[6.4-5] gas exchange in the leaf of a plant + limiting water loss

Question 1

how is gas exchange in plants similar to that of insects?

Answer 1

• no living cell is far away from external air, and therefore a source of oxygen and carbon dioxide
• diffusion takes place in the gas phase (air) which makes it more rapid than if it were in water

Question 2

what is needed for a dicotyledonous plant to be well adapted for gas exchange?

Answer 2

• high SA
• steep concentration gradient
• thin exchange surface
• short diffusion pathway

Question 3

how does a plant have a high SA?

Answer 3

high cumulative SA of palisade mesophyll cells, which have an elongated shape

Question 4

how does a plant maintain a steep concentration gradient?

Answer 4

air spaces in spongy mesophyll allow a pathway for carbon dioxide to reach the palisade cells

NB: this relies on the stomata being open

Question 5

how does a plant maintain a thin exchange surface / short diffusion pathway?

Answer 5

due to flattening of overall leaf shape

Question 6

what adaptations do insects have to reduce water loss?

Answer 6

• small SA:V ratio to minimise area over which water is lost
• waterproof coverings over body surfaces. insects have a rigid outer skeleton of chitin that is covered with a waterproof cuticle
• spiracles are the openings of the tracheae at the body surface and these 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 7

what are xerophytes?

Answer 7

plants that live in, and are therefore adapted to, areas where water is in short supply ie. dry environments (eg. cacti)

Question 8

what are the aims of the adaptations of xerophytes?

Answer 8

• minimise water loss
• maximise water gained from environment

Question 9

what adaptations do xerophytes have? (8)

Answer 9

• rolled leaves
• spines instead of leaves
• stomata in pits
• ‘hairs’ around stomata
• fewer stomata
• extensive roots - deep and wide
• thicker cuticle
• chloroplasts in stem

Question 10

why do xerophytes have rolled leaves, stomata in pits, and ‘hairs’ around stomata?

Answer 10

• limits exposure of stomata so humid air builds up around stomata
• lower concentration gradient so rate of diffusion decreases

Question 11

why do xerophytes have spines instead of leaves and fewer stomata?

Answer 11

• reduces SA for there to be stomata on, minimising water loss
• fewer stomata means fewer opportunities for water loss

Question 12

why do xerophytes have extensive roots which go deep and wide?

Answer 12

• it means they can scour water on the surface and water that is deep below because it doesn’t rain very often so they have to get the most out of it that they can
• water gained from environment is maximised

Question 13

why do xerophytes have a thicker cuticle?

Answer 13

• harder for water to evaporate, minimising water loss
• adds a layer of cuticle, therefore reducing water loss by evaporation

Question 14

why do xerophytes have chloroplasts in the stem?

Answer 14

there are fewer leaves, and plants need to photosynthesise so they have to be put somewhere