1. Transport, food storage and gas exchange in flowering plants

2. Why do plants need a transport system??
Plants are Autotrophic they make their own food by photosynthesis
Plants store this food and use it for energy in respiration
Other processes of metabolism in plants include cell division, enzyme action and hormone action
For all these processes to occur plants need to get and transport water, carbon dioxide, oxygen and certain minerals

3. Water transport in plants

4. Water uptake by roots
The region of the root with root hairs is called the Piliferous layer
These are extensions of epidermis cells but don’t have a waxy surface (cuticle) and have thin walls
How do you think this makes them suitable for absorption??
The more root hairs the more absorption can take place….. Do you think roots have many or few root hairs??

5. Osmosis
The water around soil particles is relatively pure and is called capillary water
The cytoplasm in the root hairs is full of solutes and is more concentrated than the water outside in the soil
Osmosis describes the way water will move from low concentration to high concentration

6. How does water from root hair cells get to the xylem?
Where are root hairs in relation to xylem?
What type of tissue does water need to get across to reach the xylem vessels?
Water moves across here by diffusion

7. Transverse section of root to show movement of water

8. The xylem form a hollow pipeline throughout the plant going from the roots up the stem into the petiole and from here to the leaves

9. Giant Redwoods The largest and oldest trees in the world
A single mature giant redwood can draw 650,000 litres of water up through it in one season!!
How is this possible?

11. Upward movement of water
2 mechanisms combine to cause upward movement of water
Root pressure
Transpiration

12. Root pressure
When water is drawn into roots by osmosis the extra volume causes pressure to build up
This pressure pushes water up through the xylem
However root pressure isn’t strong enough to push water through very tall plants

13. Transpiration
The loss of water by evaporation through the leaves + other aerial parts of a plant

14. The water vapour escapes through tiny holes on the undersides of leaves called STOMATA

15. Trans section of a leaf

16. Excess water in the ground tissue evaporates into air spaces in the leaf and diffuses into the atmosphere
When the cells in the ground tissue lose water they become less swollen and turgid
For this reason the cytoplasm becomes more concentrated and water from the xylem passes into the ground tissue by osmosis

18. As each water molecule is “pulled” from the xylem another water molecule is “pulled” up from the root
Water molecules

19. This pulling force is passed from water molecule to water molecule all the way down the plant
This is how water is pulled up through the plant by transpiration

20. The control of Transpiration
Leaves need to replace the water they lose in transpiration or they may wilt and die
At certain times particularly dry weather and drought it is difficult for plants to absorb water from the soil
To prevent wilting plants need to control how much water they lose in transpiration

21. 3 methods of controlling transpiration
Leaves have a waxy cuticle through which water cannot pass – this is on the upper side of a leaf as this side is more exposed and more water can evaporate here

22. 2. Stomata are normally found on the lower surface of a leaf as less evaporation occurs here

23. Each stoma has two guard cells that can open or close the stoma by changing shape

24. When are stomata open and closed??
Normally stomata open at day and close at night
This allows water vapour out and CO2 in when photosynthesis is taking place
Stomata close at night reducing water loss and CO2 intake as photosynthesis is not occurring

25. Conditions when stomata close at day
2 reasons stomata may close at day
If the plant has lost too much water
If temperatures are too high
By closing stomata the plant reduces water
loss
In dry conditions stomata remain closed for long
periods, photosynthesis cannot occur and food
crops are reduced

27. The cohesion-tension model of water transport in xylem
Two Irish Scientists working in Trinity College Henry Dixon and John Joly put forward this model in 1894
Cohesion – the sticking of similar molecules to each other , water molecules stick to each other
Adhesion – when different molecules stick together, water adheres to the walls of xylem but this force is not as great as the cohesive forces of water

28. Experiment to show the cohesion of water molecules
Capillary action is the process which causes liquid in the soil to rise up through roots and stems of plants or water to seep through a sponge. Pick a flower such as a carnation. Carefully slice the stem in half length ways. Place one half of the stem in dark blue coloured water and the other half of the stem in  dark red coloured water. You will soon notice the flower becoming half blue, half red. The flower 'sucks up' the water through narrow tubes in its stem. The capillary action overcomes the pull of gravity.

29. Outline of cohesion tension model

31. Summary points
Water evaporates from the leaf,as each water molecule evaporates another is pulled up through the thin xylem column
This pulling of water molecules puts all the water in the xylem vessels under tension
This tension is great enough to pull water to a height of 150m
Stomata open in daylight and transpiration occurs causing xylem vessels to become narrow, stems therefore are narrower at day
The strong lignin prevents xylem vessels collapsing inward
When transpiartion stops (at night) the tension is released and xylem vessels return to their normal width

32. Water movement in the leaf by day and night

33. Mineral uptake and transport

34. Plants require minerals to function normally
Calcium, magnesium, phosphates, potassium etc. enter root hairs dissolved in water
This entry requires energy so root hair cells have lots of mitochondria
Because they need energy to be transported the process is called active transport
Once inside the plant mineral get around dissolved in water that moves in the xylem

35. Uptake and transport of Carbon dioxide

36. Photosynthesis takes place mostly in the mesophyll cells in the leaf

37. Most of the Carbon dioxide comes in through the stomata from the atmosphere
It diffuses into the air spaces and then into the photosynthesising cells in the ground tissue

38. The Rate of Photosynthesis
A measure of how much CO2 is being absorbed tells us the apparent rate of photosynthesis
However CO2 is also produced by respiration in the plant and this may be used in photosynthesis
The true rate of photosynthesis is calculated by adding the carbon dioxide taken in through the stomata and the carbon dioxide formed in respiration

39. What happens to the products of photosynthesis?
Oxygen produced in photosynthesis diffuses into air spaces in the leaf and out through stomata into the atmosphere
However some of the oxygen produced is needed for respiration

40. Glucose is the carbohydrate made in photosynthesis
This may be used immediately for energy for the plant in respiration or it may be stored as starch
Some of this starch is stored in the spongy mesophyll cells in leaves

41. Starch stored in leaves is important in the diet of leaf eating animals such as horses, monkeys and cattle

42. Glucose converted to sucrose
When glucose get converted to sucrose it enters phloem sieve tubes and is transported through the plant
This sugary water is called phloem sap
An insect extracting sugary phloem sap from a plant

43. Food transport
The precise mechanism of food transport in phloem isn’t fully known
Phloem carries food to all parts of the plant, some food is sent to growth areas such as buds, flowers and roots
Food is used to form new plant structures, for respiration or it is stored as starch

44. Food storage organs in plants

45. Modified Root
In some plants with Tap roots (mainly dicots) the root becomes swollen and fleshy with stored food
This food will be used by the plant to produce flowers, seeds and fruits
Humans harvest these before this happens!
Eg Carrots, turnips, sugar beet

46. Modified stem Potato plants develop an underground stem system
The tips become swollen with stored starch
These swollen tips are called stem tubers

47. In nature a potato tuber would remain dormant in the soil over winter and in spring the buds on the tuber (better known to us as the “eye”) would grow into new plants

48. Modified Leaves
Plants such as onions, daffodils and tulips produce bulbs

49. A bulb contains a tiny underground stem which have swollen fleshy leaves attached
This stem has an apical bud and lateral buds can be seen where the leaves meet the reduced stem see fig 25.11
The entire bulb is protected by dry scaly leaves
Some bulbs are edible (onion, garlic) while others are poisonous (daffodil, tulip) this prevents organisms in the soil from eating them!

51. Modified Petioles
Celery and Rhubarb are modified petioles

52. Gas Exchange in the leaf

53. Stomata Function of Stomata is Gas exchange
Plants need carbon dioxide for photosynthesis
Carbon dioxide diffuses in through the stomata from the atmosphere

54. Huge number of stomata on a leaf
About 50,000 per cm2 !
That’s the size of this
This increases the rate of gas exchange
Carbon dioxide diffuses through the intercellular air spaces to the mesophyll cells

55. Intercellular air spaces
Mesophyll cells

56. How do air spaces affect gas exchange??
Air spaces increase the internal surface area
The inner surface are is about 20 times more than the outer surface area of a leaf
Think of a sponge

57. What gas exits the leaves??
Photosynthesis produces oxygen which diffuses out through the mesophyll cells and air spaces then out through the stomata
When the earth was first formed there was no oxygen plants are responsible for releasing oxygen into the atmosphere (it now makes up about 20% of air!)

58. What else exits the leaf through stomata??
Water vapour in transpiration

59. Recap – when are stomata open and closed??
Open at day for gas exchange
Closed at night to reduce water loss

60. Gas exchange in stems
Cells on the inside of stem and trunks need oxygen for respiration
When they respire they produce carbon dioxide these gases need to get in + out
The dermis and bark are thick and do not allow substances in and out easily

61. Lenticles are openings in the barks of trees and shrubs that allow gas exchange

62. Stomatal opening and closing
Each stomata has a pair of kidney shaped guard cells
These guard cells open and close stomata by changing shape
Remember this animation

63. Wall of guard cells are thick on the insides this causes them to curve when they absorb water

64. When water gets into guard cells by osmosis they curve and open to reveal a wide stomata
When guard cells lose water they shrink and the gap between them (the stomata closes)

66. Control of Stomata opening and closing
The concentration of carbon dioxide in the air spaces of the leaf plays a huge role in controlling the opening and closing of stomata
High levels of CO2 cause the stomata to close
Low levels of CO2 cause the stomata to open

67. Evening/Night time Photosynthesis stops when there is no light
The mesophyll cells are no longer absorbing CO2 so it builds up in the air spaces and the stomata close

68. Day time When it starts getting light the mesophyll cells absorb CO2
This means the levels of CO2 in the air spaces drops so the stomata open again

70. Proof!
If a leaf is placed in a dark chamber with no CO2 the stomata will open
What does this tell us?

71. The exact mechanism of how CO2 controls stomatal opening is not fully understood
Other factors are also involved
The uptake and loss of patassium ions by guard cells
An internal clock that tells guard cells to open and close at regular intervals