3 - water movement through a plant
TRANSCRIPT
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
WATER RELATIONS
of the
WHOLE PLANT
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Transport in Vascular Plants
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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 at the level of the whole plant.
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Minerals
H2O
H2O
CO2 O2
Sugar
Light
CO2
O2
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Solute Transport
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Selective Permeability of Membranes: A Review
• The selective permeability of the plasma
membrane controls movement of solutes into and
out of the cell.
• Specific transport proteins enable plant cells to
maintain an internal environment different from
their surroundings.
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The Central Role of Proton Pumpsin Solute Transport
• Proton pumps in plant cells create a hydrogen ion
gradient that is a form of potential energy that can
be harnessed to do work.
• They contribute to a voltage known as a
membrane potential.
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CYTOPLASM
ATP
EXTRACELLULAR FLUID
Proton pump
generates mem-
brane potential
and 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.
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CYTOPLASM EXTRACELLULAR FLUID
Cations ( , for
example) are
driven into the cell
by the membrane
potential.
Transport protein
Membrane potential and cation uptake
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• In the mechanism called cotransport, a transport
protein couples the passage of one solute to the
passage of another.
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Cell accumulates
anions ( ,
for example) by
coupling their
transport to; the
inward diffusion
of through a
cotransporter.
Cotransport of anions
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• The “coattail” effect of cotransport is also
responsible for the uptake of the sugar sucrose by
plant cells
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Plant cells can
also accumulate
a neutral solute,
such as sucrose
( ), by
cotransporting
down the
steep proton
gradient.
Cotransport of a neutral solute
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Water Transport
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The Central Role of Water Potentialin Water Movement
• To survive, plants must balance water uptake and
loss.
• Osmosis determines the net uptake or water loss
by a cell.
-- affected by solute concentration and pressure.
-- regulated by the semi-permeable membrane.
-- governed by water potential gradient.
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• 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
(i.e., less negative) to regions of lower water
potential (i.e., more negative).
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How Solutes and Pressure Affect Water Potential
• Both pressure and solute concentration affect
water potential.
• The solute potential of a solution is proportional to
the number of dissolved molecules.
• 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.
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Addition ofsolutes
0.1 Msolution
Purewater
H2O
= 0 MPa = –0.23 MPa
P = 0
S = –0.23
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• Physical pressure increases water potential.
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H2O
= 0 MPa = 0 MPa
P = 0.23S = –0.23
Applyingphysicalpressure
H2O
= 0 MPaS = –0.23
P = 0.30
= 0.07 MPa
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• Negative pressure decreases water potential.
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Negativepressure
H2O
= –0.23 MPa
P = 0
S = –0.23
= –0.30 MPa
P = –0.30
S = 0
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• 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 become plasmolyzed.
• (The protoplasts of flaccid cells are in contact with their walls but lack turgor pressure.)
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P = –0.9 MPa
P = 0
S = –0.9
Water Relations in Plant Cells
P = –0.9 MPa
P = 0
S = –0.9
initial conditions: cellular > environmental
Plasmolyzed cellat osmoticequilibrium with its surroundings
0.4 M sucrose solution:P = –0.7 MPa
P = 0
S = –0.7
Initial flaccid cell:
P = 0 MPa
P = 0
S = 0
Distilled water:
P = 0 MPa
P = 0.7
S = –0.7
Turgid cellat osmoticequilibriumwith its surroundings
initial conditions: cellular < environmental
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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.
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• Turgor loss in plants causes wilting, which can be
reversed when the plant is watered.
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Aquaporin Proteins and Water Transport
• Aquaporins are transport proteins in the cell
membrane that allow the passage of water.
• Aquaporins do not affect water potential, but
rather the rate at which water diffuses down its
water potential gradient.
- regulated by phosphorylation of these proteins induced
by changes in second messengers (e.g., Ca 2+).
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Three Major Compartments of Vacuolated Plant Cells
• Transport is also regulated by the compartmental
structure of plant cells.
• The plasma membrane directly controls the traffic
of molecules into and out of the protoplast
• The plasma membrane is a barrier between two
major compartments, the cell wall and the
cytosol.
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• The third major compartment in most mature plant
cells is the vacuole, a large organelle that
occupies as much as 90% or more of the
protoplast’s volume.
• The vacuolar membrane regulates transport
between the cytosol and the vacuole.
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• In most plant tissues, the cell walls and cytosol
are continuous from cell to cell.
• The cytoplasmic continuum is called the symplast.
• The apoplast is the continuum of cell walls and extracellular spaces.
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Cell compartments
Plasmodesma
Plasma membrane
Cell wall
Cytosol
Vacuole
Key
Symplast
Apoplast
Vacuolar membrane(tonoplast)
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Functions of the Symplast and Apoplast in Transport
• Water and minerals can travel through a plant by
three routes:
– Transmembrane route: out of one cell, across
a cell wall, and into another cell.
– Symplastic route: via the continuum of
cytosol
– Apoplastic route: via the the cell walls and
extracellular spaces
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Transmembrane route
Key
Symplast
Apoplast
Symplastic route
Transport routes between cells
Apoplastic route
Apoplast
Symplast
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Bulk Flow in Long-Distance Transport
• In bulk flow, movement of fluid in the xylem and
phloem is driven by pressure differences at
opposite ends of the xylem vessels and sieve
tubes.
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Absorption of Water by
Root Systems
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Roots absorb water and minerals from the soil
• Water and mineral salts from the soil enter the
plant through the epidermis of roots and ultimately
flow to the shoot system.
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Casparian strip
Endodermal cellPathway alongapoplast
Pathway throughsymplast
Casparian strip
Plasmamembrane
Apoplasticroute
Symplasticroute
Roothair
Vessels(xylem)
Cortex
EndodermisEpidermis Vascular cylinder
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The Roles of Root Hairs, Mycorrhizae, and Cortical Cells
• Much of the absorption of water and minerals
occurs near root tips, where the epidermis is
permeable to water and 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
with fungi, which facilitate absorption of water and
minerals from the soil.
• Roots and fungi form mycorrhizae, symbiotic
structures consisting of plant roots united with
fungal hyphae.
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2.5 mmMycorrhizae
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• After soil solution enters the roots, the extensive
surface area of cortical cell membranes
enhances uptake of water and selected minerals.
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The Endodermis: A Selective Sentry
• 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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• 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
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WATER TRANSLOCATION:Transpiration and Ascent
of the Xylem Sap
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Ascent of Xylem Sap
• Water and minerals ascend from roots 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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Bulk Flow in Long-Distance Transport
• movement of fluid in the xylem and phloem is driven
by pressure differences at opposite ends of the xylem
vessels and sieve tubes.
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Factors Affecting the Ascent of Xylem Sap
• Xylem sap rises to heights of more than 100 m in
the tallest plants.
• Is the sap pushed upward from the roots, or is it
pulled upward by the leaves?
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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 sometimes results in guttation, the
exudation of water droplets on tips of grass blades or
the leaf margins of some small, herbaceous eudicots.
Guttation
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Pulling Xylem Sap: The Transpiration-Cohesion Tension Mechanism
• Water is pulled upward by negative pressure in
the xylem.
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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.
• Transpiration produces negative pressure
(tension) in the leaf, which exerts a pulling force
on water in the xylem, pulling water into the leaf.
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Upperepidermis
MesophyllAir
space
Cuticle
Lowerepidermis
Cuticle CO2 O2 CO2
XylemO2
Stoma
Evaporation
Evaporation
Water film
Airspace
Cytoplasm
Cell wall
Vacuole
Air-waterinterface
High rate of transpiration
Low rate of transpiration
Y = –10.00 MPaY = –0.15 MPa
Cell wall
Airspace
Water Movement out of the Leaf
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Water Movement thru Leaf Xylem: Governed by Water Potential
• Water flows from the xylem of the leaves, where water
potential is least negative, toward air, where water
potential is most negative.
Y gradients :
tracheids > sheath
sheath > apoplast
sheath > spongy mesophyll
spongy mesophyll > apoplast
apoplast > stoma
(outside air -10 MPa)
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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 to the root
tips and even into the soil solution.
• Transpirational pull is facilitated by cohesion and
adhesion.
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Ascent of Xylem Sap by Bulk Flow: A Take-Home Lesson
• The movement of xylem sap against gravity is
maintained by the transpiration-cohesion-
tension mechanism.
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Xylemsap
Mesophyllcells
Stoma
Watermolecule
AtmosphereTranspiration
Xylemcells
Adhesion Cellwall
Cohesion,byhydrogenbonding
Cohesion andadhesion inthe xylem
Watermolecule
Wate
r p
ote
nti
al g
rad
ien
t
Roothair
Soilparticle
WaterWater uptakefrom soil
Trunk xylem = –0.8 Mpa
Root xylem = –0.6 MPa
Leaf (air spaces)= –7.0 MPa
Outside air = –100.0 MPa
Leaf (cell walls) = –1.0 MPa
Soil= –0.3 MPa
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Ascent of Xylem Sap: Summary
• H-bonding forms an unbroken chain of water molecules
extending from leaves all the way to the soil.
• Y gradient: the force that drives the ascent of xylem sap.
• For the bulk flow over long distance, the Y gradient is due
mainly to pressure potential gradient (Yp).
• Transpiration results in the Yp at the leaf end of the xylem
being lower than the Yp at the root end.
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Stomata
20 µm
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Stomata: Major Pathways for Water Loss
• About 90% 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.
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Stomata help regulate the rate of transpiration
• Leaves generally have broad surface areas and
high surface-to-volume ratios.
• These characteristics increase photosynthesis and
increase water loss through stomata.
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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 absorption
through the roots, the plant will lose water and wilt.
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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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Cells turgid/Stoma open
Changes in guard cell shape and stomatal opening and closing(surface view)
Radially orientedcellulose microfibrils
Vacuole
Cell
wall
Guard cell
Cells flaccid/Stoma closed
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Changes in turgor pressure that open and close
stomata result primarily from the reversible uptake
and loss of potassium ions by the guard cells.
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Cells turgid/Stoma open
Role of potassium in stomatal opening and closing
Cells flaccid/Stoma closed
H2O
H2O
H2OH2O
H2O H2O
H2O
H2O
H2O
H2O
K+
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Xerophyte Adaptations That Reduce Transpiration
• Xerophytes are plants adapted to arid climates.
• They have leaf modifications that reduce the rate
of transpiration.
• Their stomata are concentrated on the lower leaf
surface, often in depressions that provide shelter
from dry wind.
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Cuticle Upper epidermal tissue
Lower epidermaltissue
Trichomes(“hairs”)
Stomata100 µm