Transcript Transports

Chapter 36
• Transport in Plants
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• Pathways for Survival
• For vascular plants
– The evolutionary journey onto land
involved the differentiation of the
plant body into roots and shoots
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• Vascular tissue
– Transports nutrients throughout a plant; such
transport may occur over long distances
Figure 36.1
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•
Transport occurs on 3 scales
1. Transport of water and solutes by individual cells,
such as root hairs
2. Short-distance transport of substances from cell to
cell
3. Long-distance transport within xylem and phloem
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• Types of transport
4 Through stomata, leaves
take in CO2 and expel O2.
The CO2 provides carbon for
photosynthesis. Some O2
produced by photosynthesis
is used in cellular respiration.
CO2
H2O
O2
5 Sugars are produced by
photosynthesis in the leaves.
Light
Sugar
3 Transpiration, the loss of water
from leaves (mostly through
stomata), creates a force within
leaves that pulls xylem sap upward.
6 Sugars are transported as
phloem sap to roots and other
parts of the plant.
2 Water and minerals are
transported upward from
roots to shoots as xylem sap.
1 Roots absorb water
and dissolved minerals
from the soil.
O2
H2O
Minerals
Figure 36.2
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CO2
7 Roots exchange gases
with the air spaces of soil,
taking in O2 and discharging
CO2. In cellular respiration,
O2 supports the breakdown
of sugars.
Central Role of Proton Pumps
• Create a H+ (proton) gradient PE that can be
harnessed to do work
•  membrane potential
EXTRACELLULAR FLUID
CYTOPLASM
–
ATP
–
–
+
+
H+
H+
+
H+
H+
H+
H+
Figure 36.3
–
–
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+
H+
H+
+
Proton pump generates
membrane potential
and H+ gradient.
• Cotransport
– Coupled transport
H+
–
+
–
+
–
+
H+
H+
H+
H+
H+
H+
H+
–
+
–
+
–
+
(b) Cotransport of anions
Figure 36.4b
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H+
H+
H+
H+
Cell accumulates
–
anions ( NO,3 for
example) by
coupling their
transport to the
inward diffusion
of H+ through a
cotransporter.
• “coattail” effect of cotransport
– Responsible for the uptake of sucrose by plant
cells
–
H+
H+
+
H+
H+
–
+
–
+
Plant cells can
also accumulate a
neutral solute,
such as sucrose
H+
H+
S
–+
H
H+
H+
–
–
+
+
H+
–
(c) Contransport of a neutral solute
steep proton
gradient.
H+
S
+
Figure 36.4c
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( S ), by
cotransporting
H+ down the
H+
Effects of Differences in Water Potential
• Plants must balance water uptake and loss
• Osmosis
– Determines uptake or loss
– Affected by solute conc. & pressure
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• Water potential (y)
– Solute concentration and pressure
 Direction of movement of water
• Water
– Flows f/ high y to low y
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• Solute potential
– Proportional to # of dissolved molecules
• Pressure potential
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• Addition of solutes
– Reduces y
(a)
0.1 M
solution
Pure
waer
H2O
y = 0 MPa
Figure 36.5a
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yP = 0
yS = 0.23
y = 0.23 MPa
• Application of pressure
– Increases
y
(b)
(c)
H2O
H2O
y = 0 MPa
yP = 0.23
yS = 0.23
y = 0 MPa
Figure 36.5b, c
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y = 0 MPa
yP = 0.30
yS = 0.23
y = 0.07 MPa
• Negative pressure
– Decreases y
(d)
H2O
yP = 0.30
yS = 0
y = 0.30 MPa
Figure 36.5d
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yP = 0
yS = 0.23
y = 0.23 MPa
y
– Affects uptake and loss of water by plant cells
•
Flaccid cells in higher solute concentration
– Cells lose water  plasmolyzed
Initial flaccid cell:
yP = 0
yS = 0.7
0.4 M sucrose solution:
yP = 0
yS = 0.9
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
y = 0.9 MPa
yP = 0
yS = 0.9
Figure 36.6a
y = 0.9 MPa
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y = 0.7 MPa
• If Flaccid cell solution w/ low conc
– Cell gains water  turgid
Initial flaccid cell:
yP = 0
yS = 0.7
y = 0.7 MPa
Distilled water:
yP = 0
yS = 0
y = 0 MPa
Turgid cell
at osmotic equilibrium
with its surroundings
yP = 0.7
yS = 0.7
y = 0 MPa
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Turgor loss (wilting)
– Reversed when the plant is watered
Figure 36.7
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Aquaporin Proteins
– Transport proteins in the cell membrane that
allow the passage of water
– Do not affect y
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• Plasma membrane
– Controls traffic of molecules into and out of the
protoplast
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• Vacuolar membrane
– Regulates transport between the cytosol and the
vacuole
Cell wall
Transport proteins in
the plasma membrane
regulate traffic of
molecules between
the cytosol and the
cell wall.
Cytosol
Vacuole
Plasmodesma
(a)
Figure 36.8a
Transport proteins in
the vacuolar
membrane regulate
traffic of molecules
between the cytosol
and the vacuole.
Vacuolar membrane
(tonoplast)
Plasma membrane
Cell compartments. The cell wall, cytosol, and vacuole are the three main
compartments of most mature plant cells.
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• Cell walls and cytosol continuous from cell to cell
• Cytoplasmic continuum
 symplast
• Extracellular continuum
 apoplast
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Key
Symplast
Apoplast
Transmembrane route
Apoplast
The symplast is the
continuum of
cytosol connected
by plasmodesmata.
Symplast
Symplastic route
The apoplast is
the continuum
of cell walls and
extracellular
spaces.
Apoplastic route
(b) 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.
Figure 36.8b
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Bulk Flow in Long-Distance Transport
• Movement of fluid in the xylem and phloem is
driven by pressure differences
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• Water and mineral salts f/ soil epidermis of
roots shoot system
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Lateral transport
Casparian strip
Endodermal cell
Pathway along
apoplast
Pathway
through
symplast
1 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.
Casparian strip
2 Minerals and water that cross
the plasma membranes of root
hairs enter the symplast.
1
Plasma
membrane
Apoplastic
route
Vessels
(xylem)
2
3 As soil solution moves along
the apoplast, some water and
minerals are transported into
the protoplasts of cells of the
epidermis and cortex and then
move inward via the symplast.
Symplastic
route
Root
hair
4 Within the transverse and radial walls of each endodermal cell is the
Figure 36.9
Casparian strip, a belt of waxy material (purple band) that blocks the
passage 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.
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Epidermis
Cortex Endodermis Vascular cylinder
5 Endodermal cells and also parenchyma cells within the
vascular cylinder discharge water and minerals into their
walls (apoplast). The xylem vessels transport the water
and minerals upward into the shoot system.
Root hairs account for much of the surface area of roots
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Mycorrhizae
• Mutually beneficial relationships (symbiosis) with fungal
hyphae, facilitate the absorption of water and minerals
2.5 mm
Figure 36.10
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The Endodermis: A Selective Sentry
• Innermost layer of cells in the root cortex
– Surrounds the vascular cylinder and functions
as the last checkpoint for the selective
passage of minerals from the cortex into the
vascular tissue
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• Waxy Casparian strip of the endodermal wall
blocks apoplastic transfer
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• Water and minerals ascend from roots to
shoots through the xylem
• Plants lose an enormous amount of water
through transpiration, loss of water f/ leaves
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•
Xylem sap
– Rises to heights of more than 100 m
in the tallest plants
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Pushing Xylem Sap: Root Pressure
• At night, when transpiration is very low
– 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
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• Root pressure results in guttation, (exudation)
of water f/ tips of leaf
Figure 36.11
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Pulling Xylem Sap: The Transpiration-CohesionTension Mechanism
• Water is pulled upward by negative pressure in
the xylem
• Based on cohesion of water molecules which is
based on hydrogen bonding
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Transpirational Pull
• Water vapor in the airspaces of a leaf
– Diffuses down its water potential gradient and
exits the leaf via stomata
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• Transpiration produces negative pressure
(tension) in the leaf
– Pulls on water in the xylem, pulling water into
the leaf
3 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.
Y = –0.15 MPa Y = –10.00 MPa
Cell wall
Airspace
Cuticle
Upper
epidermis
Air-water
interface
Low rate of High rate of
transpiration transpiration
Cytoplasm
Evaporation
Mesophyll
Airspace
Lower
epidermis
Cuticle
Figure 36.12
Airspace
Cell wall
Evaporation
Water film
Vacuole
CO2 O2 Xylem CO2 O2 Stoma
Water vapor
Water vapor
1 In transpiration, water vapor (shown as
blue dots) diffuses from the moist air spaces of the
leaf to the drier air outside via stomata.
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2 At first, the water vapor lost by
transpiration is replaced by
evaporation from the water film
that coats mesophyll cells.
Cohesion and Adhesion in the Ascent of Xylem Sap
• Transpirational pull on xylem sap
– Transmitted f/ leaves to roots and even into the
soil solution
– Facilitated by cohesion and adhesion
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• Ascent of xylem sap
Xylem
sap
Outside air Y
= –100.0 MPa
Leaf Y (air spaces)
= –7.0 MPa
Transpiration
Leaf Y (cell walls)
= –1.0 MPa
Atmosphere
Xylem
cells
Water potential gradient
Trunk xylem Y
= – 0.8 MPa
Mesophyll
cells
Stoma
Water
molecule
Cohesion
and adhesion
in the xylem
Cohesion,
by
hydrogen
bonding
Root
hair
Soil Y
= – 0.3 MPa
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Cell
wall
Water
molecule
Root xylem Y
= – 0.6 MPa
Figure 36.13
Adhesion
Soil
particle
Water uptake
from soil
Water
• Stomata help regulate the rate of transpiration
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Stomata regulate transpiration rate, also:
• Photosynthetic rate
• Water loss rate
20 µm
Figure 36.14
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Stomata
• About 90% of the water a plant loses through
stomata
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• Stomata flanked by guard cells
– Control the diameter of the stoma by changing
shape
(a) 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 increase
in length more than width when turgor increases.
The two guard cells are attached at their tips, so the
increase in length causes buckling.
Cells turgid/Stoma open Cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Figure 36.15a
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• Chgs. in turgor pressure open and close
stomata
– From uptake and loss of K+ by the guard cells
(b) Role of potassium in stomatal opening and closing.
The transport of K+ (potassium ions, symbolized
here as red dots) across the plasma membrane and
vacuolar membrane causes the turgor changes of
guard cells.
H2O
K+
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Figure 36.15b
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H2O
H2O
Xerophytes
• Plants adapted to arid climates
• Leaf modifications
•
 reduce transpiration
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• Stomata of xerophytes concentrated on the
lower leaf surface, often located in depressions
Cuticle
Figure 36.16
Lower epidermal
tissue
Upper epidermal tissue
Trichomes
(“hairs”)
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Stomata
100 m
Translocation
• Transport of organic nutrients (e.g. sugar) in
the plant
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Sugar Movement
• Phloem sap
– Aqueous sucrose soln.
– Travels from a sugar source to a sugar sink
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• Sugar source
– Sugar producer, e.g. leaves
• Sugar sink
– Consumer or storer of sugar, e.g.tuber or bulb
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• Sugar loaded into sieve-tube members  
Mesophyll cell
Companion
(transfer) cell
Cell walls (apoplast)
Plasma membrane
Plasmodesmata
(a)
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.
Figure 36.17a
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Mesophyll cell
Bundlesheath cell
Phloem
parenchyma cell
Sieve-tube
member
• Phloem loading requires active transport
• Proton pumping and cotransport of sucrose
and H+
High H+ concentration
Cotransporter
H+
Proton
pump
S
(b)
A chemiosmotic mechanism is responsible for
the 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.
Figure 36.17b
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Key
ATP
H+
Sucrose
H+
S
Low H+ concentration
Apoplast
Symplast
Pressure Flow
• Sap moves through a sieve tube by bulk flow
driven by positive pressure
Vessel
Sieve tube
(xylem
phloem)
)
H2O
Source cell
1
(leaf)
dots) into the sieve tube
at the source reduces
water potential inside the
sieve-tube members. This
causes the tube to take
up water by osmosis.
Sucrose
1
H2O
Pressure flow
Transpiration stream
2
Sink cell
(storage
root)
4
3
H2O
Figure 36.18
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Sucrose
Loading of sugar (green
2
This uptake of water
generates a positive
pressure that forces
the sap to flow along
the tube.
3
The pressure is relieved
by the unloading of sugar
and the consequent loss
of water from the tube
at the sink.
4
In the case of leaf-to-root
translocation, xylem
recycles water from sink
to source.
• Pressure flow explains why phloem sap always
flows from source to sink
EXPERIMENT
To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermiclike 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.
25 m
Sievetube
member
Sap
droplet
Stylet
Aphid feeding
RESULTS
Figure 36.19
CONCLUSION
SieveTube
member
Sap droplet
Stylet in sieve-tube Severed stylet
member
exuding sap
The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.
The results of such experiments support the pressure flow hypothesis.
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