1. Chapter 25: Transport of Water and Nutrients in Plants
AP Biology

2. Plants require nutrients and water
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. 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 3

4. 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 transportThe 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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5. Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss
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6. Plants Acquire Mineral Nutrients from Soil
Ion exchange makes nutrients available to plants
Soil organisms contribute to plant nutrition
N-fixing bacteria (Rhizobium)
Mycrorrhizae (symbiotic association with fungi)
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7. Water and Solutes are Transported in the Xylem by Transpiration-Cohesion-Tension
Differences in Water potential govern the direction of water movement (see next slides for review)
Water and ions move across the root cell plasma membrane
Membrane proteins required!! (need to move across hydrophobic membrane and often against gradient)
Aquaporins
Ion channels and proton pumps
Ion exchange makes nutrients available to plants
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9. Water Potential and 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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10. 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
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
Water moves in the direction from higher water potential to lower water potential
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11. 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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12. 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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13. 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

14. 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
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15. 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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16. Water and Solutes are Transported in the Xylem by Transpiration-Cohesion-Tension
Differences in Water potential govern the direction of water movement (see next slides for review)
Water and ions move across the root cell plasma membrane
Membrane proteins required!! (need to move across hydrophobic membrane and often against gradient)
Aquaporins
Ion channels and proton pumps
Ion exchange makes nutrients available to plants
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18. 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
Plant cells use the energy of H gradients to cotransport other solutes by active transport
In animals, membrane potential is established through pumping Na by sodium-potassium pumps
Plant cell membranes have ion channels that allow only certain ions to pass
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19. 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
The concentration of essential minerals is greater in the roots than soil because of active transport
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20. Different mechanisms transport substances over short or long distances
Three transport routes for water and solutes are:
The apoplastic route, through cell walls and extracellular spaces
The symplastic route, through the cytosol (plasmodesmata)
The transmembrane route, across cell membrane
This requires the use of transport proteins to allow passage of anything but gases
Allows for selectivity!!! (only point in plant transport where selectivity can occur)
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21. 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

22. Water and ions pass to the xylem by way of the Apoplast and symplast:
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 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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24. 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

25. 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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26. Animation: Transport in Roots Right-click slide / select “Play”
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27. Transpiration drives the transport of water and minerals from roots to shoots via the xylem
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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29. 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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30. 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
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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31. 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 31

32. 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
Water molecules are attracted to each other through cohesion
Cohesion makes it possible to pull a column of xylem sap
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33. Animation: Water Transport
Right-click slide / select “Play”
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34. Animation: Transpiration
Right-click slide / select “Play”
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35. 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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36. Bulk flow is how solutes are transported through xylem
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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37. 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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38. 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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39. 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
This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells
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40. 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

42. 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
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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43. 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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44. Adaptations That Reduce Evaporative Water Loss
Xerophytes are plants adapted to arid climates
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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45. 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

46. Sugars are transported from sources to sinks via the phloem (Phloem Pressure Flow)
The products of photosynthesis are transported through phloem by the process of translocation
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47. 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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48. 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 exported 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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49. 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 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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50. The Pressure Flow Model

51. 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

52. 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)

53. Animation: Translocation of Phloem Sap in Summer Right-click slide / select “Play”
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54. Animation: Translocation of Phloem Sap in Spring Right-click slide / select “Play”
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