Gas exchange - Plants

Question 1

Describe what happens when a guard cells opens their stomata

Answer 1

Carbon dioxide can diffuse into the leaf, oxygen and water vapour can diffuse out.

Question 2

Describe what happens when a guard cell closes its stomata

Answer 2

The rate of gas exchange into and out of the leaf is reduced.

Question 3

Describe how gas exchange occurs in the stomata

Answer 3

Gases diffuse into the air spaces in the spongy mesophyll from the stomata, before moving into the other cells. (or vice versa if moving out).

Question 4

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf

Answer 4

1. (Carbon dioxide enters) via stomata;
2. (Stomata opened by) guard cells;
3. Diffuses through air spaces;
4. Down diffusion gradient;

Question 5

Explain how a leaf is adapted for efficient exchange

Answer 5

• Thin epidermis therefore there is a shorter diffusion distance
• Many stomata can open therefore increasing the surface area over which exchange can take place
• Air spaces within the spongy mesophyll therefore diffusion can take place more quickly
(less dense = higher rate of diffusion)
• Many cells in the mesophyll therefore there is a shorter diffusion distance between the air spaces and the mesophyll cells
• Leaves are broad and flat therefore they have a greater surface area

Question 6

Why is water loss a bad thing

Answer 6

• less photosynthesis
• less glucose
• less respiration
• less ATP
• less hydrolysis

Question 7

How does the thick waxy cuticle reduce water loss

Answer 7

• It’s hydrophobic, which reduces permeability
• It’s thick, which increases the diffusion distance

Question 8

How does closing the stomata reduce water loss

Answer 8

It reduces the SA of the leaf available for exchange

Question 9

Describe what is meant by xerophytes

Answer 9

Plants that live in extremely hot, dry conditions

Question 10

Explain how marram grass is adapted to reduce water loss
Structure link function

Answer 10

• Thick waxy cuticle therefore there is a longer diffusion distance and the leaf is less permeable
• Stomata in pits therefore increasing the diffusion distance
• Leaf hairs therefore water vapour gets trapped, reducing the concentration gradient, reducing evaporation
• Rolled leaf therefore reducing wind movement, reducing the concentration gradient, reducing evaporation

Question 11

Cacti have spines instead of leaves. Explain how this reduces water loss.

Answer 11

• The spines deter predators making it less likely that the stem will be damaged, reducing the risk of water loss
• Having spines instead of leaves reduce the surface area therefore reducing the rate of evaporation