1. Resource Acquisition and Transport in Vascular Plants
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
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3. Figure 36.1
Figure 36.1 Plants or pebbles?

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
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5. Concept 36.1: 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
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6. Xylem transports water and minerals from roots to shoots
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
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7. H2O H2O and minerals Figure 36.2-1
Figure 36.2 An overview of resource acquisition and transport in a vascular plant.
H2O and minerals

8. CO2 O2 H2O O2 H2O and minerals CO2 Figure 36.2-2
Figure 36.2 An overview of resource acquisition and transport in a vascular plant. O2
H2O and minerals
CO2

9. CO2 O2 Light Sugar H2O O2 H2O and minerals CO2 Figure 36.2-3
Figure 36.2 An overview of resource acquisition and transport in a vascular plant. O2
H2O and minerals
CO2

10. Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss
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11. 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
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12. 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
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13. Figure 36.3 24 32 42 29 40 16 11 19 21 27 3 34 8 6 14 13 26
Shoot apical meristem 1 5 22 9 18
Buds 10 4 2 31 17 7 12
Figure 36.3 Emerging phyllotaxy of Norway spruce. 23 15 20 25 28
1 mm

14. Figure 36.3a
Figure 36.3 Emerging phyllotaxy of Norway spruce.

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

16. 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
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17. Ground area covered by plant
Figure 36.4
Ground area covered by plant
Figure 36.4 Leaf area index.
Plant A Leaf area  40% of ground area (leaf area index  0.4)
Plant B Leaf area  80% of ground area (leaf area index  0.8)

18. 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
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19. Shoot height and branching pattern also affect light capture
There is a trade-off between growing tall and branching
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20. 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
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21. Mutualisms with fungi helped plants colonize land
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
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22. Figure 36.5
Roots
Figure 36.5 A mycorrhiza, a mutualistic association of fungus and roots.
Fungus

23. Concept 36.2: Different mechanisms transport substances over short or long distances
There are two major pathways through plants
The apoplast
The symplast
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24. 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
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25. 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
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26. Cell wall Apoplastic route Cytosol Symplastic route
Figure 36.6
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Figure 36.6 Cell compartments and routes for short-distance transport.
Plasmodesma
Apoplast
Plasma membrane
Symplast

27. 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
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28. Figure 36.7 Solute transport across plant cell plasma membranes.
CYTOPLASM
EXTRACELLULAR FLUID
(a) H+ and membrane potential  +
ATP  H+ +
Hydrogen ion  + H+ H+ H+ H+ H+  + H+
Proton pump H+  +  +
(b) H+ and cotransport of neutral solutes S H+ H+  + H+ H+  + H+ H+ S H+ S H+ H+ H+ S S  + S H+ 
H+/sucrose cotransporter +
Sucrose (neutral solute)  +  +
(c) H+ and cotransport of ions H+ H+
NO3  +
NO3  + H+ H+ H+ H+
Nitrate H+
NO3 H+
Figure 36.7 Solute transport across plant cell plasma membranes.
NO3  +
NO3
NO3  + H+
H+NO3 cotransporter H+  + H+  +
(d) Ion channels K+
Potassium ion  + K+ K+  + K+ K+ K+ K+  +
Ion channel  +

29. CYTOPLASM EXTRACELLULAR FLUID Hydrogen ion Proton pump  + H+  + ATP
Figure 36.7a
CYTOPLASM
EXTRACELLULAR FLUID  + H+  +
Hydrogen ion
ATP  + H+ H+ H+ H+ H+ H+  +
Figure 36.7 Solute transport across plant cell plasma membranes.
Proton pump H+  +
(a) H+ and membrane potential

30. Plant cells use the energy of H gradients to cotransport other solutes by active transport
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31. H+/sucrose cotransporter Sucrose (neutral solute)
Figure 36.7b  + H+ S H+  + H+ H+  + H+ H+ H+ S S H+ H+ H+ S S S  + H+ 
Figure 36.7 Solute transport across plant cell plasma membranes. +
H+/sucrose cotransporter
Sucrose (neutral solute)  +
(b) H+ and cotransport of neutral solutes

32. Nitrate H+NO3 cotransporter  + H+ H+ NO3  + NO3  H+ + H+ H+ H+
Figure 36.7c  + H+ H+
NO3  +
NO3  + H+ H+ H+ H+
Nitrate H+ H+
NO3
NO3 
NO3 +
NO3  + H+
Figure 36.7 Solute transport across plant cell plasma membranes.
H+NO3 cotransporter H+  H+ +
(c) H+ and cotransport of ions

33. Plant cell membranes have ion channels that allow only certain ions to pass
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34.  + K+ Potassium ion  + K+  K+ + K+ K+ K+ K+  + Ion channel  +
Figure 36.7d  + K+
Potassium ion  + K+  K+ + K+ K+ K+ K+  +
Ion channel
Figure 36.7 Solute transport across plant cell plasma membranes.  +
(d) Ion channels

35. 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
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36. Water potential determines the direction of movement of water
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
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37. Ψ = 0 MPa for pure water at sea level and at room temperature
Water potential is abbreviated as Ψ and measured in a unit of pressure called the megapascal (MPa)
Ψ = 0 MPa for pure water at sea level and at room temperature
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38. How Solutes and Pressure Affect Water Potential
Both pressure and solute concentration affect water potential
This is expressed by the water potential equation: Ψ  ΨS  ΨP
The solute potential (ΨS) of a solution is directly proportional to its molarity
Solute potential is also called osmotic potential
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39. Pressure potential (ΨP) is the physical pressure on a solution
Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast
The protoplast is the living part of the cell, which also includes the plasma membrane
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40. 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
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41. Figure 36.8
Solutes have a negative effect on  by binding water molecules.
Positive pressure has a positive effect on  by pushing water.
Solutes and positive pressure have opposing effects on water movement.
Negative pressure (tension) has a negative effect on  by pulling 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 right arm makes  lower there, resulting in net movement of water to the right arm:
Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm:
In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:
Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm:
Figure 36.8 Effects of solutes and pressure on water potential () and water movement.
Positive pressure
Positive pressure
Negative pressure
Pure water
Solutes
Solutes
Membrane
H2O
H2O
H2O
H2O

42. Solutes have a negative effect on  by binding water molecules.
Figure 36.8a
Solutes have a negative effect on  by binding water molecules.
Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm:
Pure water at equilibrium
Pure water
Figure 36.8 Effects of solutes and pressure on water potential () and water movement.
Solutes
Membrane
H2O
H2O

43. Positive pressure has a positive effect on  by pushing water.
Figure 36.8b
Positive pressure has a positive effect on  by pushing water.
Applying positive pressure to the right arm makes  higher there, resulting in net movement
of water to the left arm:
Positive pressure
Pure water at equilibrium
Figure 36.8 Effects of solutes and pressure on water potential () and water movement.
H2O
H2O

44. Solutes and positive pressure have opposing effects on water movement.
Figure 36.8c
Solutes and positive pressure have opposing effects on water movement.
In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:
Positive pressure
Pure water at equilibrium
Figure 36.8 Effects of solutes and pressure on water potential () and water movement.
Solutes
H2O
H2O

45. Pure water at equilibrium
Figure 36.8d
Negative pressure (tension) has a negative effect on  by pulling water.
Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm:
Negative pressure
Pure water at equilibrium
Figure 36.8 Effects of solutes and pressure on water potential () and water movement.
H2O
H2O

46. 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
Video: Plasmolysis
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47. Figure 36.9 Water relations in plant cells.
Initial flaccid cell: P  S
0.7 
0.4 M sucrose solution:  
0.7 MPa
Pure water: P  P  S 
0.9 S 
Plasmolyzed cell at osmotic equilibrium with its surroundings 
0.9 MPa   
0 MPa
Turgid cell at osmotic equilibrium with its surroundings P  P 
0.7 S
0.9  S 
0.7 
0.9 MPa   
0 MPa
Figure 36.9 Water relations in plant cells.
(a) Initial conditions: cellular   environmental 
(b) Initial conditions: cellular   environmental 

48. Plasmolyzed cell at osmotic equilibrium with its surroundings 
Figure 36.9a
Initial flaccid cell: P  S
0.7 
0.4 M sucrose solution: 
0.7 MPa  P  S
0.9 
Plasmolyzed cell at osmotic equilibrium with its surroundings 
0.9 MPa  P  S
0.9 
Figure 36.9 Water relations in plant cells. 
0.9 MPa 
(a) Initial conditions: cellular   environmental 

49. Turgid cell at osmotic equilibrium with its surroundings
Figure 36.9b
Initial flaccid cell: P  S
0.7  
0.7 MPa 
Pure water: P  S  
0 MPa 
Turgid cell at osmotic equilibrium with its surroundings P
0.7  S
0.7 
Figure 36.9 Water relations in plant cells. 
0 MPa 
(b) Initial conditions: cellular   environmental 

50. 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
Video: Turgid Elodea
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51. 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
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52. 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
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53. Concept 36.3: 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
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54. 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
Animation: Transport in Roots
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55. The concentration of essential minerals is greater in the roots than soil because of active transport
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56. 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
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57. 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
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58. Pathway along apoplast
Figure 36.10
Casparian strip
Endodermal cell
Pathway along apoplast
Pathway through symplast
Plasma membrane
Casparian strip
Apoplastic route
Figure Transport of water and minerals from root hairs to the xylem.
Vessels (xylem)
Symplastic route
Root hair
Epidermis
Endodermis
Vascular cylinder (stele)
Cortex

59. Vascular cylinder (stele)
Figure 36.10a
Plasma membrane
Casparian strip
Apoplastic route
Figure Transport of water and minerals from root hairs to the xylem.
Vessels (xylem)
Symplastic route
Root hair
Epidermis
Endodermis
Vascular cylinder (stele)
Cortex

60. Pathway along apoplast
Figure 36.10b
Casparian strip
Endodermal cell
Pathway along apoplast
Pathway through symplast
Figure Transport of water and minerals from root hairs to the xylem.

61. Water and minerals now enter the tracheids and vessel elements
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
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62. 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?
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63. 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
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64. Figure 36.11
Figure Guttation.

65. Positive root pressure is relatively weak and is a minor mechanism of xylem bulk flow
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66. 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
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67. 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
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68. This negative pressure pulls water in the xylem into the leaf
The transpirational pull on xylem sap is transmitted from leaves to roots
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69. Microfibrils in cell wall of mesophyll cell Mesophyll Air space
Figure 36.12
Cuticle
Xylem
Upper epidermis
Microfibrils in cell wall of mesophyll cell
Mesophyll
Air space
Figure Generation of transpirational pull.
Lower epidermis
Cuticle
Stoma
Microfibril (cross section)
Water film
Air-water interface

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

71. Water molecule Root hair Soil particle Water Water uptake from soil
Figure 36.13a
Water molecule
Root hair
Soil particle
Figure Ascent of xylem sap.
Water
Water uptake from soil

72. Adhesion by hydrogen bonding Xylem cells
Figure 36.13b
Adhesion by hydrogen bonding
Xylem cells
Cell wall
Figure Ascent of xylem sap.
Cohesion by hydrogen bonding
Cohesion and adhesion in the xylem

73. Xylem sap Mesophyll cells Stoma Water molecule Atmosphere
Figure 36.13c
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere
Figure Ascent of xylem sap.
Transpiration

74. 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
Animation: Water Transport
Animation: Transpiration
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75. 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
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76. 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
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77. 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
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78. Concept 36.4: 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
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79. Figure 36.14
Figure An open stoma (left) and closed stoma (LMs).

80. Figure 36.14a
Figure An open stoma (left) and closed stoma (LMs).

81. Figure 36.14b
Figure An open stoma (left) and closed stoma (LMs).

82. 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
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83. 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
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84. Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed
Figure 36.15
Guard cells turgid/ Stoma open
Guard cells flaccid/ 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
Figure Mechanisms of stomatal opening and closing.
H2O K
H2O
H2O
H2O
H2O
H2O
(b) Role of potassium in stomatal opening and closing

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

86. This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells
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87. Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed
Figure 36.15b
Guard cells turgid/ Stoma open
Guard cells flaccid/ Stoma closed
H2O
H2O
H2O
H2O
H2O K
H2O
H2O
H2O
Figure Mechanisms of stomatal opening and closing.
H2O
H2O
(b) Role of potassium in stomatal opening and closing

88. 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
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89. 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
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90. 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
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91. Adaptations That Reduce Evaporative Water Loss
Xerophytes are plants adapted to arid climates
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92. Oleander leaf cross section
Figure 36.16
Ocotillo (leafless)
Oleander leaf cross section
Cuticle
Upper epidermal tissue
Ocotillo after heavy rain
Oleander flowers
100 m
Trichomes (“hairs”)
Crypt
Stoma
Lower epidermal tissue
Figure Some xerophytic adaptations.
Ocotillo leaves
Old man cactus

93. Ocotillo (leafless) Figure 36.16a
Figure Some xerophytic adaptations.

94. Ocotillo after heavy rain
Figure 36.16b
Ocotillo after heavy rain
Figure Some xerophytic adaptations.

95. Ocotillo leaves Figure 36.16c
Figure Some xerophytic adaptations.
Ocotillo leaves

96. Oleander leaf cross section
Figure 36.16d
Oleander leaf cross section
Cuticle
Upper epidermal tissue
Figure Some xerophytic adaptations.
100 m
Trichomes (“hairs”)
Crypt
Stoma
Lower epidermal tissue

97. Oleander flowers Figure 36.16e
Figure Some xerophytic adaptations.

98. Figure 36.16f
Figure Some xerophytic adaptations.
Old man cactus

99. 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
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100. Concept 36.5: Sugars are transported from sources to sinks via the phloem
The products of photosynthesis are transported through phloem by the process of translocation
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101. 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
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102. 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
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103. Companion (transfer) cell High H concentration Cotransporter
Figure 36.17
Key
Apoplast
Symplast
Companion (transfer) cell
High H concentration
Cotransporter
Mesophyll cell
Proton pump H
Cell walls (apoplast)
Sieve-tube element S
Plasma membrane
Plasmodesmata
Figure Loading of sucrose into phloem.
ATP
Sucrose H H S
Bundle- sheath cell
Phloem parenchyma cell
Mesophyll cell
Low H concentration
(a)
(b)

104. Companion (transfer) cell Mesophyll cell
Figure 36.17a
Key
Apoplast
Symplast
Companion (transfer) cell
Mesophyll cell
Cell walls (apoplast)
Sieve-tube element
Plasma membrane
Plasmodesmata
Figure Loading of sucrose into phloem.
Bundle- sheath cell
Phloem parenchyma cell
Mesophyll cell
(a)

105. High H concentration Cotransporter Proton pump H S ATP Sucrose H H
Figure 36.17b
High H concentration
Cotransporter
Proton pump H S
Figure Loading of sucrose into phloem.
ATP
Sucrose H H S
Low H concentration
(b)

106. 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
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107. 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
Animation: Translocation of Phloem Sap in Summer
Animation: Translocation of Phloem Sap in Spring
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108. 1 Loading of sugar 2 Uptake of water 3 Unloading of sugar 4
Figure 36.18
Sieve tube (phloem)
Source cell (leaf)
Vessel (xylem) 1
Loading of sugar
H2O 1
Sucrose
H2O 2 2
Uptake of water
Bulk flow by positive pressure
Bulk flow by negative pressure 3
Unloading of sugar
Figure Bulk flow by positive pressure (pressure flow) in a sieve tube.
Sink cell (storage root) 4
Water recycled 4 3
Sucrose
H2O

109. 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
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110. Stylet in sieve-tube element Separated stylet exuding sap
Figure 36.19
EXPERIMENT
25 m
Sieve- tube element
Sap droplet
Figure Inquiry: Does phloem sap contain more sugar near sources than sinks?
Stylet
Sap droplet
Aphid feeding
Stylet in sieve-tube element
Separated stylet exuding sap

111. Sap droplet Aphid feeding Figure 36.19a
Figure Inquiry: Does phloem sap contain more sugar near sources than sinks?
Aphid feeding

112. Stylet in sieve-tube element
Figure 36.19b
25 m
Sieve- tube element
Stylet
Figure Inquiry: Does phloem sap contain more sugar near sources than sinks?
Stylet in sieve-tube element

113. Separated stylet exuding sap
Figure 36.19c
Sap droplet
Figure Inquiry: Does phloem sap contain more sugar near sources than sinks?
Separated stylet exuding sap

114. Concept 36.6: The symplast is highly dynamic
The symplast is a living tissue and is responsible for dynamic changes in plant transport processes
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115. 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
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116. Plasmodesma Virus particles Cell wall 100 nm Figure 36.20
Figure Virus particles moving cell to cell through a plasmodesma connecting turnip leaf cells (TEM).

117. Phloem: An Information Superhighway
Phloem is a “superhighway” for systemic transport of macromolecules and viruses
Systemic communication helps integrate functions of the whole plant
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118. 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)
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119. Figure 36.UN01
Turgid
Figure 36.UN01 In-text figure, p. 770

120. Figure 36.UN02
Wilted
Figure 36.UN02 In-text figure, p. 770

121. H2O CO2 O2 O2 CO2 Minerals H2O Figure 36.UN03
Figure 36.UN03 Summary figure, Concept 36.1 O2
CO2
Minerals
H2O

122. Figure 36.UN04
Figure 36.UN04 Test Your Understanding, question 10

123. Figure 36.UN05
Figure 36.UN05 Appendix A: answer to Test Your Understanding, question 10