Transcript Water potential

Chapter 25: Transport of Water and
Nutrients in Plants
AP Biology
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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Figure 36.2-3
CO2
H2O
O2
Light
Sugar
O2
H2O
and
minerals
CO2
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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• Adaptations in each species represent
compromises between enhancing
photosynthesis and minimizing water loss
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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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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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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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• 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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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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• 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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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:
Positive
pressure
In this example, the effect
of adding solutes is
offset by positive
pressure, resulting in no
net movement of water:
Positive
pressure
Applying negative
pressure to the right arm
makes  lower there,
resulting in net movement
of water to the right arm:
Negative
pressure
Pure
water
Membrane
H2O
Solutes
Solutes
H2O
H2O
H2O
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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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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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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Short-Distance Transport of Solutes Across
Plasma Membranes
• Plasma membrane permeability controls shortdistance 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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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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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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Figure 36.6
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Plasma membrane
Apoplast
Symplast
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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Figure 36.10
Casparian strip
Pathway along Endodermal
cell
apoplast
Pathway
through
symplast
Plasma
membrane
Casparian strip
Apoplastic
route
Symplastic
route
Vessels
(xylem)
Root
hair
Epidermis
Endodermis
Cortex
Vascular cylinder
(stele)
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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Animation: Transport in Roots
Right-click slide / select “Play”
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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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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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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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Figure 36.13
Xylem sap
Outside air 
 100.0 MPa
Mesophyll cells
Stoma
Leaf  (air spaces)
 7.0 MPa
Trunk xylem 
 0.8 MPa
Water potential gradient
Leaf  (cell walls)
 1.0 MPa
Water molecule
Transpiration Atmosphere
Xylem
cells
Adhesion by
hydrogen bonding
Cell wall
Cohesion and
adhesion in
the xylem
Cohesion by
hydrogen bonding
Water molecule
Root hair
Trunk xylem 
 0.6 MPa
Soil 
 0.3 MPa
Soil particle
Water uptake
from soil
Water
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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Animation: Water Transport
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Animation: Transpiration
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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
• 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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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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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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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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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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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
K
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
(b) Role of potassium in stomatal opening and closing
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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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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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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Figure 36.16
Ocotillo
(leafless)
Oleander leaf cross section
Cuticle
Upper epidermal tissue
100 m
Ocotillo after
heavy rain
Oleander
flowers
Trichomes Crypt Stoma
(“hairs”)
Ocotillo leaves
Old man cactus
Lower epidermal
tissue
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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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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• 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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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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The Pressure Flow Model
Figure 36.18
Sieve Source cell
tube (leaf)
(phloem)
Vessel
(xylem)
H2O
1
1 Loading of sugar
Sucrose
H2O
Bulk flow by negative pressure
Bulk flow by positive pressure
2
2 Uptake of water
3 Unloading of sugar
Sink cell
(storage
root)
4 Water recycled
3
4
H2O
Sucrose
Figure 36.17
Key
Apoplast
Symplast
Mesophyll cell
Companion
(transfer) cell
Cell walls (apoplast)
Plasma membrane
High H concentration
H
Proton
pump
Sieve-tube
element
Cotransporter
S
Plasmodesmata
ATP
BundleMesophyll cell sheath cell
(a)
Phloem
parenchyma cell
H
Low H concentration
(b)
H
Sucrose
S
Animation: Translocation of Phloem Sap in Summer
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Animation: Translocation of Phloem Sap in Spring
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