6.5- LIMITING WATER LOSS

Question 1

In terrestrial organisms i.e. insects + plants where can problems arise from?

Answer 1

opposing needs of efficient gas-exchange system + requirement to conserve water

Question 2

What are the features that make a good gas-exchange system also? (hint water loss)

Answer 2

also same features that increase water loss

Question 3

What must terrestrial organisms be able to do?

Answer 3

limit their water loss without compromising efficiency of gas-exchange system

Question 4

Where are the gas exchange systems of terrestrial organisms?

Answer 4

inside the body

Question 5

What is the air at the exchange surface of terrestrial organisms like?

Answer 5

more or less 100% saturated with water vapour

Question 6

As the air at the exchange surfaces of terrestrial organisms is more or less 100% saturated with water vapour, what does this mean?

Answer 6

there’s less evaporation of water from exchange surface

Question 7

Where do most insects live and what is this called?

Answer 7

live on land-terrestrial

Question 8

What is the problem for all terrestrial organisms?

Answer 8

water easily evaporates from surface of their bodies + they can become dehydrated

Question 9

What does efficient gas exchange require?

Answer 9

thin, permeable surface with large area

Question 10

What does a thin, permeable surface with a large area need for efficient gas exchange conflict with?

Answer 10

need to conserve water

Question 11

What adaptations have insects evolved to reduce water loss? (3)

Answer 11

small SA:V
waterproof coverings over their body surface
spiracles

Question 12

How does a small SA:V reduce water loss in insects?

Answer 12

minimise area over which water lost

Question 13

What is the waterproof covering in insects?

Answer 13

rigid outer skeleton of chitin that’s covered with waterproof cuticle

Question 14

What are spiracles?

Answer 14

opening of tracheae at body surface

Question 15

Why can spiracles be closed?

Answer 15

reduce water loss

Question 16

What does the closing of spiracles conflict with?

Answer 16

need for oxygen

Question 17

As closing of spiracles conflicts with need for oxygen, when does closing of oxygen largely occur?

Answer 17

occurs largely when insect at rest

Question 18

What do the features of insects (small SA:V, waterproof covering, spiracles) mean insects cannot do?

Answer 18

cannot use their body surface to diffuse respiratory gases in way single-celled organisms does

Question 19

What do insects have as respiratory gases cannot diffuse through their body surface?

Answer 19

tracheae

Question 20

What does the tracheae do?

Answer 20

carry air containing oxygen directly to tissues

Question 21

Why can’t plants have a small SA:V?

Answer 21

they photosynthesis + photosynthesis requires large leaf SA

Question 22

Why does photosynthesis require a large leaf SA?

Answer 22

for capture of light + for exchange of gases

Question 23

How do terrestrial plants limit water loss? (2)

Answer 23

waterproof covering over parts of leaves + ability to close stomata when necessary

Question 24

What have certain plants with restricted supply of water evolved?

Answer 24

range of other adaptations to limit water loss through transportation

Question 25

What are plants that have restricted supply of water that have evolved adaptations to limit water loss through transpiration called?

Answer 25

xerophytes

Question 26

What are xerophytes?

Answer 26

plants that adapted to living in areas where water is in short supply

Question 27

What is the main way of plants to survive in habitats where there’s a high rate of water loss + limited water supply?

Answer 27

reduce rate at which water can be lost through evaporation

Question 28

Where does the vast majority of water lost through in plants?

Answer 28

through leaves

Question 29

What modifications do the leaves of xerophytes have? (5)

Answer 29

thick cuticle
rolling up of leaves
hairy leaves 
stomata in pits or grooves 
reduced SA:V of leaves

Question 30

Why is a thick cuticle a modification of leaves of xerophytes?

Answer 30

less water can escape

Question 31

Where do most leaves have their stomata largely, or entirely confined to?

Answer 31

lower epidermis

Question 32

What does the rolling of leaves do for xerophytes?

Answer 32

protect lower epidermis from outside helps to trap region of still air within rolled leaf

Question 33

What does the region of still air in the rolled leaves becoming saturated in?

Answer 33

saturated with water vapour + so has very high water potential

Question 34

How does the rolled leaves prevent water loss in xerophytes?

Answer 34

no water potential gradient between inside + outside of lead and so no water loss

Question 35

Example of leaf with thick cuticle?

Answer 35

holly

Question 36

Example of leaf that rolls its leaves?

Answer 36

marram grass

Question 37

What does a thick layer of hair on leaves, especially on the lower epidermis do?

Answer 37

trap still, moist air next to leaf surface

Question 38

How do hairy leaves prevent water loss?

Answer 38

water potential gradient between inside + outside of leaves reduced + so less water lost by evaporation

Question 39

Example of plant that has hairy leaves?

Answer 39

one type of heather plant

Question 40

How do stomata in pits or grooves reduce water loss?

Answer 40

trap still, moist water next to leaf + reduce water potential gradient

Question 41

Example of plant with stomata in pits or groves?

Answer 41

pine trees

Question 42

What happens to rate of diffusion the smaller the SA:V?

Answer 42

slower the rate of diffusion

Question 43

What shaped leaves would reduce water loss?

Answer 43

small + roughly circular in cross-section

Question 44

What is the reduction in surface area of leaves balanced against?

Answer 44

balanced against need for sufficient area of photosynthesis to meet requirement of plant