1. Transport in Plants
Water flow in plants

2. Water and Minerals
Transported in xylem
Transpiration
Evaporation, adhesion, and
cohesion
Relies on negative pressure
Sugars
Transported in phloem
Bulk flow
Calvin cycle in leaves loads
sucrose into phloem
Relies on positive pressure
Gas Exchange
Photosynthesis
CO2 in and O2 out
Cellular Respiration
O2 in and CO2

3. The Importance of Transpiration
Helps to keep the plant cool in hot
weather – a method of evaporative
cooling.
It supports photosynthesis and
encourages the exchange of gases,
helping maintain levels of CO2 and O2 in the atmosphere.
It also-
Plays a significant role in the water cycle and releases approx. 10% of water back into the environment.
Produces 200 to 1,000 lbs of water for each pint of solid material produced by the plant
Creates water vapor that forms into fog and clouds

4. Water enters into the root hair cells by osmosis.
The root hair cell is hypertonic (higher osmotic pressure) to the surrounding soil water, ie the cell has a lower water molecule concentration.
Water moves from cell to cell through the root cortex by osmosis along a concentration
gradient; this means that each cell
is hypertonic to the one before it.
In the center of the root, the
water enters the xylem vessels.
Water may move by diffusion
through cell walls and intercellular
spaces. 4

5. Cohesion and adhesion in the transpiration stream

6. Hydrogen Bonds and Cohesion
Each water molecule is hydrogen bonded to four others. H O
The hydrogen bond has ~ 5% of the strength of a covalent bond. However, when many hydrogen bonds form, the resulting union can be sufficiently strong and quite stable.
Adhesion is the tendency of molecules of different kinds to stick together. Water sticks to the cellulose molecules in the walls of the xylem, counteracting the force of gravity.

7. As water molecules evaporate out the stomata in the leaves, cohesion pulls the water molecules up from the roots to replace the lost water
Adhesion of water molecules to the cell wall fights gravity and aids in the pull of the water molecules up the xylem.

8. 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 vaporization
Increases the kinetic energy so faster diffusion
Warms the air so lowers the  of the air, so  gradient is steeper

9. 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.

10. 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

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

12. Movement of Water Through the Stomata
Water moves from a higher (less negative) water potential to a lower (more negative) water potential
H2O
Diffusion shells
Always a compromise between photosynthesis and transpiration
A leaf may transpire more than its weight in water in a day…this loss must balance a plant’s need for CO2 for photosynthesis

13. Wilting
If water lost by transpiration is greater than water uptake via the roots the plant cells become flaccid and the plant wilts.
Subject to copyright clearance a suitable image showing plants wilting could be inserted here.
e.g. one similar to that found at:
When the guard cells are flaccid the stomata close

14. Leaf section The upper epidermis has no stomata
Subject to copyright clearance a suitable image showing a leaf section could be inserted here.
e.g. one similar to that found at:
The lower epidermis has stomata.
The guard cells control the opening and closing of the stomata

15. Lower Epidermis of Tradescantia

16. Surface view of leaf epidermis showing the guard cells which are flaccid and the stoma closed.

17. A single stoma and guard cells
Guard cells flaccid
Guard cells turgid
Stoma open
Stoma closed

18. 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

19. 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

20. 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+

21. 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

22. 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
Fleshy leaves to hold water
Silver surface to reflect sun

23. 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

24. 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
This reduces the  gradient so reduces water loss from the plant

25. 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

26. 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

27. The End

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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)