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Transcript of Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint TextEdit Art...
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint TextEdit Art Slides forBiology, Seventh Edition
Neil Campbell and Jane Reece
Chapter 36Chapter 36
Transport in Vascular Plants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.1 Coast redwoods (Sequoia sempervirens)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.2 An overview of transport in a vascular plant (layer 1)
Minerals
H2O
H2O
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.2 An overview of transport in a vascular plant (layer 2)
Minerals
H2O
CO2 O2
H2O
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.2 An overview of transport in a vascular plant (layer 3)
Minerals
H2O
CO2 O2
H2O Sugar
Light
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.2 An overview of transport in a vascular plant (layer 4)
Minerals
H2O CO2
O2
CO2 O2
H2O Sugar
Light
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.3 Proton pumps provide energy for solute transport
CYTOPLASM EXTRACELLULAR FLUID
ATP
H+
H+ H+
H+
H+
H+
H+
H+
Proton pump generates membrane potentialand H+ gradient.
–
–
–
–
– +
+
+
+
+
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.4 Solute transport in plant cells+
CYTOPLASMEXTRACELLULAR FLUID
Cations ( for example) are driven into the cell by themembrane potential.
Transport protein
K+
K+
K+
K+
K+ K+
K+
K+
–
–
– +
+
(a) Membrane potential and cation uptake
H+
H+
H+
H+
–
–
+
+
H+
H+
H+
H+
H+
H+
H+
H+
NO3–
NO 3 –
NO3–
NO 3
–
NO3
–
NO 3 – –
–
– +
+
+
(b) Cotransport of anions
H+
H+
H+
H+
H+
H+
H+H+
H+ H+
H+
H+
Plant cells canalso accumulate a neutral solute,such as sucrose( ), bycotransporting down thesteep protongradient.
S
S
S
SS
S
S
H+
(c) Cotransport of a neutral solute
–
–
– +
+
+
–
–
–
+
+
+
–
–
–
+
+
+
Cell accumulates anions (NO3
–, for example) by coupling their transport to theinward diffusion of H+ through acotransporter.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.5 Water potential and water movement: an artificial model
= –0.23 MPa
(a)
0.1 Msolution
(d)(c)(b)
P = 0
H2OH2O
H2O H2O
S = –0.23
= –0.23 MPa
S = –0.23
= 0 MPa
P = 0.23S = –0.23
= –0.07 MPa
P = 0.30S = 0
= –0.30 MPa
P = –0.30S = –0.23P = 0
= 0 MPa = 0 MPa = 0 MPa
Purewater
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Figure 36.6 Water relations in plant cells
s = –0.9
(a)
0.4 M sucrose solution: = 0s = –0.9
= –0.9 MPa
= 0s = –0.7
= –0.7 MPa
Initial flaccid cell:
= 0s = 0
= 0 MPa
Distilled water:
Plasmolyzed cell at osmotic equilibriumwith its surroundings = 0
= –0.9 MPa
= 0.7s = –0.7
= 0 MPa
Turgid cellat osmotic equilibriumwith its surroundings
Initial conditions: cellular > environmental . The cellloses water and plasmolyzes. After plasmolysis is complete, the water potentials of the cell and its surroundings are the same.
Initial conditions: cellular < environmental . There is a net uptake of water by osmosis, causing the cell tobecome turgid. When this tendency for water to enter is offset by the back pressure of the elastic wall, water potentials are equal for the cell and its surroundings. (The volume change of the cell is exaggerated in this diagram.)
(b)
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Figure 36.7 A watered Impatiens plant regains its turgor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 36.8 Cell compartments and routes for short-distance transport
Transport proteins inthe plasma membrane
regulate traffic ofmolecules betweenthe cytosol and the
cell wall.
Transport proteins inthe vacuolarmembrane regulatetraffic of moleculesbetween the cytosoland the vacuole.
Plasmodesma Vacuolar membrane(tonoplast)Plasma membrane
Cell compartments. The cell wall, cytosol, and vacuole are the three maincompartments of most mature plant cells.
Key
Symplast
Apoplast
The symplast is thecontinuum of
cytosol connectedby plasmodesmata.
The apoplast isthe continuumof cell walls andextracellularspaces.
Apoplast
Transmembrane route
Symplastic route Apoplastic 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.
Cell wallCytosol
Vacuole
(a)
(b)
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Figure 36.9 Lateral transport of minerals and water in roots
1
2
3
Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this 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 the Casparian strip, a belt of waxy material (purple band) that blocks thepassage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder.
Endodermal cells and also parenchyma cells within thevascular cylinder discharge water and minerals into theirwalls (apoplast). The xylem vessels transport the waterand 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
Endodermis
4 5
2
1
34 5
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Figure 36.10 Mycorrhizae, symbiotic associations of fungi and roots
2.5 mm
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Figure 36.11 Guttation
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Figure 36.12 The generation of transpirational pull in a leaf
Evaporation causes the air-water interface to retreat farther into the cell wall and become more curved as the rate of transpiration increases. As the interface becomes more curved, the water film’s pressure becomes more negative. This negative pressure, or tension, pulls water from the xylem, where the pressure is greater.
CuticleUpperepidermis
Mesophyll
Lowerepidermis
CuticleWater vapor
CO2 O2 Xylem CO2 O2
Water vapor
Stoma
Evaporation
At first, the water vapor lost bytranspiration is replaced by evaporation from the water film that coats mesophyll cells.
In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata.
Airspace
Cytoplasm
Cell wall
Vacuole
EvaporationWater film
Low rate oftranspiration
High rate oftranspiration
Air-waterinterface
Cell wall
Airspace
= –0.15 MPa = –10.00 MPa
Vacuole
3
1 2
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Figure 36.13 Ascent of xylem sap
XylemsapOutside air
= –100.0 MPa
Leaf (air spaces) = –7.0MPa
Leaf (cell walls) = –1.0 MPa
Trunk xylem = – 0.8 MPa
Wat
er p
ote
nti
al g
rad
ien
t
Root xylem = – 0.6 MPa
Soil = – 0.3 MPa
MesophyllcellsStoma
Watermolecule
Atmosphere
Transpiration
Xylemcells Adhesion Cell
wall
Cohesion,byhydrogenbonding
Watermolecule
Roothair
Soilparticle
Water
Cohesion and adhesionin the xylem
Water uptakefrom soil
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Figure 36.14 Open stomata (left) and closed stomata (colorized SEM)
20 µm
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Figure 36.15 The mechanism of stomatal opening and closing
Cells flaccid/Stoma closedCells turgid/Stoma open
H2O
Radially oriented cellulose microfibrils
Cellwall
VacuoleGuard cell
H2O
H2OH2O
H2O
K+
Changes in guard cell shape and stomatal opening and closing (surface view). Guard cells of a typical angiosperm are illustrated in their turgid (stoma open)and flaccid (stoma closed) states. The pair of guard cells buckle outward when turgid. Cellulose microfibrils in the walls resist stretching and compression in the direction parallel to the microfibrils. Thus, the radial orientation 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)
Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane andvacuolar membrane causes the turgor changes of guard cells.
(b) H2O H2O
H2O
H2O
H2O
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Figure 36.16 Structural adaptations of a xerophyte leaf
Lower epidermaltissue
Trichomes(“hairs”)
Cuticle Upper epidermal tissue
Stomata 100m
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Figure 36.17 Loading of sucrose into phloem
Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sieve-tube members and their companion cells.
(a)
Mesophyll cellCell walls (apoplast)
Plasma membranePlasmodesmata
Companion(transfer) cell
Sieve-tubemember
Mesophyll cellPhloem parenchyma cell
Bundle-sheath cell
High H+ concentration Cotransporter
Protonpump
ATPKey
SucroseApoplast
Symplast
H+
A chemiosmotic mechanism is responsible forthe active transport of sucrose into companion cells and sieve-tube members. Proton pumps generate an H+ gradient, which drives sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell.
(b)
H+
Low H+ concentration
H+
S
S
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Figure 36.18 Pressure flow in a sieve tubeVessel(xylem)
H2O
H2O
Sieve tube(phloem)
Source cell(leaf)
Sucrose
H2O
Sink cell(storageRoot)
1
Sucrose
Loading of sugar (green dots) into the sieve tube at thesource reduces water potential inside the sieve-tube members. This causes the tube to take up waterby osmosis.
2
4 3
1
2 This uptake ofwater generates a positive pressurethat forces the sap to flow along the tube.
The pressure isrelieved by theunloading of sugar and the consequentloss of water from the tubeat the sink.
3
4 In the case of leaf-to-roottranslocation,xylem recycleswater from sinkto source.
Tra
ns
pir
ati
on
str
ea
m
Pre
ss
ure
flo
w
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Figure 36.19 What causes phloem sap to flow from source to sink?
Aphid feeding Stylet in sieve-tube member
Severed styletexuding sap
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 different points between a source and sink.
EXPERIMENT
The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.
RESULTS
The results of such experiments support the pressure flow hypothesis.
CONCLUSION
Sap dropletStylet
Sapdroplet
25 m
Sieve-tubemember