1. Transpiration
Transpiration is the loss of water from a plant by evaporation
Water can only evaporate from the plant if the water potential is lower in the air surrounding the plant
Most transpiration occurs via the leaves
Most of this transpiration is via the stomata.

2. How Transpiration is Measured
Water evaporates from the plant
A Simple Potometer
1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’
Leafy shoot cut under water
Air tight seals
Capillary tube
Plastic tubing
Movement of meniscus is measured over time
Graduated scale

3. How Transpiration is Measured
The rate of water loss from the shoot can be measured under different environmental conditions
Water is pulled up through the plant
volume of water taken up in given time
Limitations
measures water uptake
1’’’’’’’’2’’’’’’’’3’’’’’’’’4’’’’’’’’5’’’’’’’’6’’’’’’’’7’’’’’’’’8’’’’’’’’9’’’’’’’’10’’’’’’’’11’’’’’’’’12’’’’’’’’13’’’’
cutting plant shoot may damage plant
plant has no roots so no resistance to water being pulled up

4. 6 Environmental Factors Affecting Transpiration
Relative humidity:- air inside leaf is saturated (RH=100%). The lower the relative humidity outside the leaf the faster the rate of transpiration as the  gradient is steeper
Air Movement:- increase air movement increases the rate of transpiration as it moves the saturated air from around the leaf so the  gradient is steeper.
Temperature:- increase in temperature increases the rate of transpiration as higher temperature
Provides the latent heat of vaporisation
Increases the kinetic energy so faster diffusion
Warms the air so lowers the  of the air, so  gradient is steeper

5. 4. Atmospheric pressure:- decrease in atmospheric pressure increases the rate of transpiration.
5. Water supply:- transpiration rate is lower if there is little water available as transpiration depends on the mesophyll cell walls being wet (dry cell walls have a lower ). When cells are flaccid the stomata close.
6. Light intensity :- greater light intensity increases the rate of transpiration because it causes the stomata to open, so increasing evaporation through the stomata.

6. Intrinsic Factors Affecting the Rate of Transpiration.
Leaf surface area
Thickness of epidermis and cuticle
Stomatal frequency
Stomatal size
Stomatal position

7. The Effect of Wind Speed on the Rate of Transpiration
Stomatal transpiration rate
/ gcm-2s-1
In still air closing the stomata is less effective in controlling the transpiration rate
moving air
still air 10 20
Stomata diameter/µm

8. Moving Air Removes the Boundary Layer of Water Vapour From the Leaf
Still air
Moving air
Saturated air accumulates around leaf
Water vapour is removed from the leaf surface
cross section through a leaf
Lower 
Boundary layer
the  gradient is increased, so faster rate of water evaporation via the stomata

9. Movement of Water Through the Stomata
Water moves from a higher (less negative) to a lower (more negative) water potential
H2O
Diffusion shells

10. Increase in stomatal frequency increases the rate of transpiration
Boundary layer
stoma
If the distance between the stomata is less than 10 X the pore diameter the diffusion shells overlap
So increasing the number of stomata per unit area will have no further effect on transpiration

11. Wilting
If water lost by transpiration is greater than water uptake via the roots the plant cells become flaccid and the plant wilts.
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When the guard cells are flaccid the stomata close

12. Leaf section The upper epidermis has no stomata
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The lower epidermis has stomata.
The guard cells control the opening and closing of the stomata

13. Lower Epidermis of Tradescantia
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14. Surface view of leaf epidermis showing the guard cells which are flaccid and the stoma closed.
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15. A single stoma and guard cells
Guard cells flaccid
Guard cells turgid
Stoma open
Stoma closed
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16. The guard cells control the opening and closing of the stomata
Guard cells flaccid
Guard cells turgid
Thin outer wall
Thick inner wall
Stoma closed
Stoma open

17. Regulating Stomatal Opening:-the potassium ion pump hypothesis
Guard cells flaccid K+
K+ ions have the same concentration in guard cells and epidermal cells 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+ K+ K+
Stoma closed

18. Regulating Stomatal Opening:-the potassium ion pump hypothesis
H2O
Increased concentration of K+ in guard cells K+ K+
Lowers the  in the guard cells K+ K+ K+ K+ K+ K+ K+
Water moves in by osmosis, down  gradient K+ K+ K+

19. Guard cells turgid Stoma open
Increased concentration of K+ in guard cells
H2O
H2O K+ K+
Lowers the  in the guard cells K+
H2O
H2O K+ K+ K+ K+ K+ K+
H2O
H2O K+
Water moves in by osmosis, down  gradient K+ K+
Stoma open

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

21. Adaptations to Reduce Water Loss in Xerophytes
Thick waxy cuticle to reduce evaporation
Reduced leaf area e.g.needles
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

22. Cross Section of Marram Grass Leaf
Leaf is rolled with sunken stomata on the inside
Hairs trap water vapour
Water evaporating via the stomata collects in the air trapped in the rolled leaf
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This reduces the  gradient so reduces water loss from the plant

23. Adaptation to Increase Water Uptake in Xerophytes
Deep extensive root system to maximise water uptake
Accumulation of solutes in the root system to reduce the , so making the  gradient from the soil to the root cells steeper
Some very shallow roots to absorb dew which condenses on the soil at night

24. 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

25. 24h Cycle of Stomatal Opening and Closing
12.00
Why is this cycle an advantage to most plants?
09.00
15.00
18.00
06.00
21.00
3.00
24.00

26. Click on the marks above
Questions
What is transpiration? Give three environmental factors which will increase transpiration rate. (2marks)
Explain how potassium ions are moved into the guard cells in light, and how this affects the guard cells and stomata. (6marks)
Give three adaptations a xerophyte may have to reduce transpiration and explain how they do this. (4marks)
Plants close their stomata at night and some also close their stomata around mid day. Explain why this is advantageous to the plant (2marks)
finish
Click on the marks above
to check your answer

27. Transpiration is the loss of water from a plant by evaporation
Answer Q 1
Transpiration is the loss of water from a plant by evaporation
Higher temperature, increased air movement, lower humidity
Back to question

28. Potassium ions are pumped into the guard cells by active transport
Answer Q 2
Potassium ions are pumped into the guard cells by active transport
against the concentration gradient
this lowers the water potential inside the guard cells
water is drawn in by osmosis
from the surrounding cells which have a higher water potential/down the water potential gradient
guard cells swell and become turgid
guard cells bend
causing the stomata to open
any 6 from the above
Back to question

29. Thick waxy cuticle on leaves reduces evaporation
Answer Q3
Any three from:-
Thick waxy cuticle on leaves reduces evaporation
Curled leaves reduce evaporation by trapping humid air inside the curl so reducing the water potential gradient
Reduced leaf area, e.g. spines, reduces the area from which evaporation can occur
Hairy leaves -trap a layer of humid air around the leaf,so reducing the water potential gradient
Sunken stomata – moist air trapped above stomata, so reducing the water potential gradient
Back to question

30. Answer Q4
Stomata closed at night when there is no light for photosynthesis, so reducing water loss by evaporation/transpiration via the stomata
Closing stomata at mid day, which is the hottest part of the day, is an advantage in hot dry environments, as transpiration is reduced.
Back to question

31. Can you think of any synoptic links?
Water potential
Osmosis
Active transport
Respiration (energy required for active transport)
Photosynthesis (light and CO2 required for photosynthesis, CO2 enters via stomata, water used in photosynthesis)