Control of Water Loss
Biology: AQA A Level, topic 3.2.3
Insects
Water is essential to life. Water can be lost from exchange surfaces during gas exchange. Terrestrial insects can minimise water loss without sacrificing rates of gas exchange too much through the following adaptations:
Spiracles
- Terrestrial insects can close their spiracles to prevent water loss.
- There are also hairs around the spiracles.
- The hairs decrease the water potential gradient between inside the trachea and the environment.
- A lower water potential gradient decreases the rate of diffusion.
Waxy cuticle
- Terrestrial insects also have a waxy cuticle on their body which is waterproof.
- The cuticle prevents water loss.
Xerophytic Plants
Xerophytic plants are specialised to dry habitats where water availability is low. The plants have evolved a number of features to prevent water loss.
Fewer stomata
- Xerophytic plants have evolved to have fewer stomata to avoid dehydration.
- The stomata of most plants remain open during the day to allow the exchange of carbon dioxide and oxygen for photosynthesis.
- Water is also lost through the stomata when they are open. In dry environments, this could dehydrate the plant.
Sunken stomata
- The stomata of xerophytes are also sunk into pits in the epidermis.
- The pits trap moist air. This decreases the water potential gradient between the inside of the leaves and the outside.
- The lower water potential gradient slows the rate of diffusion of water out of the stomata.
Hairs
- Xerophytic plants have fine hairs covering the epidermis.
- The hairs trap moist air in the same way as the sunken pits.
- The hairs decrease the rate of water loss.
Curled leaves
- The leaves of xerophytes are often curled up so that the stomata are sheltered from the wind.
- Wind increases the rate of diffusion of water from outside the plant.
- The curling of the leaves helps reduce the effect of wind and so slows the rate of water loss.
Cuticle
- The leaves and stems of xerophytic plants are covered in a waxy cuticle.
- The cuticle is waterproof and so reduces the rate of evaporation of water.
Test yourself on Control of Water Loss
What is the benefit of the waxy cuticle on the body of terrestrial insects?
Which of the following statements about xerophytic plants are true?
Which of these are NOT a feature of xerophytic plants to control water loss?