6.5 - Limiting water loss

Question 1

What is the problem for insects when it comes to water loss

Answer 1

• most insects are terrestrial, so water is very easily evaporated from their body surface
• For efficient gas exchange it also requires a thin, permeable surface with a large surface area
  —> these conflicts with need to conserve water

Question 2

What are the adaptations that insects have in order to reduce water loss

Answer 2

• Small SA:Vol (minimises the area over which water is lost)
• Waterproof covering over their body surface (e.g. insects have a rigid outer skeleton of chitin that is covered with a waterproof cuticle)
• Spiracles (they’re the openings of the tracheae at the body surface, they open/close to reduce water loss. Because it conflicts with the need for oxygen —> closes when insect is at rest)

Question 3

What do the specialised features to prevent water loss in insects therefore mean for insects

Answer 3

• they cannot use their body surface to diffuse respiratory gases in the way a single-celled organism does
• Instead they have an internal network of tubes called tracheae that carry air containing oxygen directly to the tissues

Question 4

What is the problem for plants when it comes to water loss

Answer 4

• whilst they also have a waterproof covering, they can’t have a small SA:Vol
• This is because they need a large SA for photosynthesis

Question 5

How do plants limit water loss

Answer 5

• waterproof covering over parts of the leaves
• The ability to open/close stomata when necessary
• Xerophytic plants have also evolved to have a range of adaptations to limit water loss through transpiration

Question 6

What are xerophytes

Answer 6

• plants that are adapted to living in areas where water is in short supply
• Without these adaptations, they’d become desiccated and die

Question 7

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

Answer 7

• reduce the rate at which water can be lost through evaporation
• Modifications for this are usually found in the leaves

Question 8

What are some of the modifications in the leaves of xerophytes in order to reduce water loss by evaporation

Answer 8

• A thick cuticle
• Rolling up of leaves
• Hair leaves
• Stomata in the pits or grooves
• A reduced SA:Vol of the leaves

Question 9

How does a thick cuticle help reduce water loss

Answer 9

• Although a waxy cuticle on leaves forms a waterproof barrier, up to 10% of water loss can still occur by this route
• The thicker the cuticle, the less water can escape
• An example of this would be a holly

Question 10

How does rolling up of leaves reduce water loss

Answer 10

• Most leaves have their stomata largely, or entirely confined to the lower epidermis
• The rolling of leaves = protects the lower epidermis from the outside and helps to trap a region of still air within the rolled leaf
• This region becomes saturated with water vapour and so has a very high water potential
• Therefore there is no water potential gradient between the inside and outside of the leaf and therefore no water loss
• An example of this would be be that Marram grass rolls its leaves

Question 11

How does Hairy leaves reduce water loss

Answer 11

• A thick layer of hairs on leaves, especially on the lower epidermis, traps still moist air next to the leaf surface
• The water potential gradient between the inside and the outside of the leaves is reduced and therefore less water is lost by evaporation
• An example of this would be a heather plant

Question 12

How does stomata in pits or grooves of plants reduce water loss

Answer 12

• These again trap still moist air next to the leaf and reduce the water potential gradient
• An example of this would be a pine tree

Question 13

How does a reduced SA:Vol ratio of the leaves reduce water loss

Answer 13

• because the smaller the SA:Vol ratio = the slower the rate of diffusion
• By having leaves that are small and roughly circular in cross-section, as in pine needles, 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 for photosynthesis to meet the requirements of the plant

Question 14

List 2 reasons why plants growing on sand dunes need to have Xerophytic features even though there is plentiful rain

Answer 14

• The rain rapidly drains through the sand out of reach of the roots
• sand dunes are usually in windy situations, which reduces water potential and so increases the water potential gradient , leading to increased water loss

Question 15

Explain in terms of water potential why salt marsh plants have difficulty absorbing water, despite having plenty around their roots

Answer 15

The soil solution is very salty, i.e. it has a very low water potential, making it difficult for root hairs to draw water in by osmosis

Question 16

Explain why plants in cold regions have difficulty obtaining water from soil for much of the year

Answer 16

Because in winter the water in the soil is frozen and therefore cannot be absorbed by osmosis

Question 17

Plants living in cold regions often reduce water loss by having leaves with a small SA:Vol. This reduces the SA available to capture light for photosynthesis. Photosynthesis is, in part, an enzyme-controlled process. Suggest a reason why having a smaller leaf area doesn’t reduce the rate of photosynthesis in the same way as it would for plants in warmer climates

Answer 17

• Being enzyme-controlled, photosynthesis is influenced by temperature
• in cold climates enzymes work slowly and this limits the rate of photosynthesis
• therefore there is a reduced need for light as photosynthesis is taking place only slowly
• In warm climates, photosynthesis occurs rapidly and therefore a larger leaf area is needed to capture sufficient light