Limiting Water Loss

Question 1

What must terrestrial organisms do in order to survive?

Answer 1

They must limit the water loss without compromising the efficiency of their gas-exchange system.

Question 2

Where are the gas exchange surfaces of terrestrial organisms located?

Answer 2

Inside the body.

Question 3

What is the air at the exchange surface saturated with?

Answer 3

Water vapour.

Question 4

Why is it a good thing that the air at the exchange surface is more or less saturated with water vapour?

Answer 4

There is less evaporation of water from the exchange surface.

Question 5

What is meant by terrestrial?

Answer 5

It’s on land

Question 6

What are the three things that insects have evolved to reduce water loss?

Answer 6

Small surface area to volume ratio;
Waterproof coverings;
Spiracles

Question 7

How has a small surface area to volume ratio reduce water loss in insects?

Answer 7

It minimises the area over which water is lost.

Question 8

Where are the waterproof coatings on insects?

Answer 8

The waterproof coverings are over there body-surface normally a rigid outer skeleton of chitin is covered with a waterproof cuticle.

Question 9

What are spiracles?

Answer 9

Spiracles are at the openings of the trachea at the body-surface and can be closed to reduce water loss. This conflicts with the need for oxygen and so largely occurs when the insect is at rest.

Question 10

Why can’t insects use their body surface to diffuse respiratory gases in the same way a single-celled organism does?

Answer 10

Because of their adaptions to reduce water loss.

Question 11

Why can’t plants have a small surface area to volume ratio?

Answer 11

Photosynthesis requires a large leaf surface area for the capture of light and the exchange of gases.

Question 12

Do plants have waterproof coverings?

Answer 12

Yes

Question 13

What have xerophytes evolved to do?

Answer 13

Limit water loss through transpiration.

Question 14

In what type of environment are xerophytes adapted to live?

Answer 14

They are adapted to live in areas where water is in short supply.

Question 15

List the five modifications of plants to reduce water loss from evaporation.

Answer 15

Thick cuticle;
Rolling up of leaves;
Hairy leaves;
Stomata in pits or grooves;
Reduced surface area to volume ratio of the leaves

Question 16

How is the thick of cuticle beneficial to a plant?

Answer 16

Less water can escape.

Question 17

How is the rolling up of leaves beneficial to a plant?

Answer 17

This 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. There is no water potential gradient between the inside and outside of the leaf and therefore no water loss.

Question 18

How is having hairy leaves are beneficial to a plant?

Answer 18

It traps still, my stairs 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.

Question 19

What do stomata in pits or grooves do?

Answer 19

Trap still, moist air next to the leaf and reduce the water potential gradient.

Question 20

How does having a reduced surface area to volume ratio of the leaves benefit the plant?

Answer 20

Small and roughly circular in cross-section leaves such as pineneedles have a limited rate of water loss and this reduction in surface area is balanced against the need for sufficient area for photosynthesis to meet the requirements of the plant.