Limiting water loss

Question 1

What does terrestrial mean?

Answer 1

Live on land.

Question 2

What are the problems for terrestrial organisms?

Answer 2

Water can be easily evaporated from the surface of their body so they can become dehydrated.
But efficient gas exchange requires a thin, permeable surface with a large SA so it conflicts the idea of conserving water.

Question 3

How have insects evolved to reduce water loss?

Answer 3

• Small SA:V to minimise the area over which water is lost.
• Waterproof covering over their body surfaces. For insects, the covering is a rigid outer skeleton of chitin that is covered with waterproof cuticle.
• Spiracles - they can be closed to reduce water loss but insects need oxygen to diffuse in so it usually closes when the insect is at rest.

Question 4

Why can’t plants have a small SA:V?

Answer 4

Because they need a large SA for photosynthesis.

Question 5

What are some of the adaptations of xerophytic plants?

Answer 5

• Thick waxy cuticles - Reduce evaporation from the epidermis and it forms a waterproof barrier.
• Small leaves/spines - Smaller SA for evaporation.
• Fewer stomata - Less transpiration because there’s less place for the water to escape.
• Stomata sunken in pits of the epidermis - Layer of humid air can be trapped around the stomata to reduce transpiration. It also reduces the concentration gradient of water between the leaf and the air so reduces evaporation from the leaf.
• Hairs around the stomata or over the whole leaf
  surface - Layer of humid air can be trapped near stomata which reduces the water potential gradient between the inside and outside of the leaf so less water is lost through evaporation.
• Curled leaves with the stomata inside, protecting them from wind because windy conditions increase the rate of diffusion and evaporation.

Question 6

What is a xerophyte?

Answer 6

Plants that live in dry, warm and windy conditions which have to adapt to limit water loss through transpiration.