Resource Acquisition and Transport in Vascular Plants

1. Chapter 36
Resource Acquisition and
Transport in Vascular Plants

2. Overview: Underground Plants
• Stone plants (Lithops) are adapted to life in the
desert
– Two succulent leaf tips are exposed above
ground; the rest of the plant lives below ground

3. Figure 36.1

4. • The success of plants depends on their ability to
gather and conserve resources from their
environment
• The transport of materials is central to the
integrated functioning of the whole plant

5. Adaptations for acquiring resources were
key steps in the evolution of vascular
plants
• The algal ancestors of land plants absorbed water,
minerals, and CO2 directly from the surrounding
water
• Early nonvascular land plants lived in shallow
water and had aerial shoots
• Natural selection favored taller plants with flat
appendages, multicellular branching roots, and
efficient transport

6. • The evolution of xylem and phloem in land plants
made possible the long-distance transport of
water, minerals, and products of photosynthesis
• Xylem transports water and minerals from roots to
shoots
• Phloem transports photosynthetic products from
sources to sinks

7. Figure 36.2-3
CO2 O2
Light
H2O Sugar
O2
H2O
and CO2
minerals

8. • Adaptations in each species represent
compromises between enhancing photosynthesis
and minimizing water loss

9. Shoot Architecture and Light Capture
• Stems serve as conduits for water and nutrients
and as supporting structures for leaves
• There is generally a positive correlation between
water availability and leaf size

10. • Phyllotaxy, the arrangement of leaves on a stem,
is specific to each species
• Most angiosperms have alternate phyllotaxy with
leaves arranged in a spiral
• The angle between leaves is 137.5° and likely
minimizes shading of lower leaves

11. Figure 36.3
42 24 32
29 40
16
11 19
21 3 27
34 8
6
13 14
26 Shoot 1
5 apical 22
18 meristem 9
Buds
10 2 4
31 17
23 7 12
15
20 25
28
1 mm

12. • Light absorption is affected by the leaf area index,
the ratio of total upper leaf surface of a plant
divided by the surface area of land on which it
grows
• Self-pruning is the shedding of lower shaded
leaves when they respire more than
photosynthesize

13. Figure 36.4
Ground area
covered by plant
Plant A Plant B
Leaf area = 40% Leaf area = 80%
of ground area of ground area
(leaf area index = 0.4) (leaf area index = 0.8)

14. • Leaf orientation affects light absorption
• In low-light conditions, horizontal leaves capture
more sunlight
• In sunny conditions, vertical leaves are less
damaged by sun and allow light to reach lower
leaves

15. • Shoot height and branching pattern also affect
light capture
• There is a trade-off between growing tall and
branching

16. Root Architecture and Acquisition of
Water and Minerals
• Soil is a resource mined by the root system
• Taproot systems anchor plants and are
characteristic of gymnosperms and eudicots
• Root growth can adjust to local conditions
– For example, roots branch more in a pocket of
high nitrate than low nitrate
• Roots are less competitive with other roots from
the same plant than with roots from different plants

17. • Roots and the hyphae of soil fungi form mutualistic
associations called mycorrhizae
• Mutualisms with fungi helped plants colonize land
• Mycorrhizal fungi increase the surface area for
absorbing water and minerals, especially
phosphate

18. Figure 36.5
Roots
Fungus

19. Different mechanisms transport substances
over short or long distances
• There are two major pathways through plants
– The apoplast
– The symplast

20. The Apoplast and Symplast: Transport
Continuums
• The apoplast consists of everything external to
the plasma membrane
• It includes cell walls, extracellular spaces, and the
interior of vessel elements and tracheids
• The symplast consists of the cytosol of the living
cells in a plant, as well as the plasmodesmata

21. • Three transport routes for water and solutes are
– The apoplastic route, through cell walls and
extracellular spaces
– The symplastic route, through the cytosol
– The transmembrane route, across cell walls

22. Figure 36.6
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Plasma membrane Apoplast
Symplast

23. Short-Distance Transport of Solutes Across
Plasma Membranes
• Plasma membrane permeability controls short-
distance movement of substances
• Both active and passive transport occur in plants
• In plants, membrane potential is established
through pumping H+ by proton pumps
• In animals, membrane potential is established
through pumping Na+ by sodium-potassium pumps

24. Figure 36.7 (a) H+ and membrane potential
CYTOPLASM EXTRACELLULAR FLUID
− +
− H+ Hydrogen ion
ATP +
− + H+
H+
H+
H+
H +
− + H +
Proton pump − H+
+
− + + (b) H+ and cotransport of neutral
S H+ − H solutes
+ H+ H+
− + H+
H+ H+
S S
H+
H+ H+
S S S
− + H+
− + Sucrose
H+/sucrose
cotransporter − + (neutral solute)
− + (c) H+ and cotransport of ions
H+ − H+
NO 3 − + −
− + H+ NO 3
H + H+
H+ Nitrate
H+
NO 3
− H+
− NO −
NO3 − NO
3 − + 3
− + H+
H+NO3−
H+ H
+
− +
cotransporter
− + (d) Ion channels
K+ Potassium ion
− +
K+
K+ − + K+
K+
K+ K+
− +
Ion channel − +

25. Figure 36.7a
CYTOPLASM EXTRACELLULAR FLUID
− +
H+ Hydrogen ion
ATP − +
− + H+
H+
H+
H+
H+ − + H+
Proton pump − H+
+
(a) H+ and membrane potential

26. • Plant cells use the energy of H+ gradients to
cotransport other solutes by active transport

27. Figure 36.7b
− +
S H −
+ H+
+ H+
H+
− + H+
H+ H+
S S
H+
H+ H+
S S S
− + H+
− +
H+/sucrose Sucrose
cotransporter − + (neutral solute)
(b) H+ and cotransport of neutral solutes

28. Figure 36.7c
− +
H + − H+
NO 3 − + −
− + H+ NO 3
H+ H+
H+ Nitrate
H+
− H+
NO 3
− NO
NO3 − NO
3 − + 3
−
− + H+
H+NO3− H+
− + H+
cotransporter
(c) H+ and cotransport of ions

29. • Plant cell membranes have ion channels that allow
only certain ions to pass

30. Figure 36.7d
− +
K+ Potassium ion
− +
K +
K+ − + K+
K+
K+ K+
− +
Ion channel − +
(d) Ion channels

31. Short-Distance Transport of Water Across
Plasma Membranes
• To survive, plants must balance water uptake and
loss
• Osmosis determines the net uptake or water loss
by a cell and is affected by solute concentration
and pressure

32. • Water potential is a measurement that combines
the effects of solute concentration and pressure
• Water potential determines the direction of
movement of water
• Water flows from regions of higher water potential
to regions of lower water potential
• Potential refers to water’s capacity to perform work

33. • Consider a U-shaped tube where the two arms are
separated by a membrane permeable only to
water
• Water moves in the direction from higher water
potential to lower water potential

34. Figure 36.8
Solutes have a negative Positive pressure has a Solutes and positive Negative pressure
effect on ψ by binding positive effect on ψ by pressure have opposing (tension) has a negative
water molecules. pushing water. effects on water effect on ψ by pulling
movement. water.
Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium Pure water at equilibrium
H2O H2O H2O H2O
Adding solutes to the Applying positive In this example, the effect Applying negative
right arm makes ψ lower pressure to the right arm of adding solutes is pressure to the right arm
there, resulting in net makes ψ higher there, offset by positive makes ψ lower there,
movement of water to resulting in net movement pressure, resulting in no resulting in net movement
the right arm: of water to the left arm: net movement of water: of water to the right arm:
Positive Positive Negative
pressure pressure pressure
Pure
water
Solutes Solutes
Membrane
H2O H2O H2O H2O

35. Water Movement Across Plant Cell
Membranes
• Water potential affects uptake and loss of water by
plant cells
• If a flaccid cell is placed in an environment with a
higher solute concentration, the cell will lose water
and undergo plasmolysis
• Plasmolysis occurs when the protoplast shrinks
and pulls away from the cell wall

36. Turgor
pressure
and
plasmolysis

37. • If a flaccid cell is placed in a solution with a lower
solute concentration, the cell will gain water and
become turgid
• Turgor loss in plants causes wilting, which can be
reversed when the plant is watered

38. Aquaporins: Facilitating Diffusion of
Water
• Aquaporins are transport proteins in the cell
membrane that allow the passage of water
• These affect the rate of water movement across
the membrane

39. Long-Distance Transport: The Role of Bulk
Flow
• Efficient long distance transport of fluid requires
bulk flow, the movement of a fluid driven by
pressure
• Water and solutes move together through
tracheids and vessel elements of xylem, and
sieve-tube elements of phloem
• Efficient movement is possible because mature
tracheids and vessel elements have no cytoplasm,
and sieve-tube elements have few organelles in
their cytoplasm

40. Transpiration drives the transport of
water and minerals from roots to shoots
via the xylem
• Plants can move a large volume of water from
their roots to shoots

41. Absorption of Water and Minerals by Root
Cells
• Most water and mineral absorption occurs near
root tips, where root hairs are located and the
epidermis is permeable to water
• Root hairs account for much of the surface area of
roots
• After soil solution enters the roots, the extensive
surface area of cortical cell membranes enhances
uptake of water and selected minerals

42. • The concentration of essential minerals is greater
in the roots than soil because of active transport

43. Transport of Water and Minerals into the
Xylem
• The endodermis is the innermost layer of cells in
the root cortex
• It surrounds the vascular cylinder and is the last
checkpoint for selective passage of minerals from
the cortex into the vascular tissue

44. • Water can cross the cortex via the symplast or
apoplast
• The waxy Casparian strip of the endodermal wall
blocks apoplastic transfer of minerals from the
cortex to the vascular cylinder
• Water and minerals in the apoplast must cross the
plasma membrane of an endodermal cell to enter
the vascular cylinder

45. Figure 36.10
Casparian strip
Pathway along Endodermal
apoplast cell
Pathway
through
symplast
Plasma
membrane Casparian strip
Apoplastic
route
Vessels
(xylem)
Symplastic Root
route hair
Epidermis Endodermis Vascular cylinder
(stele)
Cortex

46. • The endodermis regulates and transports needed
minerals from the soil into the xylem
• Water and minerals move from the protoplasts of
endodermal cells into their cell walls
• Diffusion and active transport are involved in this
movement from symplast to apoplast
• Water and minerals now enter the tracheids and
vessel elements

47. Bulk Flow Transport via the Xylem
• Xylem sap, water and dissolved minerals, is
transported from roots to leaves by bulk flow
• The transport of xylem sap involves transpiration,
the evaporation of water from a plant’s surface
• Transpired water is replaced as water travels up
from the roots
• Is sap pushed up from the roots, or pulled up by
the leaves?

48. Pushing Xylem Sap: Root Pressure
• At night root cells continue pumping mineral ions
into the xylem of the vascular cylinder, lowering
the water potential
• Water flows in from the root cortex, generating
root pressure
• Root pressure sometimes results in guttation, the
exudation of water droplets on tips or edges of
leaves

49. Figure 36.11

50. • Positive root pressure is relatively weak and is a
minor mechanism of xylem bulk flow

51. Pulling Xylem Sap: The Cohesion-Tension
Hypothesis
• According to the cohesion-tension hypothesis,
transpiration and water cohesion pull water from
shoots to roots
• Xylem sap is normally under negative pressure, or
tension

52. Transpirational Pull
• Water vapor in the airspaces of a leaf diffuses
down its water potential gradient and exits the leaf
via stomata
• As water evaporates, the air-water interface
retreats further into the mesophyll cell walls
• The surface tension of water creates a negative
pressure potential

53. • This negative pressure pulls water in the xylem
into the leaf
• The transpirational pull on xylem sap is
transmitted from leaves to roots
© 2011 Pearson Education, Inc.

54. Figure 36.12
Cuticle Xylem
Upper
epidermis
Microfibrils in
cell wall of
Mesophyll mesophyll cell
Air
space
Lower
epidermis
Cuticle
Stoma
Microfibril Water Air-water
(cross section) film interface

55. Figure 36.13
Xylem sap
Outside air ψ Mesophyll cells
= −100.0 MPa
Stoma
Leaf ψ (air spaces) Water molecule
= −7.0 MPa Transpiration Atmosphere
Leaf ψ (cell walls)
Adhesion by
= −1.0 MPa Xylem hydrogen bonding
Water potential gradient cells
Cell wall
Trunk xylem ψ
Cohesion by
= −0.8 MPa Cohesion and hydrogen bonding
adhesion in
the xylem
Water molecule
Root hair
Trunk xylem ψ
= −0.6 MPa Soil particle
Soil ψ Water uptake Water
= −0.3 MPa from soil

56. Figure 36.13a
Water molecule
Root hair
Soil particle
Water
Water uptake
from soil

57. Figure 36.13b
Adhesion by
Xylem hydrogen bonding
cells
Cell wall
Cohesion by
Cohesion and hydrogen bonding
adhesion in
the xylem

58. Figure 36.13c
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere
Transpiration

59. Adhesion and Cohesion in the Ascent of
Xylem Sap
• Water molecules are attracted to cellulose in
xylem cell walls through adhesion
• Adhesion of water molecules to xylem cell walls
helps offset the force of gravity

60. • Water molecules are attracted to each other
through cohesion
• Cohesion makes it possible to pull a column of
xylem sap
• Thick secondary walls prevent vessel elements
and tracheids from collapsing under negative
pressure
• Drought stress or freezing can cause cavitation,
the formation of a water vapor pocket by a break
in the chain of water molecules

61. Xylem Sap Ascent by Bulk Flow: A Review
• The movement of xylem sap against gravity is
maintained by the transpiration-cohesion-tension
mechanism
• Bulk flow is driven by a water potential difference
at opposite ends of xylem tissue
• Bulk flow is driven by evaporation and does not
require energy from the plant; like photosynthesis
it is solar powered

62. • Bulk flow differs from diffusion
– It is driven by differences in pressure potential,
not solute potential
– It occurs in hollow dead cells, not across the
membranes of living cells
– It moves the entire solution, not just water or
solutes
– It is much faster

63. The rate of transpiration is regulated by
stomata
• Leaves generally have broad surface areas and
high surface-to-volume ratios
• These characteristics increase photosynthesis and
increase water loss through stomata
• Guard cells help balance water conservation with
gas exchange for photosynthesis

64. Figure 36.14

65. Stomata: Major Pathways for Water Loss
• About 95% of the water a plant loses escapes
through stomata
• Each stoma is flanked by a pair of guard cells,
which control the diameter of the stoma by
changing shape
• Stomatal density is under genetic and
environmental control

66. Mechanisms of Stomatal Opening and
Closing
• Changes in turgor pressure open and close
stomata
– When turgid, guard cells bow outward and the
pore between them opens
– When flaccid, guard cells become less bowed and
the pore closes

67. Figure 36.15
Guard cells turgid/ Guard cells flaccid/
Stoma open Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
(a) Changes in guard cell shape and stomatal opening
and closing (surface view)
H2O H2O H2O
H2O
H2O
K+ H2O
H2O
H2O H2O H2O
(b) Role of potassium in stomatal opening and closing

68. Figure 36.15a
Guard cells turgid/ Guard cells flaccid/
Stoma open Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
(a) Changes in guard cell shape and stomatal opening
and closing (surface view)

69. • This results primarily from the reversible uptake
and loss of potassium ions (K+) by the guard cells

70. Figure 36.15b
Guard cells turgid/ Guard cells flaccid/
Stoma open Stoma closed
H2O H2O H2O
H 2O
K+ H2O
H2O
H2O
H2O H2O H2O
(b) Role of potassium in stomatal opening and closing

71. Stimuli for Stomatal Opening and Closing
• Generally, stomata open during the day and close
at night to minimize water loss
• Stomatal opening at dawn is triggered by
– Light
– CO2 depletion
– An internal “clock” in guard cells
• All eukaryotic organisms have internal clocks;
circadian rhythms are 24-hour cycles

72. • Drought, high temperature, and wind can cause
stomata to close during the daytime
• The hormone abscisic acid is produced in
response to water deficiency and causes the
closure of stomata

73. Effects of Transpiration on Wilting and
Leaf Temperature
• Plants lose a large amount of water by
transpiration
• If the lost water is not replaced by sufficient
transport of water, the plant will lose water and wilt
• Transpiration also results in evaporative cooling,
which can lower the temperature of a leaf and
prevent denaturation of various enzymes involved
in photosynthesis and other metabolic processes

74. Adaptations That Reduce Evaporative
Water Loss
• Xerophytes are plants adapted to arid climates

75. Figure 36.16
Ocotillo
(leafless)
Oleander leaf cross section
Cuticle Upper epidermal tissue
Ocotillo after
heavy rain
Oleander
100 µm
flowers
Trichomes Crypt Stoma Lower epidermal
(“hairs”) tissue
Ocotillo leaves
Old man cactus

76. • Some desert plants complete their life cycle during
the rainy season
• Others have leaf modifications that reduce the rate
of transpiration
• Some plants use a specialized form of
photosynthesis called crassulacean acid
metabolism (CAM) where stomatal gas exchange
occurs at night

77. Sugars are transported from sources to sinks
via the phloem
• The products of photosynthesis are transported
through phloem by the process of translocation

78. Movement from Sugar Sources to Sugar
Sinks
• In angiosperms, sieve-tube elements are the
conduits for translocation
• Phloem sap is an aqueous solution that is high in
sucrose
• It travels from a sugar source to a sugar sink
• A sugar source is an organ that is a net producer
of sugar, such as mature leaves
• A sugar sink is an organ that is a net consumer or
storer of sugar, such as a tuber or bulb

79. • A storage organ can be both a sugar sink in
summer and sugar source in winter
• Sugar must be loaded into sieve-tube elements
before being exposed to sinks
• Depending on the species, sugar may move by
symplastic or both symplastic and apoplastic
pathways
• Companion cells enhance solute movement
between the apoplast and symplast

80. Figure 36.17
Key
Apoplast
Symplast
Companion High H+ concentration
Mesophyll cell Cotransporter
(transfer) cell
Proton H+
Cell walls (apoplast) Sieve-tube pump
element S
Plasma membrane
Plasmodesmata
ATP Sucrose
H+ H+
Bundle- Phloem S
Mesophyll cell sheath cell parenchyma cell Low H+ concentration
(a) (b)

81. • In many plants, phloem loading requires active
transport
• Proton pumping and cotransport of sucrose and H +
enable the cells to accumulate sucrose
• At the sink, sugar molecules diffuse from the
phloem to sink tissues and are followed by water

82. Bulk Flow by Positive Pressure: The
Mechanism of Translocation in Angiosperms
• Phloem sap moves through a sieve tube by bulk
flow driven by positive pressure called pressure
flow

83. Figure 36.18
Sieve Source cell
Vessel tube (leaf)
(xylem) (phloem) 1 Loading of sugar
H2O 1 Sucrose
H2O
2
Bulk flow by positive pressure
Bulk flow by negative pressure
2 Uptake of water
3 Unloading of sugar
Sink cell
(storage
root)
4 Water recycled
4 3
Sucrose
H2O

84. • The pressure flow hypothesis explains why
phloem sap always flows from source to sink
• Experiments have built a strong case for pressure
flow as the mechanism of translocation in
angiosperms
• Self-thinning is the dropping of sugar sinks such
as flowers, seeds, or fruits

85. The symplast is highly dynamic
• The symplast is a living tissue and is responsible
for dynamic changes in plant transport processes

86. Changes in Plasmodesmata
• Plasmodesmata can change in permeability in
response to turgor pressure, cytoplasmic calcium
levels, or cytoplasmic pH
• Plant viruses can cause plasmodesmata to dilate
so viral RNA can pass between cells

87. Figure 36.20
Plasmodesma Virus
particles
Cell wall
100 nm

88. Phloem: An Information Superhighway
• Phloem is a “superhighway” for systemic transport
of macromolecules and viruses
• Systemic communication helps integrate
functions of the whole plant

89. Electrical Signaling in the Phloem
• The phloem allows for rapid electrical
communication between widely separated organs
– For example, rapid leaf movements in the
sensitive plant (Mimosa pudica)

90. Figure 36.UN01
Turgid

91. Figure 36.UN02
Wilted

92. Figure 36.UN03
CO2 H2O
O2
O2
CO2
Minerals H2O

93. Figure 36.UN05