement of air or water over the surface replaces the fluid and keeps the concentration gradient steep.
Waterproofing and moisture
Waterproofing and moisture exchange surfaces are moist so gases dissolve, but this causes water loss, a problem addressed in the lesson on gas exchange versus water loss.
Section 4 of 9Linking to function
When explaining an adaptation, link it to Fick's law: rate of diffusion is proportional to (surface area x concentration difference) / thickness.
Linking to function
For instance: 'the alveoli are very numerous so the surface area is large, which increases the rate of diffusion of oxygen'.
Always state both the feature and the effect on the rate.
Where a question mentions an organism that is more active, you can add that it needs a higher rate of exchange, so there are more or longer exchange surfaces or a more efficient ventilation system.
Section 5 of 9Exam application
You may be given data such as the number of gill lamellae or the length of the gut in different species, and asked to relate them to lifestyle.
Exam application
A more active fish such as tuna has more and longer lamellae and a higher blood flow than a sluggish bottom-dwelling fish since it needs more oxygen.
Use comparative language: more, larger, longer.
Also look out for questions about how a structure achieves a balance, such as stomata opening for gas exchange but closing to reduce water loss.
Section 6 of 9Specialised transport and ventilation in larger organisms
Exchange surfaces alone are not enough.
1
A fish needs a circulatory system to move oxygen from the gills to the tissues, and a mammal's circulatory system carries oxygen from the lungs to every cell and brings carbon dioxide back.
2Mass flow
These transport systems are examples of mass flow: they move substances over long distances much faster than diffusion could.
3
In single-celled and flat organisms diffusion alone suffices.
4
In an insect, the tracheal system brings air directly to the cells, so the circulatory fluid (haemolymph) does not transport oxygen.
5
In plants, gas exchange is directly with air spaces so no transport of gases is needed, but water and sugars need the xylem and phloem.
6
When a question asks why a particular organism lacks a system, relate the answer to its SA:V ratio, its activity level, and the diffusion distance in its body.
Section 7 of 9Exam-style questions
Explain how the structure of the alveoli is adapted for efficient gas exchange. [4]4 marks
11. Many alveoli give a large surface area.22. Walls are one cell thick, giving a short diffusion path.33. Surrounded by a dense capillary network which removes oxygen and maintains the concentration gradient.44. Ventilation replaces the air, maintaining the gradient of oxygen and carbon dioxide.A flatworm is 1 mm thick and has no gas exchange organs. Explain why it does not need them. [2]2 marks
1. It is thin and flat so it has a large SA:V ratio.
2. The diffusion distance to all cells is short, so diffusion across the body surface is sufficient.
Suggest why a fish species that swims fast has more gill lamellae than one that lives on the sea bed. [2]2 marks
1. Fast swimmers need more oxygen for respiration.
2. More lamellae give a larger surface area for faster oxygen uptake.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Exchange surface | Region where gases or nutrients are exchanged with the environment |
| Ventilation | Movement of air or water over an exchange surface |
| Alveolus | Microscopic air sac of the lung where gas exchange occurs |
| Capillary network | Dense network of tiny blood vessels |
| Fick's law | Rate of diffusion is proportional to area x gradient / thickness |
| Diffusion path | Distance over which a substance diffuses |
Section 9 of 9Exam technique
1
Describing an adaptation without explaining its effect on the rate of diffusion.
2
Saying that exchange surfaces are thick to protect them.
3
Stating that large surface area increases the concentration gradient.
4
Thinking ventilation and diffusion are the same - ventilation moves the medium, diffusion moves the gas across the surface.
5
Missing that the transport system also maintains gradients.
Marked practiceQuizzes for this lesson
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