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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 36
Resource Acquisition and
Transport in Vascular Plants
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
Concept 36.1: 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 transport
© 2011 Pearson Education, Inc.
• The 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
© 2011 Pearson Education, Inc.
Figure 36.2-3
CO2
H2O
O2
Light
Sugar
O2
H2O
and
minerals
CO2
• Adaptations in each species represent
compromises between enhancing photosynthesis
and minimizing water loss
© 2011 Pearson Education, Inc.
Shoot Architecture and Light Capture
• Stems serve as conduits for water and nutrients
and as supporting structures for leaves
• There is generally a positive correlation between
water availability and leaf size
• Phyllotaxy, the arrangement of leaves on a stem,
is specific to each species
• Leaf orientation affects light absorption
• In low-light conditions, horizontal leaves capture
more sunlight
• In sunny conditions, vertical leaves are less
damaged by sun and allow light to reach lower
leaves
© 2011 Pearson Education, Inc.
Figure 36.4
Ground area
covered by plant
Plant A
Leaf area  40%
of ground area
(leaf area index  0.4)
Plant B
Leaf area  80%
of ground area
(leaf area index  0.8)
• Shoot height and branching pattern also affect
light capture
• There is a trade-off between growing tall and
branching
© 2011 Pearson Education, Inc.
Root Architecture and Acquisition of
Water and Minerals
• Soil is a resource mined by the root system
• Taproot systems anchor plants and are
characteristic of gymnosperms and eudicots
• Root growth can adjust to local conditions
– For example, roots branch more in a pocket of
high nitrate than low nitrate
• Roots are less competitive with other roots from
the same plant than with roots from different plants
© 2011 Pearson Education, Inc.
• Roots and the hyphae of soil fungi form mutualistic
associations called mycorrhizae
• Mutualisms with fungi helped plants colonize land
• Mycorrhizal fungi increase the surface area for
absorbing water and minerals, especially
phosphate
© 2011 Pearson Education, Inc.
Figure 36.5
Roots
Fungus
Concept 36.2: Different mechanisms
transport substances over short or long
distances
• There are two major pathways through plants
– The apoplast
– The symplast
© 2011 Pearson Education, Inc.
The Apoplast and Symplast: Transport
Continuums
• 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
© 2011 Pearson Education, Inc.
• Three transport routes for water and solutes are
– The apoplastic route, through cell walls and
extracellular spaces
– The symplastic route, through the cytosol
– The transmembrane route, across cell walls
© 2011 Pearson Education, Inc.
Figure 36.6
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Plasma membrane
Apoplast
Symplast
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
• In animals, membrane potential is established
through pumping Na by sodium-potassium pumps
© 2011 Pearson Education, Inc.
Figure 36.7
CYTOPLASM



ATP
EXTRACELLULAR FLUID
+
H+ Hydrogen ion
+
+ H+
H+
H+
H+
H+

Proton pump 

H+ 
H+

+
H+
+ +
H
+
+ H+
H+
H+
H+



H+
+ H+
+ Sucrose
+ (neutral solute)
K+
K+



+
+ H+
H+
+
+
+
+
Nitrate
H+
H+
Potassium ion
K+
K+
K+

Ion channel 
(c) H+ and cotransport of ions
+
H+
+
+ H+
H+
H+


H+NO3
cotransporter 
H+
(b) H+ and cotransport of neutral
solutes
H+
H+


H+/sucrose

cotransporter
K+
H+
+
H+
H+
(a) H+ and membrane potential
K+
+
+
(d) Ion channels
Figure 36.7a
CYTOPLASM
ATP

EXTRACELLULAR FLUID
+

+

+
H+
H+
Hydrogen ion
H+
H+
H+
H+

Proton pump 
+
+
(a) H+ and membrane potential
H+
H+
• Plant cells use the energy of H gradients to
cotransport other solutes by active transport
© 2011 Pearson Education, Inc.
Figure 36.7b
H+
H+

+

+

+
H+
H+
H+
H+
H+
H+


H+/sucrose
cotransporter 
H+
H+
+ H+
+
Sucrose
+
(neutral solute)
(b) H+ and cotransport of neutral solutes
Figure 36.7c
H+
H+

+

+

+ H+
H+
H+
H+
H+

+

+ H+
H+NO3
cotransporter 
+
(c) H+ and cotransport of ions
H+
Nitrate
H+
H+
• Plant cell membranes have ion channels that allow
only certain ions to pass
© 2011 Pearson Education, Inc.
Figure 36.7d
K+
K+
K+

+

+

+
Potassium ion
K+
K+
K+
Ion channel
(d) Ion channels
K+

+

+
Short-Distance 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
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
• Consider a U-shaped tube where the two arms are
separated by a membrane permeable only to
water
• Water moves in the direction from higher water
potential to lower water potential
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Figure 36.9
Initial flaccid cell:
0.4 M sucrose solution:
Plasmolyzed
cell at osmotic
equilibrium with
its surroundings
P  0
S  0.9
  0.9 MPa
P  0
S  0.9
  0.9 MPa
(a) Initial conditions: cellular   environmental 
P  0
S  0.7
  0.7 MPa
Pure water:
P  0
S  0
  0 MPa
Turgid cell
at osmotic
equilibrium with
its surroundings
P  0.7
S  0.7
  0 MPa
(b) Initial conditions: cellular   environmental 
• If a flaccid cell is placed in a solution with a lower
solute concentration, the cell will gain water and
become turgid
• Turgor loss in plants causes wilting, which can be
reversed when the plant is watered
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Long-Distance Transport: The Role of Bulk
Flow
• 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
© 2011 Pearson Education, Inc.
Concept 36.3: Transpiration drives the
transport of water and minerals from
roots to shoots via the xylem
• Plants can move a large volume of water from
their roots to shoots
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Animation: Transport in Roots
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
• The concentration of essential minerals is greater
in the roots than soil because of active transport
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
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)
• The endodermis regulates and transports needed
minerals from the soil into the xylem
• Water and minerals move from the protoplasts of
endodermal cells into their cell walls
• Diffusion and active transport are involved in this
movement from symplast to apoplast
• Water and minerals now enter the tracheids and
vessel elements
© 2011 Pearson Education, Inc.
Bulk Flow Transport via the Xylem
• Xylem sap, water and dissolved minerals, is
transported from roots to leaves by bulk flow
• The transport of xylem sap involves transpiration,
the evaporation of water from a plant’s surface
• Transpired water is replaced as water travels up
from the roots
• Is sap pushed up from the roots, or pulled up by
the leaves?
© 2011 Pearson Education, Inc.
Pushing Xylem Sap: Root Pressure
• At night 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
• Root pressure sometimes results in guttation, the
exudation of water droplets on tips or edges of
leaves
© 2011 Pearson Education, Inc.
Figure 36.11
• Positive root pressure is relatively weak and is a
minor mechanism of xylem bulk flow
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• This negative pressure pulls water in the xylem
into the leaf
• The transpirational pull on xylem sap is
transmitted from leaves to roots
© 2011 Pearson Education, Inc.
Figure 36.12
Cuticle
Xylem
Upper
epidermis
Mesophyll
Air
space
Microfibrils in
cell wall of
mesophyll cell
Lower
epidermis
Cuticle
Stoma
Microfibril
Water Air-water
(cross section) film interface
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
Figure 36.13a
Water molecule
Root hair
Soil particle
Water uptake
from soil
Water
Figure 36.13b
Xylem
cells
Adhesion by
hydrogen bonding
Cell wall
Cohesion and
adhesion in
the xylem
Cohesion by
hydrogen bonding
Figure 36.13c
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere
Transpiration
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
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Animation: Water Transport
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Animation: Transpiration
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
• Water molecules are attracted to each other
through cohesion
• Cohesion makes it possible to pull a column of
xylem sap
• Thick secondary walls prevent vessel elements
and tracheids from collapsing under negative
pressure
• Drought stress or freezing can cause cavitation,
the formation of a water vapor pocket by a break
in the chain of water molecules
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
Concept 36.4: 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
© 2011 Pearson Education, Inc.
Figure 36.14
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
Figure 36.15a
Guard cells turgid/
Stoma open
Radially oriented
cellulose microfibrils
Guard cells flaccid/
Stoma closed
Cell
wall
Vacuole
Guard cell
(a) Changes in guard cell shape and stomatal opening
and closing (surface view)
• This results primarily from the reversible uptake
and loss of potassium ions (K) by the guard cells
© 2011 Pearson Education, Inc.
Figure 36.15b
Guard cells turgid/
Stoma open
H 2O
K
Guard cells flaccid/
Stoma closed
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H2O
H 2O
H 2O
(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
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• 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
© 2011 Pearson Education, Inc.
Adaptations That Reduce Evaporative
Water Loss
• Xerophytes are plants adapted to arid climates
© 2011 Pearson Education, Inc.
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
• 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
© 2011 Pearson Education, Inc.
Concept 36.5: Sugars are transported from
sources to sinks via the phloem
• The products of photosynthesis are transported
through phloem by the process of translocation
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• 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 exposed 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
© 2011 Pearson Education, Inc.
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
• 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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Animation: Translocation of Phloem Sap in Summer
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Animation: Translocation of Phloem Sap in Spring
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
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
• The pressure flow hypothesis explains why
phloem sap always flows from source to sink
• Experiments have built a strong case for pressure
flow as the mechanism of translocation in
angiosperms
• Self-thinning is the dropping of sugar sinks such
as flowers, seeds, or fruits
© 2011 Pearson Education, Inc.
Figure 36.19
EXPERIMENT
25 m
Sievetube
element
Sap
droplet
Aphid feeding
Stylet
Sap droplet
Stylet in sieve-tube Separated stylet
exuding sap
element
Concept 36.6: The symplast is highly
dynamic
• The symplast is a living tissue and is responsible
for dynamic changes in plant transport processes
© 2011 Pearson Education, Inc.
Changes in Plasmodesmata
• Plasmodesmata can change in permeability in
response to turgor pressure, cytoplasmic calcium
levels, or cytoplasmic pH
• Plant viruses can cause plasmodesmata to dilate
so viral RNA can pass between cells
© 2011 Pearson Education, Inc.
Figure 36.20
Plasmodesma
Virus
particles
Cell wall
100 nm
Phloem: An Information Superhighway
• Phloem is a “superhighway” for systemic transport
of macromolecules and viruses
• Systemic communication helps integrate
functions of the whole plant
© 2011 Pearson Education, Inc.
Electrical Signaling in the Phloem
• The phloem allows for rapid electrical
communication between widely separated organs
– For example, rapid leaf movements in the
sensitive plant (Mimosa pudica)
© 2011 Pearson Education, Inc.