Part I Explain the need for transport systems in multicellular plants Describe the distribution of xylem and phloem tissue in roots, stems and leaves Explain the absorption process in roots Describe transport mechanisms Part II List factors that affects rate transpiration Describe xerophyte properties List the series of events that leads to translocation

1. Moving water, minerals and sugars
Jorge Melo

2. Introduction
 Plants use photosynthesis
to convert light energy to
chemical energy
 Simple organic
substances, such as CO2,
H2O and ions are used in
their raw form to produce
glucose and other
carbohydrates.

3. Introduction
 How does the plants obtain H2O and CO2?
 Does the plants have a circulatory system like us?

4. Introduction
 American sequoias (giant redwood)
 Height of 60 meters
 How these trees lift water?

5. Objectives
 Explain the need for transport systems in multicellular
plants
 Describe the distribution of xylem and phloem tissue
in roots, stems and leaves
 Explain the absorption process in roots
 Describe transport mechanisms

6. Plants have two separate transport tissues
 Xylem tissue: Water and ions travel upwards
Roots Stems Leaves Flowers Fruits
 Phloem tissue: Sucrose and other assimilates travel upwards
and downwards
 Movement of water in the xylem and phloem is by mass flow.
Everything travels in the same direction within each of column
of xylem or phloem
 Note that neither plant transport system carries O2 or CO2

7. Xylem and Phloem
 2 distinct transport systems
 In both the walls of the tubes are further thickened by
the addition of:
 Cellulose (organic compound –polysaccharide)
 Lignin (woody material)

9. Water transport in 3 parts
 Transpiration (or evapo-transpiration) is the
transport of water and minerals from roots to leaves.
It involves three basic steps:
1- Absorption at the roots.
2 - Capillary action in the xylem vessels.
3 - Evaporation at the leaf.

10. Roots
 Root hair
 Single-celled extensions of some cells
 Very thin (200-250 µm)
 A single root can have thousands
 Increases the surface area
 Absorbs water by osmosis

11. Roots
 Osmosis
 Movement of H2O molecules from an area
of high concentration to an area of lower
concentration
 lower solute concentration in the soil
 Higher solute concentration in the root
 High water potential in soil
 Low water potential in roots

12. Two different routs
 Apoplast pathway:
 When H2O soaks through the cell walls and then seeps
across the root from cell wall to cell wall and through
the spaces between cells
 Symplast pathway:
 When H2O enters the cell walls and moves from cell to
cell by osmosis
 Or through strands of cytoplasm that makes direct
connection between adjacent cells- plasmodesmata

13.  When water reaches the stele the apoplast pathway is
blocked.
 Endodermis cells (stele) have suberin (waterproof)

14.  Casparian strip: belt of
waxy material, allows only
minerals in the symplast to
pass into the vascular
cylinder through
the plasma membrane of
endodermal cells.
 Cells in the vascular
cylinder transport water
and minerals throughout
the plant.

18. Task 1

19. Xylem
 Long narrow cells
 Xylem elements
 Start as living cells
(nucleus, cell wall)
 Then differentiated into
specialised structures
and died
 No living material
 Just empty shells

20. Protoxylem: The first one to
Xylem be developed behind root
and shoot tips. Lignin added
 Primary xylem in rings and spirals to form
annular vessels (rings).
Metaxylem: more
mature and walls are
fully lignin.
Secondary thickening
 Secondary Xylem The seasonal growth of the
xylem shows up as annual
rings. The ring from the
previous year transports little
water but is useful for
support.

21. Primary xylem
Protoxylem Metaxylem

22. Root cross section

23. Stem

24. Leaf

25. Leaf cross section

26. Task 2

27. Palisade and spongy mesophyll cells have very
large internal surface for gas exchange.
As the carbon dioxide concentration in the air is
so low (0.04%), the surfaces are large so that
enough can be absorbed for photosynthesis.

29. The air inside leaves is always fully saturated with
water vapour.
Usually, the air outside is less saturated than this
and so a concentration gradient for water vapour
exists between the air spaces and the outside.
Water vapour therefore diffuses down this
humidity gradient.
The pathway with the least resistance is
through the stomata. It is open during the
day to allow CO2 in and water out. In most
plants it is closed at night.

30. How does the water goes up?

31. Transpiration drives the
movement of water in plants
• The loss of water from leaves by transpiration
causes water to travel upwards through the
plant by mass flow.
• The mechanism is called ‘cohesion-tension’
and it works as follows:

32. Cohesion-tension theory
 Water loss caused by transpiration
 Causes a pulling force
 Negative pressure produced
 Transpiration pull

33. Cohesion-tension theory
2 important factors of the water:
 Cohesion: H2O molecules tend to stick together by
hydrogen bonding
 Adhesion: H2O molecules tend to stick to the inside of
the xylem

34. Cohesion-tension theory
 Root
 Absorption through osmosis
 Endodermal cells actively secrete mineral salts
 Why?
 To keep the water potential in the xylem lower
 Causing water to be drawn through the endodermis
 “pulling” of water caused by cortex cells produce
positive hydrostatic pressure inside the xylem , forcing
water upwards
 Root pressure

35. Cohesion-tension theory
 Capillarity
 Third force
 Water tends to rise inside narrow tubes by capillary
action
 Capillarity relies upon the tendency of water
molecules to stick to walls of xylem vessels by
adhesion.
 This force may be important in the upward movement
of water in small plants but no relevance in large trees

36. How does the water goes up?
 Transpiration pull (negative
pressure)
 Root pressure (positive pressure)
 Capillarity (small plants)
2 important factors of the water:
 Cohesion: H2O molecules tend to
stick together
 Adhesion: H2O molecules tend to
stick to the inside of the xylem

37. Transpiration
 Spongy mesophyll cells are not tightly packed
 Air spaces are direct contact with the air outside the
leaf, through small pores called stomata
 If air outside the leaf contains less H2O vapour then
inside
 There is a H2O potential
gradient from the air
spaces inside the leaf to
the outside

39. Task 3

40. Moving water, minerals and sugars
Jorge Melo

42. How does the water goes up?
 Transpiration pull (negative
pressure)
 Root pressure (positive pressure)
 Capillarity (small plants)
2 important factors of the water:
 Cohesion: H2O molecules tend to
stick together
 Adhesion: H2O molecules tend to
stick to the inside of the xylem

46. Objectives
 List factors that affects rate transpiration
 Describe xerophyte properties
 List the series of events that leads to translocation

47. Potometer
 Measures the water
absorption
 Estimate the rate of
transpiration
 Air/water tight
 Water transpired
 Water entering to xylem

48. Factors affecting rate of transpiration
Light intensity:
 Affects the opening and
closing of the stomata
 ROT
 Indirect effect

49. Factors affecting rate of transpiration
Humidity:
 Humid atmosphere
 Contains a lot of H2O
molecules
 Reduction of the water
potential gradient between
the air spaces and atmosphere
 ROT decreases
 Low humidity increases ROT

50. Factors affecting rate of transpiration
Temperature:
 Temperature
 kinetic energy
 Rate of diffusion through
the stomata pores
 Air is able to hold more
water molecules at higher
temperatures
 ROT

51. Factors affecting rate of transpiration
Wind speed:
 Still air makes the H2O
molecules to accumulate
around the stomata pores
(leaves)
 Reduces the H2O
potential gradient and
slows the ROT
 Wind disperse H2O
molecules
 gradient in H2O
potential ROT

52. Xerophytes Vs Mesophytes

53. xerophytes
 A plant adapted to live in dry conditions
 They have a range of adaptations to reduce the loss of
water vapour by transpiration.

54. xerophytes
 Leaves
 Small to reduce the surface area
 Thick to reduce surface area: volumes ratio

55. xerophytes
 Sunken Stomata

56. xerophytes
 Stomata
 Set deep inside the leaf so that
they are at the base of a
depression full of water vapour
 Some plants open their stomata
at night to store and absorb CO2

57. xerophyte
 Thick waxy
cuticles
 reduce water loss
through the
epidermis

58. Xerophytes
 Rolling up of leaves
 Lower surface faces
inside and traps
humid air next to the
stomata
 Varies with conditions

59. Xerophytes
 Leaf hairs
 Trap damp air
 Reduces air
movement
 Cut down
transpiration

60. Task 1

62. Transport in the Phloem
• Most photosynthesis occurs in the leaves.
• The reactions take place in the chloroplasts.
• The compounds that the plant makes are
called assimilates.
• Many of these are exported form the leaves
to the rest of the plant in the phloem.

63. Sources and Sinks
• The transport of these assimilates is called
translocation.
• This literally means ‘from place to place’.
• Assimilates are loaded in the phloem in the
leaves, they are often called sources.
• They are transported to other parts of the plant, such
as roots, stems, flowers, fruits and seeds. These are
called sinks.

66. Movement in the Phloem in an active
transport
• The transport of these
assimilates is called
translocation
• Sucrose and other
assimilates travel
throughout a plant in
phloem sieve tubes.
• These are made from
cells called sieve
elements.

67. Sieve tube
• Made of sieve
elements
• Living cells
• No nucleous
• Ribosomes or
tonoplast
• Diameter 10 15 um
• End walls: sieve plates
• Large pores

68. Alongside sieve tubes
are companion cells.
Mesophyll cells in the
leaf are close to veins
containing sieve tubes.
Sucrose travels to
the phloem
companion cells in
two ways.

69. From cell to cell through
the plasmodesmata.
Along cell walls in the
mesophyll.
Carrier proteins in the
cell surface membranes
of companion cells
actively pump sucrose
into the cytoplasm.
From here it passes
through plasmodesmata
into a sieve element.

70. The accumulation of sucrose
and other solutes, such as
amino acids, in sieve
elements lowers the water
potential so that water
diffuses in by osmosis from
adjacent cells and form the
xylem.
This creates pressure in
the sieve elements
causing the liquid
(phloem sap) to flow out
of the leaf.

71. Phloem sieve elements are
adapted for transport as it
has:
• End walls that have sieve
pores allowing sap to
flow freely.
• Little cytoplasm to
impede the flow of sap.
• Plasmodesmata to allow
assimilates to flow in
from companion cells.

72. Sieve elements differ
form xylem vessels
because they are alive.
They have some
cytoplasm with
organelles.
They are not
lignified, as they do
not need to
withstand the same
forces as exist in the
xylem.

73. Sucrose is unloaded at
sinks.
This is taken up by the
cells and is respired or
stored s starch.
This reduces the
concentration of
phloem sap and lowers
the pressure, so helping
to maintain a pressure
gradient form source to
sink so the sap keeps
flowing in the phloem.

75. Task