1. Resource Acquisition and Transport in Vascular Plants
Chapter 36
Resource Acquisition and Transport in Vascular Plants

2. 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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3. What adaptations must have evolved for plants to establish themselves on land?

4. 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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5. Roots Soil is a resource mined by the root system
Taproot systems anchor plants and are characteristic of gymnosperms
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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6. Roots and 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
Root hairs increase the absorbing surface area
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7. Figure 36.5
Roots
Figure 36.5 A mycorrhiza, a mutualistic association of fungus and roots.
Fungus

8. Stems
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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9. 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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10. Leaves: Site of Photosynthesis

11. Flowers: Reproductive Organs

12. Three major 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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13. 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

14. 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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15. 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

16. Plant cells use the energy of H gradients to cotransport other solutes by active transport
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17. 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

18. 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

19. Plant cell membranes have ion channels that allow only certain ions to pass
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20.  + 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

21. 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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22. 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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23. 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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24. 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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25. 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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26. 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

27. 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 ______ water and undergo plasmolysis
Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall
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28. 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 

29. If a flaccid cell is placed in a solution with a lower solute concentration, the cell will ______ water and become turgid
Turgor loss in plants causes wilting, which can be reversed when the plant is watered
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30. 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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31. 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
What causes the pressure created by the xylem sap?
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32. 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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33. 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
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34. 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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35. 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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36. 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

37. 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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38. 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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39. 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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40. Figure 36.11
Figure Guttation.

41. 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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42. 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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43. 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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44. 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

45. 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

46. 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

47. 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

48. 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
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49. Animation: Transpiration
Right-click slide / select “Play”
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50. Water molecules are attracted to each other through cohesion
Cohesion makes it possible to pull a column of xylem sap
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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51. 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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52. Figure 36.14
Figure An open stoma (left) and closed stoma (LMs).

53. 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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54. 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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55. 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

56. This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells
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57. 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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58. 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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59. 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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60. 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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61. Unlike water phloem sap can run up or down the plant
A storage organ (root) can be both a sugar sink in summer and sugar source in winter
Sugar must be loaded into sieve-tube elements before being exported to sinks
Unlike water phloem sap can run up or down the plant
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62. 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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63. 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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64. 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
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65. Animation: Translocation of Phloem Sap in Summer Right-click slide / select “Play”
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66. 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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67. Phloem: An Information Superhighway
Phloem is a “superhighway” for systemic transport of macromolecules and viruses
Systemic communication helps integrate functions of the whole plant
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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68. 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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69. 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).