6.5 Limiting water loss

Question 1

What are 3 adaptations insects have that reduce water loss?

Answer 1

1. small surface area to volume ratio
   - minimise the area over which water is lost
2. waterproof coverings
3. spiracles
   - can be closed to reduce water loss

Question 2

Why can insects not diffuse respiratory gases in the way a single-celled organism does?

Answer 2

Due to the adaptations on the body surface of insects ( small SA:V), waterproof, spiracles) to reduce water loss, they cannot rely on their body surface to diffuse respiratory gases.

Question 3

How do plants reduce water loss?

Answer 3

• waterproof covering over parts of the leaves
• ability to close stomata when necessary
• xerophyte plants evolved a way to limit water loss through transpiration

Question 4

Why can plants not have small SA:V?

Answer 4

Because they need large leaf surface area for the capture of light and for the exchange of gases required for photosynthesis.

Question 5

What is the main way of surviving in habitats where there is a high rate of water loss and a limited water supply?

Answer 5

By reducing the rate at which water can be lost through evaporation.

Question 6

What are the 5 modifications plants have to reduce water loss? (xerophytes)

Answer 6

1. thick cuticle
   - less water can escape
2. rolling up of leaves
   - protects the lower epidermis from the outside helping to trap a region of still air within the rolled leaf. This region becomes saturated with water vapour so has a high water potential. There is no water potential gradient between the inside and outside of the leaf hence no water loss.
3. hairy leaves
   - traps still, moist air next to the leaf surface. The water potential gradient between the inside and outside of the leaves is reduced and therefore less water is lost by evaporation
4. stomata in pits or grooves
   - trap moist air next to the leaf and reduce the water potential gradient
5. a reduced SA:V of the leaves
   - smaller the SA:V the slower the rate of diffusion. By having leaves that are small and roughly circular in cross-section rather than leaves that are broad and flat, the rate of water loss can be considerably reduced. This reduction in SA is balanced against the need for a sufficient area of photosynthesis to meet the requirements of the plant.