Water balance in arid environments - TCE Biology (Tasmania)
“Understand the various mechanisms that animals and plants use to maintain water balance, involving structural features and behavioural and physiological responses, in an arid environment (desert animals and xerophytes)”
In arid environments, organisms survive by reducing water loss and making the most of any water they gain. Desert animals use structural features (efficient kidneys, waterproof skin), behaviour (being nocturnal, sheltering in burrows or shade) and physiology (very concentrated urine, dry faeces, using water from food and respiration). Xerophytes, plants adapted to dry conditions, have thick waxy cuticles, sunken or protected stomata, reduced or rolled leaves, hairs that trap humid air, extensive root systems and water storage tissue, and they close their stomata in the hottest part of the day.
Jump to a section
What this dot point is asking
This dot point is part of criterion 6 (Section C of the TASC exam), under osmoregulation in animals and plants. Questions usually ask you to classify a feature as structural, behavioural or physiological and explain how it reduces water loss or increases water gain.
Three kinds of mechanism
- Structural: physical features of the body or plant (a waxy cuticle, long loops of Henle).
- Behavioural: what the organism does (being active at night, burrowing, seeking shade).
- Physiological: processes inside the body (concentrating urine, closing stomata, using metabolic water).
Desert animals
| Mechanism | Type | How it helps | Australian example |
|---|---|---|---|
| Kidneys with long loops of Henle | Structural | More water reabsorbed, producing highly concentrated urine | Spinifex hopping mouse |
| Very concentrated urine and dry faeces | Physiological | Less water lost in wastes | Spinifex hopping mouse |
| Obtaining water from food and from cellular respiration (metabolic water) | Physiological | Reduces the need to drink | Spinifex hopping mouse |
| Nocturnal activity; sheltering in burrows by day | Behavioural | Avoids the hottest, driest part of the day, reducing evaporative water loss | Spinifex hopping mouse, many desert reptiles |
| Skin grooves that draw water towards the mouth | Structural | Collects water from damp sand or dew | Thorny devil |
| Burrowing underground and sealing in a cocoon during drought | Behavioural and structural | Reduces evaporation until rain returns | Water-holding frog |
| Resting in shade in the heat of the day | Behavioural | Reduces heat gain and the need for evaporative cooling | Red kangaroo |
Xerophytes
| Feature | Type | How it reduces water loss |
|---|---|---|
| Thick, waxy cuticle | Structural | Waterproof layer reduces evaporation through the leaf surface |
| Sunken stomata, or stomata inside rolled leaves | Structural | Traps humid air, reducing the water vapour gradient (spinifex grass leaves roll up) |
| Hairs on the leaf surface | Structural | Traps a layer of humid air around the stomata |
| Reduced leaf area (small or needle-like leaves, or phyllodes) | Structural | Less surface for transpiration (many acacias have phyllodes, flattened leaf stalks) |
| Leaves hanging vertically | Structural | Less leaf surface faces the midday sun, reducing heating and evaporation (many eucalypts) |
| Deep or widespread roots | Structural | Reach more water in the soil |
| Water storage tissue | Structural | Stores water for dry periods (succulents such as pigface) |
| Stomata close in the hottest part of the day | Physiological | Reduces transpiration when the gradient is steepest |
A mark is earned by explaining how the feature reduces water loss or increases water gain, for example by reducing the water vapour concentration gradient or reducing water in urine, not just by naming it.
Features of a desert plant and a desert animal
- A plant's stomata are in pits in the leaf surface: structural; humid air in the pit reduces the gradient, so less water vapour diffuses out.
- The same plant's stomata close at midday: physiological; transpiration stops when evaporation would be greatest.
- A lizard stays in a burrow during the day: behavioural; the burrow is cooler and more humid, so less water is lost.
- A mouse produces urine several times more concentrated than a human's: physiological (with a structural basis in its long loops of Henle); less water is lost in urine.
Marker's note: name the type, then give the mechanism.
- Classifying closing stomata as structural
- Closing stomata is a physiological response; having sunken stomata is structural.
- Describing an adaptation without explaining it
- Link each feature to reducing water loss or gaining water.
- Mixing thermoregulation and water balance without linking them
- Sweating and panting cool the body but cost water, which is why many desert animals avoid needing them.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksDescribe one structural, one behavioural and one physiological mechanism that help a desert mammal maintain water balance, and explain how each works.Show worked answer →
Two marks for each mechanism (named and explained).
- Structural
- Kidneys with long loops of Henle, which allow more water to be reabsorbed from the filtrate, so very concentrated urine is produced and little water is lost.
- Behavioural
- Being nocturnal and sheltering in a burrow during the day, where it is cooler and more humid, reduces water lost through evaporation and the need to cool by panting or sweating.
- Physiological
- Producing very concentrated urine and dry faeces, and obtaining water from food and from respiration (metabolic water), so the animal may rarely need to drink.
Original4 marksExplain how two structural features of xerophytes reduce water loss by transpiration.Show worked answer →
Two marks for each feature, linked to transpiration.
Thick waxy cuticle. The wax is waterproof, so less water evaporates directly through the leaf surface.
Sunken stomata, or stomata inside rolled leaves. Humid air is trapped around the stomata, which reduces the water vapour concentration gradient between the inside of the leaf and the air, so water vapour diffuses out more slowly.
Other accepted features: hairs on the leaf surface trapping humid air; a reduced leaf surface area (small or needle-like leaves, or phyllodes); leaves hanging vertically to reduce heating; water storage tissue.