1. What substances need transporting in plants?
Oxygen?
Carbon dioxide?
Water?
Minerals?
Nutrients? Sugars,amino acids.
Hormones?

2. Transport in multicellular plants
Carbon dioxide
Needed for photosynthesis in all green parts of the plant, mainly leaves.
It diffuses from the air into leaves down the concentration gradient(high conc in air low conc in leaves)
The large surface area/volume ratio of leaves helps this
It is NOT moved by the plant transport system
CO2

3. Oxygen
All plant cells need it for respiration
Photosynthesising cells produce their own supplies
For other cells such as roots it must diffuse in from air in the soil
Excess oxygen from leaves diffuse out
Plants have much lower energy needs than animals ,so need oxygen much less rapidly
Oxygen is NOT transported around the plant.

4. Organic nutrients
Photosynthetic cells make their own nutrients
These include glucose & amino acids
These need to be transported to other parts of the plant
This involves the phloem
Inorganic ions and water
Ions such as magnesium,potassium,nitrates etc are needed by cells
These including water are taken up by the roots and transported in the xylem

5. The plant transport system
Only WATER, MINERAL IONS and ORGANIC NUTRIENTS need transporting
Because of the LOW ACTIVITY of plants these are not needed rapidly
The plant transport system is much simpler than animals
Fluids move much more slowly
There is no pump to move the fluids
The cells transporting substances are known as vascular tissue
There are two tissues consisting of systems of tubes called xylem & phloem

7. WATER TRANSPORT In a plant water moves down a water potential gradient
From HIGH water potential (ψ) in the soil to
LOW water potential (ψ) in the air around the leaves
Because of this there is a constant flow of water through the plant

8. Soil to Root Hair Water is absorbed by the roots
Most of the root is impermeable to water
Only the root hair cells have thin permeable cell walls
These are found in a region behind the root tip
They grow out between the soil particles and provide a large surface area.
They are very delicate and easily damaged,often lasting only a few days before being replaced

9. Mycorhizas Some trees have fungi growing in their roots
These associations are called mycorhizas
The fungi form a mass of fine threads which help absorb nutrients especially phosphates
Some trees growing on poor soils cannot survive without the fungi

10. Water enters the root hair cell by osmosis
The root hair cell contains dissolved nutrients and minerals, this gives it a low water potential
These minerals have been pumped into the cells by active transport
Soil water has higher water potential
Water enters the cell by osmosis from a high WP to a low WP

11. Water Transport : From root hair to xylem
Water enters the root hair cells
It then passes through the cortex and into the stele
This consists of the endodermis,pericycle and into the xylem
Water potential in the xylem is lower than in the root hair cells
Water moves down the water potential gradient

12. Root Section
Ranunculus: t.s young root
Tissue plan

13. Apoplast & Symplast
There are three routes for the water through the cortex cells
1. Apoplast pathway
The water moves through the cell walls
These are made of cellulose fibres and can soak up water
It moves between cells via the intercellular spaces or via touching cell walls.
It does not pass through any membranes, meaning mineral ions can be transported

14. 2. Symplast pathway
Here the water enters the cells
It passes through the cytoplasm
It passes from cell to cell through the cytoplasm in plasmodesmata
It is thought that this pathway is most often used
3. Vacuolar pathway
This is similar to the symplast but also includes the vacuoles
Apoplast may be used when a lot of water is being lost from the plant and flow through the root needs to be rapid

15. The endodermis
When the water reaches these cells it must follow the symplast pathway
This is because the cell walls contain a thick waterproof band called a casparian strip.
This is made of a waxy substance called suberin
This blocks the flow of water by the apoplast route

16. It is thought that this may allow control of the minerals entering the xylem as they must pass through cell membranes
Endodermis cells pump minerals into the xylem by active transport
This lowers the ψ and helps move water in to the xylem by osmosis
It may also help generate root pressure
Casparian strip also blocks water return out of the xylem

17. Apoplast & Symplast

18. How does water move up the stem?
Root pressure
Transpiration stream
Capillary action

19. Water transport: From root to leaf
Water passes from the roots into the stem and then into the leaves
It does this through the XYLEM TISSUE

21. Xylem Tissue
Xylem tissue is a group of cells that work together to transport water & minerals and support the plant
It contains several different types of cells
Xylem vessel elements
Tracheids
(Sclerenchyma) fibres
Parenchyma cells

22. Xylem vessel elements Xylem vessels transport water
They are made up of many elongated vessel elements.
These form a hollow pipe
These began life as a normal plant cell
But a substance called lignin has been laid down in the walls

23. Lignification Lignin is a complex carbohydrate
It is hard,strong and impermeable to water
It can be stained red in microscope slides
This thickening of the cell wall with lignin kills the cell
This leaves a hollow space, a lumen, through which water can pass

24. Types of lignification
Thickening starts off as circular or spiral,
then reticulate
and finally pitted

25. Types of thickening

26. Pits Pits are not open They have the original cell wall in them
This is fully permeable to water
Pits allow water to enter and leave the xylem
They can also be used to bypass blockages

27. Tracheid Also dead , hollow cells with lignified walls
Their ends are not completely open and taper
Water passes between cells via pits
They are the main water conducting tissue in more primitive plants
More modern plants(angiosperms) make more use of xylem vessels

28. Fibres Small dead elongated cells Lignified walls Small lumen
Main role is supporting the plant

29. Stem Vascular Structure

30. Vascular Bundle

31. Leaf Section(dicot TS)

32. Leaf section

33. Transpiration Stream To understand this we first need to consider:
Movement of water: Leaf to air
The mesophyll cells of the leaf have wet cell walls
Water evaporates into the air spaces saturating them
Water will move by osmosis from the xylem vessels (via pits) across the cells of the leaf to replace water lost
If the air outside the leaf has a lower water potential than inside, water vapour will diffuse out of the leaf through the stomata
This loss of water by evaporation from the aerial parts of plants is called TRANSPIRATION

34. Transpiration: the price plants pay for photosynthesis
Carbon dioxide is needed for photosynthesis
Stomata must be open so this can diffuse into the leaf
This means water is lost through the open stomata
If water is being lost too quickly the stomata may be closed
Leaves also wilt giving less surface area to lose water
Transpiration can also be important in cooling plant leaves as evaporation use heat

35. Transpiration Stream : root xylem to leaf xylem
As water evaporates from the leaf cells water replaces it from the leaf xylem vessels by osmosis
Removing water from the xylem lowers the hydrostatic pressure
This is now lower than the pressure in the roots causing water to move up the xylem (similar to being sucked up a straw)
If you suck too hard on a straw the pressure causes the walls to collapse
To prevent this xylem vessels walls are strong, lignified.
The transpiration stream is a passive process, relying on evaporation of water from the leaves.

36. Mass Flow This movement of water is called mass flow
The water molecules move all together as a mass
It can do this because the water molecules are held together by H bonds
This is called COHESION
They are also attracted to the lignin in the walls of the xylem
This is ADHESION

37. If the column of water broke an air bubble would form and this would stop the mass flow
The small diameter of the xylem vessels makes this less likely to happen

38. Capillary action
If thin tubes are placed in water the water will move some way up the tube
This is capillary action due to adhesion & cohesion

39. Root Pressure
Hydrostatic pressure at the root end of the xylem can be increased
Cells surrounding the xylem in the roots pump mineral ions into the xylem by active transport
This lowers the water potential and more water enters the xylem from the cells by osmosis
Hydrostatic pressure increases
This is not essential though and only helps

40. Measuring the rate of transpiration
The rate of transpiration varies
This will depend on:
Temperature:
The higher the temperature the faster the rate of evaporation inside the leaves and the faster the rate of diffusion out of the stomata
Water potential gradient between leaf and air
The steeper the gradient the faster the water vapour will diffuse out of the leaf

41. Moving air will remove water vapour from around the leaf
Moving air will remove water vapour from around the leaf. Keeping the gradient steep and speeding up diffusion
Dry air will mean a steeper gradient
Number of stomata
The more stomata the quicker water vapour will be lost
Position of stomata
Stomata on lower surface are not exposed to the heating effect of the sun.So less water lost
Light
Stomata open in the light.
Soil water availability
Low levels of soil water may mean wilting,plant stress and stomatal closure
Waxy cuticle
Its presence reduces water evaporation from leaf surface

42. Factors affecting the rate of transpiration
Factor affecting transpiration rate
How it affects water loss

43. Movement of meniscus each minute(mm)
Using the potometer
Label the diagram fully
The potometer does not measure the accurate uptake of water,it can only compare water uptake.Why?
Why should the leafy shot stem be cut at an angle under water and placed in the potometer?
What must the potometer be to ensure it works correctly?
Explain how you can estimate water uptake using the potometer
Explain how you can change the following conditions when using the potometer: humidity, light intensity, air currents, temperature.
Explain the role of the reservoir.
Plot a graph of your results and explain them.
condition
Movement of meniscus each minute(mm) 1 2 3 4 5

44. Using the potometer
Fill the apparatus and syringe totally with water by placing it in a bowl – air bubbles will block the water flow
Cut a shoot under water – this ensures the xylem does not get blocked with air bubbles
Place the shoot in the apparatus under water
Ensure it is a tight fit and seal with vaseline – any leaks will let air into the apparatus.
Set up on a stand
As water evaporates from the leaves water will be taken up from the apparatus to replace it.
This will not give real total of water uptake as a small amount of the water taken up will be used for photosynthesis
Measure the movement of the water level in a set time.
The water level can be reset by adding water from the syringe.
Try moving air, lower temperature, higher humidity to see their effect on transpiration rate.

45. Xerophytes Plants adapted to living in very dry areas.
Explain using examples how xerophytic plants reduce water loss by transpiration
Include:
Leaves reduced to spines
Waxy cuticle
Position of stomata
Sunken stomata
Hairy leaves
Rolled leaves

46. Phloem Tissue This tissue transports ASSIMILATES
These are substances the plants make themselves and include sugars and amino acids
Phloem tissue consists of 2 main types of cells
Sieve elements
Companion cells
There are also parenchyma cells and fibres

48. Sieve Elements
Sieve elements are the single cells which join up to form sieve tubes
These are living cells
They have a
Cellulose cell wall
Cell membrane
Only a thin lining of cytoplasm containing
Endoplasmic reticulum & mitochondria
No nucleus,chloroplasts or ribosomes

49. Sieve plates
The ends of the cells are not completely open but have a sieve plate containing open sieve pores so materials are free to move through

53. Companion cells These are next to the sieve elements
They have the “normal plant cell” structure
However they have more mitochondria and ribosomes than normal
Smaller vacuole
There are many plasomdesmata between the cells so materials can easily pass between the cells

54. Microscopy
Both light and electron microscopy show protein fibres in the phloem
It was thought these had a role to play
It has now been shown they are only a response to damage by the cells when the sections are taken

56. Translocation
The transport of soluble organic molecules within the plant (assimilates)
Takes place through the sieve tubes
It takes place by mass flow
Due to hydrostatic pressure differences at either end of the phloem
But this is an active process , not passive as in transpiration
Assimilates are actively loaded into the phloem and unloaded out of it

57. Sources and Sinks
The area where assimilates are being made is called the SOURCE
It is most often the leaf where many organic molecules are made during photosynthesis
However it could be a storage organ such as a potato tuber where sugars are being loaded into the phloem

58. Assimilates are transported from the source to the SINK
This is where the assimilates are unloaded and are being used by the plant
They might include:
A growing point of the root or stem
A storage organ in the root, building up starch
A developing fruit or flower
A nectary in a flower
As sinks could be above or below the sources materials can move in both directions in phloem

59. How translocation occurs
Loading of assimilates
The most widespread assimilate which is loaded into the phloem is sucrose
This is formed from triose sugars(C3) made in the mesophyll leaf cells in photosynthesis
This is a disaccharide that is the transport carbohydrate for plants
It moves across the leaves to the phloem via apoplast or symplast
This varies between species

60. When it reaches the companion cells it is moved into them(loaded) by active transport
A H+ protein pump moves hydrogen ions out of the companion cell by active transport
This is against their concentration gradient and requires energy from ATP
This is provided by the many mitochondria in the companion cells
This means a high concentration of H+ outside the cell
Another protein in the membrane allows the H+ ions back into the cell down their concentration gradient
This is linked to bringing sucrose into the cell
This protein is a co-transporter protein

61. Mass Flow
Sucrose can then diffuse into the sieve tube via the plasmodesmata
This lowers the water potential of the sieve tube
Water enters them by osmosis from the xylem raising the hydrostatic pressure
The sucrose solution is pushed down the sieve tubes
At the sink sucrose diffuses out of the phloem and water follows
This lowers the hydrostatic pressure

63. Unloading at the sink
The mechanism of this is unclear
sucrose may just diffuse out into the cells that need it
The sucrose may be converted to glucose & fructose by the enzyme invertase, and be used for respiration
or changed to starch and stored
In both cases its concentration will decrease maintaining a concentration gradient
In some cases it is possible that sucrose is unloaded by active transport

65. Evidence for the mechanism of translocation
Sources of evidence come from:
Light and electron microscopy
Composition of phloem sap
Speed of flow of sap and hydrostatic pressure
Potential difference readings from inside and outside cells

66. Phloem sap This is not easy to collect.
As soon as you cur phloem tissue it rapidly blocks the sieve pores with the protein plug
Within hours this is replaced by callose a polysaccharide similar to cellulose
This “clotting” mechanism stops loss of sap and entry of microbes.
Castor oil sap can be collected
Solute
Concentration (m/l)
Sucrose
250 K+ 80
Amino acids 40
Cl- 15
phosphate 10
Mg2+ 5
Na+ 2
ATP
0.5
nitrate
auxins
traces

67. Using Aphids
Aphids can place their proboscis into the phloem sieve tube
The proboscis is so fine that it does not activate the plants protective mechanism
They can then be anaesthetised and heads removed
Samples of sap will then exude from the cut end of the stylet

68. Evidence for Translocation theory
Phloem sap has a high pH about 8.0 , suggesting a low concentration of H+ ions
This would be the case if they are being removed by active transport.
There is a -150mv potential across the membrane, again supporting the removal of H+ ions, making the inside –ve.
ATP is present in the sap which would support active transport
Flow rate of sap is 10,000x faster than it would be for diffusion and match readings of hydrostatic pressure differences at source & sink
Phloem sap is under pressure and will leak out if cut

69. Evidence against
Why do the sieve pores still exist? They obstruct mass flow and you would think evolution would have removed them!
Phloem transports to several different sinks at one time, not to the one with the lowest hydrostatic pressure!

70. The differences between sieve elements and xylem vessels
Do exercise SAQ p147