Gas Exchange vs Water Loss: Xerophytes
How insects and xerophytes such as marram grass balance gas exchange against water loss, with each adaptation linked to its effect. Includes the 4-mark exam question, a worked model answer and the examiner trap on rolled leaves and reduced water loss.
Chapters (8)
Exam question [4 marks]
Marram grass is a xerophyte that grows on windy sand dunes. Insects are terrestrial. Explain how insects and marram grass are each adapted to allow gas exchange while limiting water loss.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Spiracles can close to reduce water loss, and open to allow gas exchange | "valves", "open and close", "closed when resting"; alternative for insects: small surface area to volume ratio of the body limits water loss | "spiracles are small holes" alone; "insects do not lose water" |
| 2 | Insects have a waterproof cuticle, which reduces evaporation from the body surface | "waxy / impermeable outer layer", "exoskeleton is waterproof" | "hard shell" with no reference to water loss |
| 3 | Rolled leaves / hairs / sunken stomata trap humid air, so the water potential gradient is smaller and less water vapour diffuses out | "moist air trapped round stomata", "reduced diffusion gradient", any one of the three features with this explanation; rolled leaf reduces air movement / exposed surface so humid air is not swept away | "reduces water loss" with no mention of humid air or gradient; "stomata are closed" |
| 4 | Thick waxy cuticle reduces evaporation from the leaf surface (stomata stay open so carbon dioxide can still enter) | "waterproof layer", "less water lost through the epidermis" | "cuticle stops gas exchange"; "the plant does not photosynthesise" |
Marks 1 and 2 are for insects, 3 and 4 for marram grass. Each mark names a feature and links it to less water lost. Naming a feature alone scores nothing. Xerophytes still photosynthesise and their stomata still open. Gases do not pass through the cuticle.
Model answer (4/4)
Insect spiracles can close, which reduces water loss, and open to allow gas exchange. The insect has a waterproof cuticle, which reduces evaporation from its body surface. Marram grass has rolled leaves, hairs and sunken stomata, which trap humid air, so the water potential gradient is smaller and less water vapour diffuses out. It also has a thick waxy cuticle, which reduces evaporation from the leaf, while the stomata stay open so carbon dioxide can still enter.
Examiner trap
- Describing adaptations without linking them to reduced water loss
- Saying xerophytes do not photosynthesise
- Not explaining how each adaptation reduces the diffusion gradient
Key terms
- xerophyte
- rolled leaves
- hairs
- sunken stomata
- thick cuticle
- spiracle
- waterproof cuticle
- water potential gradient
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:31The compromise
Here is the problem. A good gas exchange surface is thin, moist and has a large surface area. But that is exactly what loses water too. Wherever oxygen or carbon dioxide can diffuse in, water vapour can diffuse out. So land organisms face a compromise. They need gas exchange, but they must limit water loss.
0:54Insects: spiracles and cuticle
Start with insects. Air enters through small holes along the body called spiracles, then travels down tubes called tracheae, which end in tiny tracheoles. Each spiracle can open and close. When an insect is resting, the spiracles stay closed, so little water vapour escapes. They open when the insect needs more oxygen, or needs to get rid of carbon dioxide.
That is the compromise in action: open for gases, closed to save water.
The rest of the body is covered by a waterproof cuticle. Very little water can pass through it, so evaporation from the surface is reduced.
1:35Xerophytes in general
Now plants. A xerophyte is a plant adapted to live where water is scarce. Marram grass is a classic example. Its stomata are open at least some of the time, to let in carbon dioxide for photosynthesis. But open stomata let water vapour out, down a water potential gradient, from the moist leaf to the drier air outside.
So the trick is not to close the door. It is to shrink that gradient.
2:03Marram grass adaptations
Marram grass leaves are rolled, with the stomata on the inside. The stomata sit in pits, which we call sunken stomata. Hairs grow into the rolled space. All of this traps a layer of still, humid air next to the stomata. That humid air has a high water potential, close to the water potential inside the leaf.
So the water potential gradient between the leaf and the air is smaller, and less water vapour diffuses out. Rolling also shrinks the area exposed to the wind, which would otherwise sweep the humid air away.
A thick waxy cuticle on the outside adds a barrier, so less water evaporates straight through the leaf surface.
2:46Comparing insect and marram
Notice the pattern. In both cases, gas exchange still happens, but it is slowed down or controlled. And in both cases, water loss is reduced. The organism never wins outright. It trades some speed of gas exchange for a lot less water lost.
3:43Examiner trap and recap
The trap. Saying a feature reduces water loss is not enough. You must say how: it traps humid air, so the water potential gradient is smaller.
And remember, xerophytes still photosynthesise. Their stomata open to let carbon dioxide in. Recap. Open to gain gas, adapted to lose less water.
Next lesson, we move to the human gas exchange system, and follow the air from the human trachea down to the alveoli.
Watch first
This lesson builds on:
- Gas Exchange: Single Cells and Insects · AQA 7402 · 3.3.2
- Gas Exchange in Leaves: Mesophyll and Stomata · AQA 7402 · 3.3.2
Keep going
More from 3.3 Exchange with the environment
- Gas Exchange: Single Cells and Insects · AQA 7402 · 3.3.2
- Fish Gills and the Counter-current Principle · AQA 7402 · 3.3.2
- Gas Exchange in Leaves: Mesophyll and Stomata · AQA 7402 · 3.3.2
- Human Gas Exchange System and Alveoli · AQA 7402 · 3.3.2
All 10 3.3 Exchange with the environment lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-056.