xylem and phloem

1. Chapter 8 Transport in Plants
Ong Yee Sing
2017

2. Learning Objectives:
• To comprehend the structures and functions of the conducting tissue
(xylem tissue and phloem tissue) in plants.
• To comprehend the uptake of water and mineral salt (or inorganic
salts).
• To understand the mechanism of transport of water from the root to
the leaf.
• To understand the transport of organic nutrients.

3. 8.1 Transport
system in plants

4. Vascular bundle维管束
• Higher plants such as angiosperms
transport various kinds of nutrients,
water and mineral salts (or inorganic
salts) through the conducting tissue
called the vascular bundle.
• The vascular bundle consists of phloem
韧皮部, xylem木质部and surrounding
mechanical tissue机械组织.
• Xylem is responsible for the transport of
water and inorganic salts.
• Phloem is responsible for the transport
of organic nutrients.
• Apart from carrying out the transport of
substances, the vascular bundle also
offer support to the plant.

5. 8.1.1 Xylem

6. Evolution of water-conducting cells

7. Xylem木质部
• from Greek xulon ‘wood’ + the passive suffix -ēma
• Xylem transports water and mineral salts.
• Xylem is mainly made up of conductive elements and
non-conductive elements.
• Conductive elements are also known as trachaery
elements, which includes vessel导管 and tracheid假导管.
• Non-conductive elements includes xylem parenchyma
and xylem fibres.
Xylem of a celery petiole叶柄.

8. The death of tracheary elements

9. Vessels
• A chain of elongated
cylindrical cells that die at
maturity to form a thin, long
and hollow tube .
• Perforation plates穿孔板 are
formed on the cell calls at
the end of the cell to allow
bulk flow of water from the
roots to the leaves.
• Pits壁孔 are formed side of
the tube to allow transverse
横向 movement of water.
Vessels of a
stem of Borya
sphaerocephala
(Boryaceae).

10. Lignification of vessels
• The cell wall of vessel is thickened and
lignified木质化 to prevent walls from
collapsing from high water pressure.
• There are different type of thickening.
• The vessels also provide support for the
plant.

11. Tracheids假导管
• from medieval Latin trachea “rough”.
• Both ends of the tracheids are tapering尖细
• The lumen (pl. lumina) of tracheids is smaller.
• from Latin “opening”
• The transverse end walls间隔墙 between two
neighboring tracheids is retained.
• The transverse end walls possess pits壁孔so that
water can be transported from a tracheid to
another tracheid.
• Pits also are formed side of the tube to allow
transverse movement of water.

12. Differences of vessels and tracheids
Vessels Tracheids
Larger lumen Smaller lumen
Shorter cell Longer cell
End wall perforated End wall closed and tapered
Continuous tube No continuous tube

13. Quiz
• Which of the following plant cells transports water and minerals from
the roots to the leaves?
A) epidermal cell
B) parenchymal cell
C) sclerenchymal cell
D) vessel element
E) sieve tube cell

14. Quiz
• Which of the following transports organic nutrients, usually from the
leaves to the roots?
A) epidermal cell
B) parenchymal cell
C) sclerenchymal cell
D) xylem
E) phloem

15. 8.1.2 Phloem

16. Phloem韧皮部
• from Greek phloos ‘bark’
+ the passive suffix -
ēma .
• Phloem is responsible
for the transport of
organic nutrients.
• Phloem is chiefly made
up of sieve tubes and
companion cells.
• Phloem may also contain
phloem parenchyma and
phloem fibres.

17. Quiz
• Which of the following cells will always have at least one companion
cell associated with it?
A) parenchyma cell
B) sclerenchyma cell
C) tracheid
D) vessel element
E) sieve-tube cell

18. Formation of sieve tube and companion cell

19. Sieve tube
• The sieve tube cells are long
tubular in shape (narrow,
elongated) without nucleus.
• Only cytoplasm is present in
these cells.
• The sieve plates (end walls)
possess numerous sieve pores.
• The side of each sieve tube is
the companion cell(s) which
possess a nucleus and dense
cytoplasm.
• The companion cells can assist
the sieve tubes in the transport
of nutrients (ATP, proteins etc.).

21. Quiz
• Which of the following cells have perforated end walls and cytoplasm,
but no nuclei?
A) sclerenchyma cell
B) tracheid
C) vessel element
D) sieve-tube cell
E) companion cell

22. Movement of phloem
sap
• Thin filaments of cytoplasm
(plasmodesma strands) pass
through the sieve pores, connecting
the sieve elements with the
companion cells.
• The movement of cytoplasm
facilitate movement of material
between cells.
• The sieve tube cells in the roots are
connected to those in the stems
and these are connected to those
in the leaves.

23. Comparison of sieve tube elements and
companion cells

24. Comparison of xylem and phloem
Phloem Xylem
Function Transportation of
organic substances.
• Transportation of
water and mineral
ions.
• Support by lignin
Movement Bidirectional Unidirectional
Elements Sieve tubes elements,
companion cells,
phloem parenchyma,
phloem fibres.
Tracheids, vessel
elements, xylem
parenchyma, xylem
fibres.
Nature of
tissue
Living tissue with little
cytoplasm but no
nucleus / tonoplast.
Dead tissue at maturity
so it is hollow with no
cell contents.

26. Conclusion

28. Quiz
• Which of the following comparisons is NOT correct?
A) dermal tissue--epidermal cell
B) ground tissue--parenchyma and sclerenchyma cells
C) vascular tissue--xylem and phloem
D) xylem--tracheids and vessel elements
E) phloem--guard cells and vessel elements

29. 8.2 Absorption
and transport
of water and
mineral salts

30. Cross-section of roots
表皮
皮层
中柱
内皮层
中柱鞘
木髓

31. Ranunculus Root Cross Section
Casparian strip
凯氏带

32. Absorption of water and mineral salt
• The water in the soil enters the root through osmosis
• Mineral salts are absorbed by means of facilitated diffusion and
active transport.

33. 8.2.1 Root and water uptake

34. Root hair根毛
(root hair zone)• The root hair zone is the zone
where a root absorbs water
most actively.
• The root hair does not contain
cuticle角质层.
• Cuticles are protective,
hydrophobic, waxy coverings
• Cuticles minimize water loss and
reduce pathogen entry
• Numerous number to increase
the surface area.

36. Transportation of water and mineral salt
Epidermis
(root hair
cells)
Cortex
Endodermis
Pericycle
Xylem
Other plant
tissue

37. Quiz
• Write your name, class
number, class, and date on
the top of the paper.
• Write the name of the
components of the roots with
a blue or black pen.
• (2 marks)
A
B
D
C
E
F
G
H

38. Osmosis of water into the roots
• Generally, the concentration of the cell
sap of the root hair cell is higher than
that of the soil solution.
• Water moves from the soil into the root
hair cell through osmosis.
• The turgor pressure膨胀压 of the root
hair cell increases.
• When a plant cell is fully turgid, it will
produce an opposite pressure, the wall
pressure壁压.
• When the wall pressure is equal to the
osmotic pressure (i.e. the root hair cell
and soil solution have the same water
concentration), water will stop moving
into the root hair cell.

39. Pressures
• Turgor pressure is the pressure exerted by the
cytoplasm on the cell wall.
It is the turgor pressure in the plant cells which
helps the plants to be erect.
• Wall pressure is the pressure applied by the cell
wall on the contents of the cell. Wall pressure is
opposite to the turgor pressure.
• Osmotic pressure is the
minimum pressure which needs to be applied to
a solution to prevent the inward flow of water
across a semipermeable membrane. It is also
defined as the measure of the tendency of a
solution to take in water by osmosis.

40. Movement of water in the cells
• This results in the
concentration of the cell
sap of the root hair cell
being lower than that of
the cortical cell (cell of
cortex) next to it.
• Water goes down the
concentration gradient
by osmosis.

41. Route taken by water to travel into the xylem
• The apoplast is the space outside the
plasma membrane within which
material can diffuse freely.
• The symplast of a plant is the inner side
of the plasma membrane in which
water and low-molecular-weight
solutes can freely diffuse.
• The plasmodesmata allow the direct
flow of small molecules such as sugars,
amino acids, and ions between cells.
• Transport velocity is higher in apoplast
pathway质体外途径 compare to symplast
pathway共质体途径.
• The apoplast pathway accounts for a
higher proportion of water transport in
plant tissues than does symplastic
transport.

42. Unidirectional flow of water in the root
• The cells of the endodermis contain
casparian strip, a band-like structure
formed from the lignification木质化
and suberization栓质化 of the
endodermis cell wall.
• Lignin is a complex organic polymers
that is hydrophobic.
• Suberin is a another highly
hydrophobic waxy substance
• This strip is oily.

44. • It is able to prevent the
backflow of salt solution
from the xylem vessel to the
cortex.
• It blocks the passage of
substances through the pits
of the cell wall, so the
substances have to move
into the cells of endodermis.
• The movement of the water
and water-soluble
substances can be
controlled by the
endodermis cells.
Function of the casparian strip

45. Quiz
• Which tissue in the dicot root regulates the entrance of minerals into
the vascular cylinder?
A) epidermis
B) cortex
C) endodermis
D) root hairs
E) pericycle

46. Quiz
• The ring of waxy material that borders the endodermal cells on four
sides is known as the ______ .
A) plasmodesmata
B) Casparian strip
C) cotyledon
D) pericycle
E) vascular cambium

47. Conclusion
• Movement of water in the roots is:
Epidermis (root hair cells)  Cortex  Endodermis  Pericycle  Xylem
• Osmosis of water in the plant cells is controlled by the osmotic
pressure, turgor pressure and wall pressure.
• The cell wall of the endodermis cells contain a casparian strip, which
is lignified and subrised to prevent the backflow of salt solution from
the xylem vessel to the cortex and to block the passage of substances
through the pits of the cell wall.

48. 8.2.2 Absorption of mineral salts
by the root

49. Mineral salts and mineral ions
• Generally, mineral elements
exist in mineral salts (inorganic
salts) found in the soil.
• Salt is the product of the
reaction between an acid and a
base (other than water).
• Table salt (NaCl) is the product of
a reaction between hydrochloric
acid (HCl) and sodium hydroxide
(NaOH).
• An inorganic salt is any salt that
doesn't contain carbon.

50. Absorption of mineral
• Plants can only absorb soluble minerals as ions.
• They absorb minerals ions from the soil through their root hair cells.
• For example, potassium and nitric acid dissolve in water to form K+
and NO3
- , K and N are absorbed by the root in the form of K+ and
NO3
- respectively.

51. Energy in mineral uptake
• Absorption of mineral ions required
energy.
• The concentration of minerals in the soil is
very low.
• Active transport is required if facilitated
diffusion is not able to satisfy the need of
the cell.

52. Selective absorption of mineral ions
• The absorption of inorganic
salt is relatively independent
to water.
• Cucumber plant absorbs more
water in the dark, but the
amount of sodium and
bromine absorbed does not
change.
• The absorption of inorganic
salt is selective.
• When using ammonium
sulfate fertilizer, the plant
absorbs more sulphate ion
than ammonium ion.

53. 8.2.3
Mechanism of
the ascent of
water from the
root to the leaf

54. Mechanisms of
ascent of sap in
xylem tissue
• The ascent of water from the
root to the leaf requires the
following mechanisms:
• Root pressure
• Transpiration pull
• Adhesive-cohesion forces

55. Root pressure根压
• Root pressure is the transverse横向
osmotic pressure within the cells of
a root system that causes sap to
rise through a plant stem to the
leaves.
• There is a difference in the
concentration of the cell sap of the
root cells and that of the xylem
vessels.
• The difference in concentration
creates an osmosis gradient.
• Water will move down the
concentration gradient until
equilibrium is reach (then no net
movement).
• This will produce a pulling force
which causes water molecules to
move in by endosmosis内渗 (osmosis
toward the inside of a cell or
vessel), a.k.a the root pressure.

56. Demonstration of root pressure
Demonstration of root pressure.
吐水作用
e.g. lady’s finger (Alchemilla vulgaris)

58. Strength of root pressure
• As for tall trees, root pressure is not
strong enough to force water up to
this height.
• The ascent of water must depend on
the transpiration pull, the cohesive
forces (cohesion forces) between
water molecules and the adhesive
forces.

59. Quiz
• Which of the following statements about water movement is FALSE?
A. The flow of water depends upon air pressure and humidity.
B. Water initially moves into the root hair cells by osmosis because the
mineral content of the surrounding environment is higher than that
of the cells.
C. Water movement from the roots to the leaves is dependent upon
both adhesion and cohesion.

60. Transpirational pull蒸散牵引力
• The evaporation of water from leaves result in a suction force, which pulls water up
the xylem vessels, called the transpirational pull.
• Transpiration steam蒸散水柱, i.e. uninterrupted stream of water and solutes which is
taken up by the roots and transported via the xylem to the leaves where it evaporates
into the air.
• Transpiration pull is not affected by the height of the trees.
Evaporation
of water
Water ↓
= Concentration of cell sap ↑
Draw water from cells
deeper inside the leaf by
osmosis
Water removed
from veins
(xylem vessels)

61. Transpirational pull

63. Adhesive-cohesion forces
• A combination of two forces:
• Adhesive force
• Cohesion force
• These forces originate
principally because of coulomb
(electric charge) forces. When
two molecules are at an
intermediate distance,
their potential energy is at a
minimum, requiring the
expenditure of work to either
approximate or separate them,
whether they be of the same or
different material.

64. Cohesion of water
• from Latin cohaerere, from co-
‘together’ + haerere ‘to stick.’
• Cohesion is the mutual
attraction between like
molecules that causes them to
stick together.
• This allow the transpiration
stream remains as a continuous
stream.
Dew drops on a the stalk of a
water horsetail (Equisetum
fluviatile) formed by cohesion.

65. Adhesion of water
• from Latin verb adhaerere: ad- ‘to’
+ haerere ‘to stick.’
• Adhesion is the mutual attraction
between unlike molecules that causes
them to cling to one another.
• The adhesion of water molecules to
other molecules allow them to cling at
a higher level around the edges of the
surface.
Getty Image: Dorling Kindersle

66. Capillary action
• Capillary action is the ability of a liquid to flow
in narrow spaces without the assistance of, or
even in opposition to, external forces like
gravity.
• If the diameter of the tube is sufficiently small,
then the combination of surface tension
(which is caused by cohesion within the liquid)
and adhesive forces between the liquid and
container wall act to propel the liquid.
• As the diameter of the lumen of the xylem
vessel is small, capillary action is produced.

67. Water climbs higher in tube with smaller diameter

68. Quiz
• Which of the following capillary
tubes has the largest diameter?
A. A
B. B
C. C
When a glass capillary is placed in
liquid water, water rises up into the
capillary. The smaller the diameter of
the capillary, the higher the water
rises. The height of the water does
not depend on the angle at which the
capillary is tilted.
A B C

69. Conclusion
• The ascent of water is propelled by the following forces:
• Root pressure = the transverse osmotic pressure within the cells of a
root system that causes sap to rise through a plant stem to the leaves.
• The transpirational pull = resulted suction force created by the
evaporation of water from leaves, which pulls water up the xylem
vessels.
• The capillary action which is a result of the adhesive-cohesion forces.
• Cohesionis = the mutual attraction between like molecules that causes them
to stick together.
• Adhesion = the mutual attraction between unlike molecules that causes them
to cling to one another.

70. 8.3 Transport of organic nutrients

71. Functional of phloem
• Phloem transports organic substances
produced during photosynthesis from
the leaves to other parts of the body.
• Phloem also transport
• Some mineral salts
• Amino acids
• Vitamins
• Plant hormones

72. Transportation of photosynthetic products
• Photosynthesis produces PGAL as a product.
• PGAL is transported to other regions of the plant body by the phloem.
• In most plants, these organic products have to be converted into
soluble sucrose before they can be transported.
• Sucrose = glucose + fructose
+ water

73. Why sucrose?
• Soluble in water
• Disaccharides are soluble in
water, hence can be transported
with the phloem.
• Increase energy storage
• Sucrose is a more efficient energy
storage compound compare to
glucose and fructose.
• Starch will be even more efficient
but starch is insoluble in water.
• Removing unwanted
intermediate reactions
• Glucose is highly reactive.
• Sucrose is a non-reducing sugar,
hence it is less reactive.

74. Bidirectional translocation of sucrose
• In phloem tissue the transport
of organic nutrients is
bidirectional.
• Some are transported upwards
• to the buds, young leaves,
flowers
• to provide energy for carry out
cellular respiration and growth
• Some are transported
downward
• to the storage organ such as
roots and stems
• to be converted into starches,
lipids and fats, proteins etc. to be
stored.

75. Transportation in phloem requires energy
• There are more sucrose in the sieve cells
compare to the companion cells and other
plant tissues.
• ATP is required to pump sucrose into the
phloem against concentration gradient.
• Companion cells provide the ATP.
• Sieve cells lost most its organelles, includes
nucleus and mitochondria.

76. Experiment to test for movement of organic
nutrient
• It is difficult to obtain uncontaminated samples of phloem sap.
• In many plant species, phloem-specific protein (P-protein) provides an
almost instantaneous seal.
• Some experiment can be carried out:
• Tracer experiment
• Alphids’ stylet test

77. Tracer experiment
• Radioactive carbon isotope 14C is
used to produce 14CO2.
• The 14C is incorporated into organic
compounds during photosynthesis.
• Autoradiography can shows the
movement of these organic
compound in the phloem.
• Experiment also can be carried out
by direct injection of 14C-sucrose
into the leaves.
• When the ring of phloem is
removed, the 14C-sucrose only
travelled in one direction.Autoradiogram visualizing carbon transport to sink tissues. Picture (A) and
autoradiogram (B) of a Col-0 plant labelled with 14CO2. Leaf 8 was labelled for
5 min followed by a chase period of 1 h. The leaves are numbered as described in
Figure 2. Plant material was exposed to the film for 13 days.

78. The sap can be collected and analysed.
口器
Scale bars: top = 1 mm, bottom = 1.5 mm

79. 8.4 Transpiration

80. Transpiration 蒸散作用
• Transpiration is the process of water
movement through a plant and its
evaporation from aerial parts of the plant
body, such as leaves, stems and flowers.
• In transpiration, water is lost as vapour.
• Only a small portion (1 – 5%) of water
absorbed by terrestrial plants is used for
metabolism.
• Water is transpired out to the atmosphere
mostly through the stomata on the surface
of the laves, i.e. stomatal transpiration气孔
蒸散.
• A small amount of water can be transpired
through the cuticle layer of epidermis
(epidermal/cuticular transpiration) and
lenticels (lenticular transpiration) .

81. Function of transpiration
• Lower plant body temperature.
• Evaporation cooling
• Help in the absorption and
transport of water and mineral
salts.
• Create negative pressure
• Remove excessive water

82. Quiz
• Which one of the following is not a function of transpiration?
A. It is responsible for keeping the plant cool.
B. It causes minerals to travel up through the plant.
C. It is responsible for water travelling from the roots to the leaves.
D. It gives leaves their green colour.

83. 8.4.1 Process of
transpiration

84. Pathway of transpiration
soil
root hair
xylem
mesophyll cell
surface of mesophyll cell
air chamber
stomata
atmosphere

85. Wet walls
• Water fills the whole vacuole
and enters the cell wall
through osmosis.
• Water coats the walls of
mesophyll cells as a film.
• Humidity in the air chamber
is saturation.
• Water evaporates.
• Water diffuse out the leaves
as the relative humidity in
the atmosphere is lower than
that of the sub-stomata
chamber.

86. Quiz
• Water vapour evaporates from cells in the leaves of plants and exits
the leaves by way of tiny pores in their leaves. What is this process
called?
A. Excretion
B. Transpiration
C. Respiration
D. Mutation

87. Quiz
• The diagram shows an experiment whose
purpose is:
A. to show that water enters a plant through
its roots.
B. to show that a plant makes its own food
through photosynthesis.
C. to show that water evaporates from a leaf
by transpiration.
D. to show the path of water through a plant.

88. 8.4.2 Factors
affecting
transpiration

89. Factors
• Light intensity光强度
• Temperature温度
• Humidity湿度
• Wind speed风速

90. Light
• In bright light transpiration
increases.
• The stomata open to allow
more carbon dioxide into the
leaf for photosynthesis.

91. Temperature
• Transpiration is faster in higher
temperatures.
• Evaporation and diffusion are faster at
higher temperatures.

92. Humidity
• Transpiration is slower in humid
conditions.
• Diffusion of water vapour out of the leaf
slows down if the leaf is already
surrounded by moist air

93. Wind
• Transpiration is faster in windy
conditions.
• When there is very little wind, this
means that the layer of water vapour
directly surrounding the leaves is not
being swept away.
• When there is more wind, water
vapour is removed quickly by air
movement.
• This speeds up the diffusion of water
vapour out of the leaf.

94. Conclusion

95. Quiz
• What happens to the transpiration rate as light intensity increases?
A. It increases
B. It stays the same
C. It decreases

96. 8.5 The mechanism
of opening and
closing of stomata
on the leaf surface

97. Quiz
• The primary function of stomata is to
A. minimize water loss
B. facilitate gas exchange for photosynthesis
C. signal guard cells to open or close
D. convert sunlight into usable energy

98. Stoma
• Singular stoma, plural stomata
• from Greek στόμα, "mouth“
• Stoma is a pore, found in the epidermis
of leaves, stems, and other organs, that
allow exchange of air in the plant.
• Air enters the plant through these
openings by gaseous diffusion.
• Stoma = a paired guard cells + the pore
(stomatal aperture).
• The guard cells control the opening of
stoma.

99. Guard cells
• A pair of kidney-shaped cells with a gap
between them.
• Guard cells contain chloroplast. (Epidermis
cell do not contain chloroplast)
• The thickness of their cell wall is unevenly.
• The inner wall near the side of stoma is
thicker than the outer wall.
Transmission electron
micrographs of paradermal
sections through guard cells
of a pea (Pisum sativum).

100. Opening and closing of stomata
Stoma open Stoma closed
Guard cells absorb water Guard cells lose water
Guard cells are turgid Guard cells are flaccid
The thin outer walls stretch The thin walls is straighten
The cell curves outwards The cell is straighten

101. Quiz
• Stomata close when the guard cells:
A) lose water.
B) photosynthesis begins and the internal CO2 concentration decreases.
C) become turgid.
D) gain potassium ions.

102. Opening of stoma
• In the presence of light, the chloroplast of the guard cell carries out photosynthesis.
• The concentration of carbon dioxide in the cell decreases during the carbon fixation
process of the dark reaction.
• The reduction of carbon dioxide cause the pH value of the cell to increase.
• The glucose produced from photosynthesis promotes the synthesis of organic acids and
accumulation of potassium ions within the guard cells by active transport.
• The osmotic pressure of the guard cells rises.
• Water from the surrounding epidermal cells enters the guard cell though osmosis, causing
it to expand, and therefore the stoma is opened.

103. Closing of stoma
• In the absent of light, the guard cell do not undergo photosynthesis.
• The concentration of carbon dioxide in the cell increases during respiration.
• The reduction of carbon dioxide cause the pH value of the cell to decrease.
• The organic acids are consumed and the potassium ions exits the guard cells by
facilitated diffusion.
• The osmotic pressure of the guard cells decreases.
• Water exits the guard cell though osmosis, causing it to deflat, and therefore the
stoma is closed.

104. Mechanism of stomata movement
Light Dark
Concentration of CO2 decreases
due to photosynthesis
↓
pH increases
↓
K+ moves into the guard cells by active
transport
+
Synthesis of organic acids
↓
Osmotic pressure increases
↓
Water moves into the guard cells through
osmosis
↓
Guard cells become turgid
↓
Stomata open
Concentration of CO2 increases
due to respiration
↓
pH decreases
↓
K+ moves out of the guard cells by
diffusion
+
Consumption of organic acids
↓
Osmotic pressure decreases
↓
Water moves out of the guard cells
through osmosis
↓
Guard cells become flaccid
↓
Stomata close

105. Quiz
• The macronutrient, _______________ , is important to the operation
of stomata.
A) magnesium
B) manganese
C) sulfur
D) potassium
E) iron

107. Quiz
• The opening of the stomata is effected by all of the following except
A) oxygen concentration
B) temperature
C) light
D) carbon dioxide concentration