Plant Gas Exchange and Xerophytes

Question 1

How is the waxy cuticle adapted for gas exchange?

Answer 1

It is transparent so light can still pass through and reach chlorophyll molecules

Question 2

How is the spongy mesophyll adapted for gas exchange? (2)

Answer 2

Contains many air spaces so gases can come into contact with cells. Large SA for maximum diffusion.

Question 3

How is the palisade mesophyll layer adapted for photosynthesis?

Answer 3

Large number of chloroplasts close to the surface for maximum light absorption.

Question 4

What are stomata?

Answer 4

Small holes in the lower surface of the leaf which are opened and closed periodically by guard cells to allow gases in/out and minimise water loss.

Question 5

How are stomata opened?

Answer 5

When water is high in the plant, guard cells become turgid so they bend and open the stomata.

Question 6

How are stomata closed?

Answer 6

When water is minimal in the plant, guard cells become flaccid and close the stomata.

Question 7

What phase does diffusion in plants take place in and why?

Answer 7

Takes place in the gas phase, which is more rapid

Question 8

How do plants have a short diffusion pathway?

Answer 8

Stomata are found all over the leaf’s underside so the diffusion pathway is short. This means the plant is never far from air.

Question 9

What are the benefits of leaves having a large surface area?

Answer 9

For maximum gas exchange.

Question 10

How is the diffusion gradient maintained in a plant?

Answer 10

Mitochondria constantly use up oxygen in respiration and chloroplasts constantly use up CO2 in photosynthesis.

Question 11

Why do stomata usually close at night?

Answer 11

Photosynthesis can’t occur due to a lack of sunlight, so oxygen is not produced.
As there is no light, gas exchange is only needed for respiration, so the stomata shut to minimise water loss.

Question 12

What are xerophytes?

Answer 12

A species of plant that are well adapted to survive in water scarce environments.

Question 13

Name 7 ways that xerophytes are adapted to reduce water loss.

Answer 13

Hairy leaves
Stomata pits
Rolling of leaves
Reduced SA:Vol
Thick cuticle
Deep roots
Shallow widespread roots

Question 14

Name a way of reducing water loss without limiting photosynthesis and explain how.

Answer 14

Waxy cuticles are transparent so light can still reach the palisade mesophyll layer.
Waxy cuticles trap moisture within the plant.

Question 15

How do hairy leaves prevent water loss?
How do they affect the water potential gradient?

Answer 15

They trap still moist air by the plant, which reduces the water potential gradient and therefore less water is lost by evaporation.

Question 16

How do stomata pits prevent water loss?
How do they affect the water potential gradient?

Answer 16

Trap still moist air by the leaf and reduce the water potential gradient.

Question 17

How does rolling of leaves prevent water loss?
How does this affect water potential?

Answer 17

Protects the lower epidermis and stomata and traps still air within the leaf. This region becomes very saturated with water so has a high water potential and therefore there is no gradient between the inside and outside of the plant.

Question 18

How does reducing SA:Vol ratio reduce water loss?
(Eg: smaller leaves/spikes in cacti)

Answer 18

Smaller leaves means smaller surface area, meaning a slower rate of diffusion and therefore less water lost.

Question 19

How does a thick cuticle benefit xerophytes?

Answer 19

Reduces water loss

Question 20

How do deep roots benefit xerophytes?

Answer 20

Allow them to reach water sources deep underground

Question 21

How do shallow widespread roots benefit xerophytes?

Answer 21

Allows the plant to take advantage of sudden rainfall.