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PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 36
Transport in Vascular Plants
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Overview: Pathways for Survival
For vascular plants
The evolutionary journey onto land involvedthe differentiation of the plant body into rootsand shoots
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Vascular tissue
Transports nutrients throughout a plant; suchtransport may occur over long distances
Figure 36.1
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Concept 36.1: Physical forces drive the transport of materials in plants over a range of distances
Transport in vascular plants occurs on three scales
Transport of water and solutes by individual cells,
such as root hairs Short-distance transport of substances from cell to
cell at the levels of tissues and organs
Long-distance transport within xylem and phloem atthe level of the whole plant
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MineralsH2O CO 2
O 2
CO 2 O 2
H2O Sugar
Light
A variety of physical processes
Are involved in the different types of transportSugars are produced by
photosynthesis in the leaves.5
Sugars are transported asphloem sap to roots and other parts of the plant.
6
Through stomata, leavestake in CO 2 and expel O 2.The CO 2 provides carbon for photosynthesis. Some O 2 produced by photosynthesisis used in cellular respiration.
4
Transpiration, the loss of water from leaves (mostly through
stomata), creates a force withinleaves that pulls xylem sap upward.
3
Water and minerals aretransported upward from
roots to shoots as xylem sap.
2
Roots absorb water and dissolved minerals
from the soil.
1
Figure 36.2
Roots exchange gaseswith the air spaces of soil,taking in O 2 and dischargingCO 2. In cellular respiration,O 2 supports the breakdownof sugars.
7
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Selective Permeability of Membranes: A Review
The selective permeability of a plant cells
plasma membrane Controls the movement of solutes into and out
of the cell
Specific transport proteins
Enable plant cells to maintain an internalenvironment different from their surroundings
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The Central Role of Proton Pumps
Proton pumps in plant cells
Create a hydrogen ion gradient that is a formof potential energy that can be harnessed todo work
Contribute to a voltage known as a membranepotential
Figure 36.3
CYTOPLASM EXTRACELLULAR FLUID
ATP
H+
H+ H+H+
H+
H+H+
H+Proton pump generatesmembrane potentialand H + gradient.
+
+
+
+
+
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Plant cells use energy stored in the proton
gradient and membrane potential To drive the transport of many different solutes
+CYTOPLASM EXTRACELLULAR FLUID
Cations ( , for example) are driveninto the cell by themembrane potential.
Transport protein
K+
K+
K+
K+
K+ K+
K+
K+
+
+
(a) Membrane potential and cation uptake
+
+
Figure 36.4a
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In the mechanism called cotransport
A transport protein couples the passage of onesolute to the passage of another
Figure 36.4b
H+
H+
H+
H+
H+
H+H+
H+
H+
H+H+
H+
N O 3
N O 3
N O 3
N O 3
N O 3
N O 3
+
+
+
+
++
NO 3
(b) Cotransport of anions
H+of through acotransporter.
Cell accumulatesanions ( , for example) bycoupling their transport to theinward diffusion
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H+
H+
H+
H+
H+H+
H+
H+ H+
H+
S SS
S
S
Plant cells canalso accumulate aneutral solute,such as sucrose
( ), bycotransporting
down the
steep protongradient.
S
H+
+
+
+
++
Figure 36.4c
H+ H+S+
(c) Contransport of a neutral solute
The coattail effect of cotransport
Is also responsible for the uptake of the sugar sucrose by plant cells
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Effects of Differences in Water Potential
To survive
Plants must balance water uptake and loss
Osmosis
Determines the net uptake or water loss by acell
Is affected by solute concentration and
pressure
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Water potential
Is a measurement that combines the effects of solute concentration and pressure
Determines the direction of movement of water
Water
Flows from regions of high water potential to
regions of low water potential
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How Solutes and Pressure Affect Water Potential
Both pressure and solute concentration
Affect water potential
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The solute potential of a solution
Is proportional to the number of dissolvedmolecules
Pressure potential
Is the physical pressure on a solution
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Quantitative Analysis of Water Potential
The addition of solutes
Reduces water potential
Figure 36.5a
0.1 M solution
H2O
Purewater
P = 0 S = 0.23 = 0.23 MPa = 0 MPa
(a)
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Application of physical pressure
Increases water potential
H2O
P = 0.23 S = 0.23 = 0 MPa = 0 MPa
(b)
H2O
P = 0.30 S = 0.23 = 0.07 MPa = 0 MPa
(c)
Figure 36.5b, c
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Negative pressure
Decreases water potential
H2O
P = 0 S = 0.23 = 0.23 MPa
(d)
P = 0.30 S = 0 = 0.30 MPa
Figure 36.5d
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Water potential
Affects uptake and loss of water by plant cells
If a flaccid cell is placed in an environment witha higher solute concentration
The cell will lose water and become plasmolyzed
Figure 36.6a
0.4 M sucrose solution:
Initial flaccid cell:
Plasmolyzed cellat osmotic equilibriumwith its surroundings
P = 0 S = 0.7
P = 0 S = 0.9
P = 0 S = 0.9
= 0.9 MPa
= 0.7 MPa
= 0.9 MPa
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If the same flaccid cell is placed in a solution
with a lower solute concentration The cell will gain water and become turgid
Distilled water:
Initial flaccid cell:
Turgid cellat osmotic equilibrium
with its surroundings
P = 0 S = 0.7
P = 0 S = 0
P = 0.7 S = 0.7
Figure 36.6b
= 0.7 MPa
= 0 MPa
= 0 MPa
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Turgor loss in plants causes wilting
Which can be reversed when the plant iswatered
Figure 36.7
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Aquaporin Proteins and Water Transport
Aquaporins
Are transport proteins in the cell membranethat allow the passage of water
Do not affect water potential
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Three Major Compartments of Vacuolated Plant Cells
Transport is also regulated
By the compartmental structure of plant cells
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The plasma membrane
Directly controls the traffic of molecules intoand out of the protoplast
Is a barrier between two major compartments,the cell wall and the cytosol
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The third major compartment in most mature plantcells
Is the vacuole, a large organelle that can occupyas much as 90% of more of the protoplastsvolume
The vacuolar membrane
Regulates transport between the cytosol and the
vacuole
Transport proteins inthe plasma membrane
regulate traffic of molecules between
the cytosol and thecell wall.
Transport proteins inthe vacuolar membrane regulatetraffic of moleculesbetween the cytosoland the vacuole.
PlasmodesmaVacuolar membrane
(tonoplast)Plasma membrane
Cell wall
Cytosol
Vacuole
Cell compartments. The cell wall, cytosol, and vacuole are the three main
compartments of most mature plant cells.
(a)Figure 36.8a
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In most plant tissues
The cell walls and cytosol are continuous from cellto cell
The cytoplasmic continuum
Is called the symplast
The apoplast
Is the continuum of cell walls plus extracellular spaces
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Key
Symplast
Apoplast
The symplast is the
continuum of cytosol connectedby plasmodesmata.
The apoplast isthe continuumof cell walls andextracellular spaces.
Apoplast
Transmembrane route
Symplastic routeApoplastic route
Symplast
Transport routes between cells. At the tissue level, there are three passages:the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another.
(b)
Figure 36.8b
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Functions of the Symplast and Apoplast in Transport
Water and minerals can travel through a plantby one of three routes
Out of one cell, across a cell wall, and intoanother cell
Via the symplast
Along the apoplast
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Bulk Flow in Long-Distance Transport
In bulk flow
Movement of fluid in the xylem and phloem isdriven by pressure differences at oppositeends of the xylem vessels and sieve tubes
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Concept 36.2: Roots absorb water andminerals from the soil
Water and mineral salts from the soil
Enter the plant through the epidermis of rootsand ultimately flow to the shoot system
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Lateral transport of minerals and water in roots
Figure 36.9
1
2
3
Uptake of soil solution by thehydrophilic walls of root hairsprovides access to the apoplast.Water and minerals can thensoak into the cortex alongthis matrix of walls.
Minerals and water that crossthe plasma membranes of roothairs enter the symplast.
As soil solution moves alongthe apoplast, some water andminerals are transported intothe protoplasts of cells of theepidermis and cortex and thenmove inward via the symplast.
Within the transverse and radial walls of each endodermal cell is theCasparian strip, a belt of waxy material (purple band) that blocks thepassage of water and dissolved minerals. Only minerals already inthe symplast or entering that pathway by crossing the plasmamembrane of an endodermal cell can detour around the Casparianstrip and pass into the vascular cylinder.
Endodermal cells and also parenchyma cells within thevascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system.
Casparian strip
Pathway alongapoplast
Pathwaythroughsymplast
Plasmamembrane
Apoplasticroute
Symplasticroute
Root
hair
Epidermis Cortex Endodermis Vascular cylinder
Vessels(xylem)
Casparian strip
Endodermal cell
4 5
2
1
Th R l f R H i M hi d C i l C ll
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The Roles of Root Hairs, Mycorrhizae, and Cortical Cells
Much of the absorption of water and minerals occursnear root tips, where the epidermis is permeable towater and where root hairs are located
Root hairs account for much of the surface area of roots
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Most plants form mutually beneficial relationships withfungi, which facilitate the absorption of water andminerals from the soil
Roots and fungi form mycorrhizae, symbiotic structuresconsisting of plant roots united with fungal hyphae
Figure 36.10
2.5 mm
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Once soil solution enters the roots
The extensive surface area of cortical cellmembranes enhances uptake of water andselected minerals
Th E d d i A S l i S
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The Endodermis: A Selective Sentry
The endodermis
Is the innermost layer of cells in the root cortex
Surrounds the vascular cylinder and functionsas the last checkpoint for the selective
passage of minerals from the cortex into thevascular tissue
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Water can cross the cortex
Via the symplast or apoplast
The waxy Casparian strip of the endodermalwall
Blocks apoplastic transfer of minerals from thecortex to the vascular cylinder
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Concept 36.3: Water and minerals ascend fromroots to shoots through the xylem
Plants lose an enormous amount of water through transpiration, the loss of water vapor
from leaves and other aerial parts of the plant The transpired water must be replaced by
water transported up from the roots
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P hi X l S R t P
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Pushing Xylem Sap: Root Pressure
At night, when transpiration is very low
Root cells continue pumping mineral ions intothe xylem of the vascular cylinder, lowering thewater potential
Water flows in from the root cortex
Generating root pressure
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Root pressure sometimes results in guttation,the exudation of water droplets on tips of grassblades or the leaf margins of some small,herbaceous eudicots
Figure 36.11
P lli g X l S Th T i ti C h i
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Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism
Water is pulled upward by negative pressure inthe xylem
Transpirational Pull
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Transpirational Pull
Water vapor in the airspaces of a leaf
Diffuses down its water potential gradient andexits the leaf via stomata
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Transpiration produces negative pressure(tension) in the leaf
Which exerts a pulling force on water in thexylem, pulling water into the leaf
Evaporation causes the air-water interface to retreat farther intothe cell wall and become more curved as the rate of transpiration
increases. As the interface becomes more curved, the water filmspressure becomes more negative. This negative pressure, or tension,pulls water from the xylem, where the pressure is greater.
CuticleUpper epidermis
Mesophyll
Lower epidermis
CuticleWater vapor CO 2 O 2 Xylem CO 2 O 2
Water vapor Stoma
Evaporation
At first, the water vapor lost bytranspiration is replaced byevaporation from the water filmthat coats mesophyll cells.
In transpiration, water vapor (shown asblue dots) diffuses from the moist air spaces of theleaf to the drier air outside via stomata.
Airspace
Cytoplasm
Cell wall
VacuoleEvaporationWater film
Low rate of transpiration
High rate of transpiration
Air-water interface
Cell wallAirspace
= 0.15 MPa = 10.00 MPa
3
1 2
Figure 36.12
Air-space
Cohesion and Adhesion in the Ascent of Xylem Sap
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Cohesion and Adhesion in the Ascent of Xylem Sap
The transpirational pull on xylem sap
Is transmitted all the way from the leaves tothe root tips and even into the soil solution
Is facilitated by cohesion and adhesion
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Ascent of xylem sapXylemsap
Outside air = 100.0 MPa
Leaf (air spaces)= 7.0 MPa
Leaf (cell walls)= 1.0 MPa
Trunk xylem = 0.8 MPa
W a t e r p o
t e n t
i a l g r a
d i e n t
Root xylem = 0.6 MPa
Soil = 0.3 MPa
MesophyllcellsStomaWater molecule
Atmosphere
Transpiration
Xylem
cells Adhesion Cellwall
Cohesion,byhydrogen
bonding
Water molecule
Roothair
Soilparticle
Water
Cohesionand adhesionin the xylem
Water uptakefrom soilFigure 36.13
Xylem Sap Ascent by Bulk Flow: A Review
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Xylem Sap Ascent by Bulk Flow: A Review
The movement of xylem sap against gravity
Is maintained by the transpiration-cohesion-tension mechanism
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Concept 36.4: Stomata help regulate the rateof transpiration
Leaves generally have broad surface areas
And high surface-to-volume ratios
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Both of these characteristics
Increase photosynthesis
Increase water loss through stomata
20 m
Figure 36.14
Effects of Transpiration on Wilting and Leaf Temperature
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Effects of Transpiration on Wilting and Leaf Temperature
Plants lose a large amount of water bytranspiration
If the lost water is not replaced by absorptionthrough the roots
The plant will lose water and wilt
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Transpiration also results in evaporativecooling
Which can lower the temperature of a leaf andprevent the denaturation of various enzymesinvolved in photosynthesis and other metabolicprocesses
Stomata: Major Pathways for Water Loss
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Stomata: Major Pathways for Water Loss
About 90% of the water a plant loses
Escapes through stomata
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Each stoma is flanked by guard cells
Which control the diameter of the stoma bychanging shape
Cells flaccid/Stoma closedCells turgid/Stoma open
Radially orientedcellulose microfibrils
Cellwall
VacuoleGuard cell
Changes in guard cell shape and stomatal opening
and closing (surface view). Guard cells of a typicalangiosperm are illustrated in their turgid (stoma open)and flaccid (stoma closed) states. The pair of guardcells buckle outward when turgid. Cellulose microfibrilsin the walls resist stretching and compression in thedirection parallel to the microfibrils. Thus, the radialorientation of the microfibrils causes the cells to increasein length more than width when turgor increases.The two guard cells are attached at their tips, so the
increase in length causes buckling.
(a)
Figure 36.15a
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Changes in turgor pressure that open andclose stomata
Result primarily from the reversible uptake andloss of potassium ions by the guard cells
H2O
H2O
H2OH2O
H2O
K+
Role of potassium in stomatal opening and closing. The transport of K + (potassium ions, symbolizedhere as red dots) across the plasma membrane andvacuolar membrane causes the turgor changes of guard cells.
(b) H2O H2O
H2O
H2O
H2O
Figure 36.15b
Xerophyte Adaptations That Reduce Transpiration
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Xerophyte Adaptations That Reduce Transpiration
Xerophytes
Are plants adapted to arid climates
Have various leaf modifications that reduce therate of transpiration
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The stomata of xerophytes
Are concentrated on the lower leaf surface
Are often located in depressions that shelter the pores from the dry wind
Lower epidermaltissue
Trichomes(hairs)
Cuticle Upper epidermal tissue
Stomata 100 m
Figure 36.16
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Concept 36.5: Organic nutrients aretranslocated through the phloem
Translocation
Is the transport of organic nutrients in the plant
Movement from Sugar Sources to Sugar Sinks
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Movement from Sugar Sources to Sugar Sinks
Phloem sap
Is an aqueous solution that is mostly sucrose
Travels from a sugar source to a sugar sink
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A sugar source
Is a plant 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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Figure 36.17a
Mesophyll cellCell walls (apoplast)
Plasma membranePlasmodesmata
Companion(transfer) cell
Sieve-tubemember
Mesophyll cell
Phloemparenchyma cell
Bundle-sheath cell
Sucrose manufactured in mesophyll cells cantravel via the symplast (blue arrows) tosieve-tube members. In some species, sucroseexits the symplast (red arrow) near sievetubes and is actively accumulated from theapoplast by sieve-tube members and their companion cells.
(a)
Sugar must be loaded into sieve-tube membersbefore being exposed to sinks
In many plant species, sugar moves bysymplastic and apoplastic pathways
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A chemiosmotic mechanism is responsible for the active transport of sucrose into companion cellsand sieve-tube members. Proton pumps generatean H + gradient, which drives sucrose accumulationwith the help of a cotransport protein that couplessucrose transport to the diffusion of H + back into the cell.
(b)
High H + concentration Cotransporter
Protonpump
ATPKey
SucroseApoplast
Symplast
H+ H+
Low H + concentration
H+
S
S
Figure 36.17b
In many plants
Phloem loading requires active transport
Proton pumping and cotransport of sucroseand H +
Enable the cells to accumulate sucrose
Pressure Flow: The Mechanism of Translocation in
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Vessel(xylem)
H2O
H2O
Sieve tube(phloem)
Source cell(leaf)
Sucrose
H2O
Sink cell(storageroot)
1
Sucrose
Loading of sugar (greendots) into the sieve tube
at the source reduceswater potential inside thesieve-tube members. Thiscauses the tube to takeup water by osmosis.
2
4 3
1
2 This uptake of water generates a positivepressure that forcesthe sap to flow alongthe tube.
The pressure is relievedby the unloading of sugar and the consequent lossof water from the tubeat the sink.
3
4 In the case of leaf-to-roottranslocation, xylemrecycles water from sinkto source.
T r a n s p i r a
t i o n s t r e a m
P r e s s u r e
f l o w
Figure 36.18
Angiosperms
In studying angiosperms Researchers have concluded that sap moves
through a sieve tube by bulk flow driven bypositive pressure
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The pressure flow hypothesis explains whyphloem sap always flows from source to sink
Experiments have built a strong case for pressure flow as the mechanism of
translocation in angiosperms
Aphid feeding Stylet in sieve-tubemember
Severed styletexuding sap
Sieve-Tubemember
EXPERIMENT
RESULTS
Sap dropletStylet
Sapdroplet
25 m
Sieve-tubemember
To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic-like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at differentpoints between a source and sink.
The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.