2. Gas exchange in Plants Leaf’s structure Stomata
Mechanisms of opening of the stomata

3. Cross-section of a leaf
upper epidermis
protect internal tissues from mechanical damage and bacterial and fungal invasion

4. Cross-section of a leaf
Cuticle
a waxy layer
prevent water loss from the leaf surface

5. Cross-section of a leaf
mesophyll

6. Cross-section of a leaf
palisade mesophyll
contains many chloroplasts
columnar cells closely packed together to absorb light more efficiently

7. Cross-section of a leaf
Irregular cells loosely packed together to leave numerous large air spaces
to allow rapid diffusion of gases throughout the leaf
Cross-section of a leaf
spongy mesophyll (moist)

8. Cross-section of a leaf
same as upper epidermis except the cuticle is thinner
lower epidermis

9. Cross-section of a leaf
Stoma
(pl. Stomata)
opening which allows gases to pass through it to go into or out of the leaf

10. Cross-section of a leaf
Stomata also let water out during respiration. This process is called transpiration

11. Cross-section of a leaf
guard cells
control the size of stoma

12. Cross-section of a leaf
vascular bundle (vein)

13. Cross-section of a leaf
xylem
to transport water and mineral salts
towards the leaf

14. Cross-section of a leaf
phloem
to transport organic substances
away from the leaf

15. Xerophytes Have Special Adaptations to Reduce the Rate of Transpiration
Xerophytes are plants which live in hot, dry environments.
These cacti have reduced leaf area as the leaves are reduced to spines
Fleshy stems to hold water
Silver surface to reflect sun

16. Adaptations to Reduce Water Loss in Xerophytes
Thick waxy cuticle to reduce evaporation
Reduced leaf area e.g. needles, spines
Hairy leaves:- the hairs trap a layer of saturated air
Sunken stomata:- the pits above the stomata become saturated
Rolled leaves:- this reduces the area exposed to the air and keeps the stomata on the inside so increasing the water vapour inside the roll
Increasing the water vapour around the stomata reduces the water potential gradient so slows water loss

17. Number Name of part Job of part1
Upper epidermis
Protection 2
Cuticle
Waterproof layer 3
Stomata
Movement of gases 4
Mesophyll
Photosynthesis 5
Palisade cells 6
Spongy cells
Allows movement of gases 7
Phloem
Transport of food 8
Xylem
Transport of water and minerals

19. The guard cells control the opening and closing of the stomata
Guard cells flaccid (lack of water)
Guard cells turgid (plenty of water)
Thin outer wall
Thick inelastic inner wall
Stoma closed
Stoma open

20. We think that light, water availability and carbon dioxide concentration control the opening of the stomata.
Guard cells flaccid (lack of water)
Guard cells turgid (plenty of water)
Thin outer wall
Thick inelastic inner wall.
It’s fundamental for the movement
Stoma closed
Stoma open

21. Graph to show stomatal opening over 24 hours
Some plants close stomata during hottest time-saving water
100
Increased light intensity causes more stomata to open
stomatal opening/%
An adaptation to hot dry environments
Stomata close as the sun sets
Dawn-stomata begin to open 12 2 4 6 8 10 12 2 4 6 8 10 12

22. Conclusions: Stomata are closed when there isn’t much water in the guard cells. Water gets inside the guard cells via osmosis These become turgid and because of their inelastic inner wall they curve and open the stoma. This process is influenced by water availability, carbon dioxide concentration and light intensity. The question is.... How do plants regulate the stomatal movement?

23. Regulating Stomatal Opening: Potassium ion pump - hypothesis
Guard cells flaccid K+
K+ ions have the same concentration in guard cells and epidermal cells K+ K+ K+ K+ K+ K+ K+
Light activates K+ pumps which actively transport K+ from the epidermal cells into the guard cells K+ K+ K+ K+
K+ pumps use ATP produced during photosynthesis. It’s not a FREE process for the plant!
Stoma closed

24. Regulating Stomatal Opening: Potassium ion pump - hypothesis
H2O
Increased concentration of K+ in guard cells
Lowers the  (water potential) in the guard cells K+ K+
Water moves in by osmosis, down  (water potential) gradient K+ K+ K+ K+ K+
Stoma open K+
H2O K+ K+ K+ K+ K+
Stoma closed

25. To make sure that the guard cell don’t gain too many K+ and become too positive (therefore too turgid), some negative ions (anions) must be produced. Starch which is found in the cells is then converted into malate which is an anion. This process re-equilibrate the conditions in the guard cells.