Download (a) H

CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman •
Minorsky • Jackson
36
Transport in
Vascular
Plants
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
A Whole Lot of Shaking Going On
 Plants have various adaptations that aid in the
acquisition of resources, including water, minerals,
carbon dioxide, and light
 For example, Aspen leaves have a peculiar
adaption that causes their leaves to tremble even
in light wind
© 2014 Pearson Education, Inc.
Figure 36.1
© 2014 Pearson Education, Inc.
Figure 36.1a
© 2014 Pearson Education, Inc.
 The success of plants depends on their ability to
gather resources from their environment and
transport them to where they are needed
© 2014 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
© 2014 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
© 2014 Pearson Education, Inc.
Figure 36.2-1
H2O
H2O and
minerals
© 2014 Pearson Education, Inc.
Figure 36.2-2
CO2
O2
H2O
H2O and
minerals
© 2014 Pearson Education, Inc.
O2
CO2
Figure 36.2-3
CO2
O2
Sugar
Light
H2O
H2O and
minerals
© 2014 Pearson Education, Inc.
O2
CO2
 Adaptations in each species represent
compromises between enhancing photosynthesis
and minimizing water loss
© 2014 Pearson Education, Inc.
Shoot Architecture and Light Capture
 Stems serve as conduits for water and nutrients
and as supporting structures for leaves
 Shoot length and branching pattern affect light
capture
 There is a trade-off between growing tall and
branching
© 2014 Pearson Education, Inc.
 There is generally a positive correlation between
water availability and leaf size
 Phyllotaxy, the arrangement of leaves on a stem,
is a species-specific trait important for light capture
 Most angiosperms have alternate phyllotaxy with
leaves arranged in a spiral
 The angle between leaves is 137.5 and likely
minimizes shading of lower leaves
© 2014 Pearson Education, Inc.
Figure 36.3
42
29
16
34
21
32
24
11
19
27
3
8
6
14
13
26
Shoot
1
apical
9
meristem
5
18
Buds
31
10
23
4
2
15
28
7
20
1 mm
© 2014 Pearson Education, Inc.
40
12
25
17
22
Figure 36.3a
© 2014 Pearson Education, Inc.
Figure 36.3b
42
29
16
34
21
32
24
11
19
27
3
8
6
14
13
26
Shoot
1
apical
9
meristem
5
18
31
10
23
4
2
15
28
7
20
1 mm
© 2014 Pearson Education, Inc.
40
12
25
17
22
 The depth of the canopy, the leafy portion of all
the plants in a community, affects the productivity
of each plant
 Self-pruning, the shedding of lower shaded leaves,
occurs when they respire more than
photosynthesize
© 2014 Pearson Education, Inc.
 Light absorption is affected by the leaf area index,
the ratio of total upper leaf surface of a plant
divided by the surface area of land on which it
grows
© 2014 Pearson Education, Inc.
Figure 36.4
Ground area
covered by plant
Plant A
Leaf area = 40%
of ground area
(leaf area index = 0.4)
© 2014 Pearson Education, Inc.
Plant B
Leaf area = 80%
of ground area
(leaf area index = 0.8)
 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
© 2014 Pearson Education, Inc.
Root Architecture and Acquisition of Water and
Minerals
 Soil contains resources mined by the root system
 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
© 2014 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
© 2014 Pearson Education, Inc.
Concept 36.2: Different mechanisms transport
substances over short or long distances
 There are two major pathways through plants
 The apoplast
 The symplast
© 2014 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 all the
living cells in a plant, as well as the
plasmodesmata
© 2014 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
© 2014 Pearson Education, Inc.
Figure 36.5
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Plasma membrane
Apoplast
Symplast
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 36.6
CYTOPLASM
−
ATP
−
+
−
+
H
H+
H+
H+
Proton pump
−
+
−
+
H+
Hydrogen
ion
+
H
H+
−
−
H+
H+
−
+
H+
+
+
H+
H+
H+
S
−
H+
H+
H+
H+
−
H+/sucrose
cotransporter −
+
−
+ H+
H+
+
(c) H+ and cotransport of ions
K+
H+
−
© 2014 Pearson Education, Inc.
S
−
H+
H+
H+/NO3−
cotransporter −
H+
+
H+
+ H+
+ Sucrose
+
(neutral solute)
(b) H+ and cotransport of neutral solutes
+
+
H+ −
S
H+
(a) H+ and membrane potential
H+
−
EXTRACELLULAR
FLUID
+
+
H+
K+
Nitrate
K+
H+
K+
−
−
+
−
+
+
K+
Ion channel
(d) Ion channels
−
+
−
+
Potassium ion
K+
K+
Figure 36.6a
CYTOPLASM
ATP
−
−
−
EXTRACELLULAR
+
FLUID
+
+
+
H
H+
H+
H+
Proton pump
−
+
−
+
(a) H+ and membrane potential
© 2014 Pearson Education, Inc.
H+
Hydrogen
ion
H+
H+
H+
Figure 36.6b
H+
S
H+
S
−
+
−
−
+
+
H+
H+
H+
H+
S
−
−
H+/sucrose
cotransporter −
H+
H+
H+
H+
+ H+
+ Sucrose
+ (neutral solute)
(b) H+ and cotransport of neutral solutes
© 2014 Pearson Education, Inc.
Figure 36.6c
H+
H+
H+
H+/NO3−
−
−
+
−
+ H+
+
H+
H+
−
+
−
+ H+
H+
+
cotransporter −
(c) H+ and cotransport of ions
© 2014 Pearson Education, Inc.
H+
H+
H+
Nitrate
Figure 36.6d
K+
K+
K+
K+
−
+
−
+
−
+
K+
Ion channel
−
+
−
+
(d) Ion channels
© 2014 Pearson Education, Inc.
Potassium ion
K+
K+
 Plant cells use the energy of H gradients to
cotransport other solutes by active transport
 Plant cell membranes have ion channels that allow
only certain ions to pass
© 2014 Pearson Education, Inc.
Short-Distance Transport of Water Across
Plasma Membranes
 To survive, plants must balance water uptake and
loss
 Osmosis is the diffusion of water into or out of a
cell that is affected by solute concentration and
pressure
© 2014 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
© 2014 Pearson Education, Inc.
 Water potential is abbreviated as  and measured
in a unit of pressure called the megapascal (MPa)
   0 MPa for pure water at sea level and at room
temperature
© 2014 Pearson Education, Inc.
How Solutes and Pressure Affect Water
Potential
 Both solute concentration and pressure affect
water potential
 This is expressed by the water potential equation:
  S  P
 The solute potential (S) of a solution is directly
proportional to its molarity
 Solute potential is also called osmotic potential
© 2014 Pearson Education, Inc.
 Pressure potential (P) 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
© 2014 Pearson Education, Inc.
Water Movement Across Plant Cell Membranes
 Water potential affects uptake and loss of water by
plant cells
 If a flaccid (limp) 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
© 2014 Pearson Education, Inc.
Figure 36.7
Initial flaccid cell:
ψP = 0
ψS = −0.7
Environment
0.4 M sucrose solution:
ψP = 0
ψS = −0.9
ψ = −0.7 MPa
ψ = −0.9 MPa
Final plasmolyzed cell at osmotic
equilibrium with its surroundings:
ψP = 0
ψS = −0.9
ψ = −0.9 MPa
(a) Initial conditions: cellular ψ > environmental ψ
Initial flaccid cell:
ψP = 0
ψS = −0.7
ψ = −0.7 MPa
Final turgid cell at osmotic
equilibrium with its surroundings:
ψP = 0
ψS = −0.7
ψ =
0 MPa
(b) Initial conditions: cellular ψ < environmental ψ
© 2014 Pearson Education, Inc.
Environment
Pure water:
ψP = 0
ψS = 0
ψ = 0 MPa
Figure 36.7a
Initial flaccid cell:
ψP = 0
ψS = −0.7
Environment
0.4 M sucrose solution:
ψP = 0
ψS = −0.9
ψ = −0.7 MPa
Final plasmolyzed cell at osmotic
equilibrium with its surroundings:
ψP = 0
ψS = −0.9
ψ = −0.9 MPa
(a) Initial conditions: cellular ψ > environmental ψ
© 2014 Pearson Education, Inc.
ψ = −0.9 MPa
Video: Plasmolysis
© 2014 Pearson Education, Inc.
Figure 36.7b
Initial flaccid cell:
ψP = 0
ψS = −0.7
ψ = −0.7 MPa
Final turgid cell at osmotic
equilibrium with its surroundings:
ψP = 0.7
ψS = −0.7
ψ =
0 MPa
(b) Initial conditions: cellular ψ < environmental ψ
© 2014 Pearson Education, Inc.
Environment
Pure water:
ψP = 0
ψS = 0
ψ = 0 MPa
Video: Turgid Elodea
© 2014 Pearson Education, Inc.
 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
© 2014 Pearson Education, Inc.
Figure 36.UN02
Turgid
© 2014 Pearson Education, Inc.
Figure 36.UN03
Wilted
© 2014 Pearson Education, Inc.
Aquaporins: Facilitating Diffusion of Water
 Aquaporins are transport proteins in the cell
membrane that facilitate the passage of water
 These affect the rate of water movement across
the membrane
© 2014 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
© 2014 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
© 2014 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
© 2014 Pearson Education, Inc.
 The concentration of essential minerals is greater
in the roots than soil because of active transport
© 2014 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
© 2014 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
© 2014 Pearson Education, Inc.
Figure 36.8
Casparian strip
Endodermal cell
Pathway along
apoplast
Water
moves
upward
in vascular
cylinder
4
Pathway
through
symplast
5
Plasmodesmata
1 Apoplastic
Casparian strip
route
1
2 Symplastic
Plasma
membrane
Apoplastic
route
3
route
2
Symplastic
route
4
Root
hair
3 Transmembrane
route
Epidermis
4 The endodermis: controlled entry
to the vascular cylinder (stele)
© 2014 Pearson Education, Inc.
5
Vessels
(xylem)
Endodermis Vascular
cylinder
(stele)
Cortex
5 Transport in the xylem
Figure 36.8a
Casparian strip
1 Apoplastic
route
1
2 Symplastic
route
Plasma
membrane
Apoplastic
route
3
2
Symplastic
route
4
Root
hair
3 Transmembrane
route
Epidermis
4 The endodermis: controlled
entry to the vascular cylinder
(stele)
© 2014 Pearson Education, Inc.
5
Vessels
(xylem)
Endodermis Vascular
cylinder
(stele)
Cortex
5 Transport in the xylem
Figure 36.8b
Casparian strip
Endodermal cell
Pathway along
apoplast
Pathway
through
symplast
4
Plasmodesmata
4 The endodermis: controlled entry
to the vascular cylinder (stele)
5 Transport in the xylem
© 2014 Pearson Education, Inc.
5
Water
moves
upward
in vascular
cylinder
Animation: Transport in Roots
© 2014 Pearson Education, Inc.
 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 own 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
© 2014 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?
© 2014 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, a push of xylem sap
 Root pressure sometimes results in guttation, the
exudation of water droplets on tips or edges of
leaves
© 2014 Pearson Education, Inc.
Figure 36.9
© 2014 Pearson Education, Inc.
 Positive root pressure is relatively weak and is a
minor mechanism of xylem bulk flow
© 2014 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
© 2014 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
© 2014 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
© 2014 Pearson Education, Inc.
Figure 36.10
5 Water from xylem pulled
into cells and air spaces.
Cuticle
4 Increased surface
tension pulls
water from cells
and air spaces.
Xylem
Upper
epidermis
Mesophyll
Air
space
Microfibrils
in cell wall of
mesophyll cell
2 Water vapor
replaced
from water
film.
Lower
epidermis
Cuticle
1 Water vapor diffuses outside
via stomata.
© 2014 Pearson Education, Inc.
3 Air-water
interface
retreats.
Stoma
Microfibril
(cross section)
Water Air-water
film interface
Figure 36.11
Xylem sap
Mesophyll cells
Outside air ψ
= −100.0 MPa
Stoma
Water molecule
Atmosphere
Leaf ψ (air spaces)
= −7.0 MPa
Trunk xylem ψ
= −0.8 MPa
Trunk xylem ψ
= −0.6 MPa
Soil ψ
= −0.3 MPa
© 2014 Pearson Education, Inc.
Water potential gradient
Leaf ψ (cell walls)
= −1.0 MPa
Transpiration
Xylem
cells
Adhesion by
hydrogen bonding
Cell
wall
Cohesion
by hydrogen
bonding
Cohesion and
adhesion in
the xylem
Water molecule
Root hair
Soil particle
Water
Water uptake from soil
Figure 36.11a
Water molecule
Root hair
Soil particle
Water
Water uptake from soil
© 2014 Pearson Education, Inc.
Figure 36.11b
Xylem
cells
Adhesion by
hydrogen bonding
Cell
wall
Cohesion and
adhesion in
the xylem
© 2014 Pearson Education, Inc.
Cohesion
by hydrogen
bonding
Figure 36.11c
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere
Transpiration
© 2014 Pearson Education, Inc.
BioFlix: Water Transport in Plants
© 2014 Pearson Education, Inc.
Animation: Transpiration
© 2014 Pearson Education, Inc.
Animation: Water Transport
© 2014 Pearson Education, Inc.
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
© 2014 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 a break in
the chain of water molecules through cavitation,
the formation of a water vapor pocket
© 2014 Pearson Education, Inc.
Xylem Sap Ascent by Bulk Flow: A Review
 The movement of xylem sap against gravity is
maintained by the cohesion-tension mechanism
 Bulk flow is driven by a water potential difference
at opposite ends of xylem tissue
 Bulk flow is driven by transpiration and does not
require energy from the plant; like photosynthesis
it is solar powered
© 2014 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
© 2014 Pearson Education, Inc.
Concept 36.4: The rate of transpiration is
regulated by stomata
 Leaves generally have large surface areas and
high surface-to-volume ratios
 These characteristics increase photosynthesis, but
also increase water loss through stomata
 Guard cells help balance water conservation with
gas exchange for photosynthesis
© 2014 Pearson Education, Inc.
Figure 36.12
© 2014 Pearson Education, Inc.
Figure 36.12a
© 2014 Pearson Education, Inc.
Figure 36.12b
© 2014 Pearson Education, Inc.
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
© 2014 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
© 2014 Pearson Education, Inc.
Figure 36.13
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)
H 2O
K+
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
H 2O
(b) Role of potassium ions (K+) in stomatal
opening and closing
© 2014 Pearson Education, Inc.
H 2O
Figure 36.13a
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)
© 2014 Pearson Education, Inc.
Figure 36.13b
Guard cells turgid/
Stoma open
H2O
K+
H2O
Guard cells flaccid/
Stoma closed
H2O
H2O
H2O
H2O
H2O
H2O
(b) Role of potassium ions (K+) in stomatal
opening and closing
© 2014 Pearson Education, Inc.
H2O
H2O
 This results primarily from the reversible uptake
and loss of potassium ions (K) by the guard cells
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
 Drought, high temperature, and wind can cause
stomata to close during the daytime
 The hormone abscisic acid (ABA) is produced in
response to water deficiency and causes the
closure of stomata
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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 fleshy stems that store water or 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
© 2014 Pearson Education, Inc.
Figure 36.14
▶ Ocotillo
(Fouquieria
splendens)
▼ Oleander (Nerium oleander)
▼ Oleander (Nerium oleander)
Upper
epidermal tissue
100 µm
Thick cuticle
Trichomes
Stoma
(“hairs”) Crypt
► Old man cactus
(Cephalocereus
senilis)
© 2014 Pearson Education, Inc.
Lower epidermal
tissue
Figure 36.14a
Ocotillo (leafless)
© 2014 Pearson Education, Inc.
Figure 36.14b
Ocotillo after heavy rain
© 2014 Pearson Education, Inc.
Figure 36.14c
Ocotillo leaves
© 2014 Pearson Education, Inc.
Figure 36.14d
100 µm
Thick cuticle
Upper
epidermal tissue
Trichomes
(“hairs”)
Stoma
Crypt
Oleander leaf cross section
© 2014 Pearson Education, Inc.
Lower epidermal
tissue
Figure 36.14e
Oleander flowers
© 2014 Pearson Education, Inc.
Figure 36.14f
Old man cactus
© 2014 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
© 2014 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
© 2014 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 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
© 2014 Pearson Education, Inc.
Figure 36.15
Apoplast
Symplast
Companion
(transfer) cell
Mesophyll cell
Cell walls (apoplast)
Plasma
membrane
Sieve-tube
element
High H+ concentration
Proton
pump
Cotransporter
H+
S
Plasmodesmata
Mesophyll cell
Bundlesheath cell
Phloem
parenchyma cell
(a) Sucrose manufactured in mesophyll cells
can travel via the symplast (blue arrows)
to sieve-tube elements.
© 2014 Pearson Education, Inc.
ATP
H+
H+
Low H+ concentration
Sucrose
S
(b) A chemiosmotic mechanism is
responsible for the active transport
of sucrose.
Figure 36.15a
Companion
(transfer) cell
Mesophyll cell
Cell walls (apoplast)
Plasma
membrane
Sieve-tube
element
Plasmodesmata
Mesophyll cell
Bundlesheath cell
Phloem
parenchyma
cell
(a) Sucrose manufactured in mesophyll cells
can travel via the symplast (blue arrows)
to sieve-tube elements.
© 2014 Pearson Education, Inc.
Apoplast
Symplast
Figure 36.15b
High H+ concentration
H+
Proton
pump
ATP
Low
H+
Cotransporter
S
H+
H+
concentration
Sucrose
S
(b) A chemiosmotic mechanism is
responsible for the active transport
of sucrose.
© 2014 Pearson Education, Inc.
 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
© 2014 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
© 2014 Pearson Education, Inc.
Figure 36.16
Sieve Source cell
tube
(leaf)
1 Loading of sugar
(phloem)
H2O
Bulk flow by negative pressure
2
2 Uptake of water
3 Unloading of sugar
Sink cell
(storage
root)
3
4
H2O
© 2014 Pearson Education, Inc.
Sucrose
H2O
1
Bulk flow by positive pressure
Vessel
(xylem)
4 Recycling of water
Sucrose
Animation: Translocation of Phloem Sap in
Spring
© 2014 Pearson Education, Inc.
Animation: Translocation of Phloem Sap in
Summer
© 2014 Pearson Education, Inc.
 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, the dropping of sugar sinks such as
flowers, seeds, or fruits, occurs when there are
more sugar sinks than the sources can support
© 2014 Pearson Education, Inc.
Figure 36.17
25 µm
Sievetube
element
Sap
droplet
Aphid feeding
© 2014 Pearson Education, Inc.
Stylet
Stylet in sievetube element
Sap droplet
Separated stylet
exuding sap
Figure 36.17a
Sap
droplet
Aphid feeding
© 2014 Pearson Education, Inc.
Figure 36.17b
25 µm
Sievetube
element
Stylet
Stylet in sievetube element
© 2014 Pearson Education, Inc.
Figure 36.17c
Sap droplet
Separated stylet
exuding sap
© 2014 Pearson Education, Inc.
Concept 36.6: The symplast is highly dynamic
 The symplast is a living tissue and is responsible
for dynamic changes in plant transport processes
© 2014 Pearson Education, Inc.
Changes in Plasmodesmatal Number and Pore
Size
 Plasmodesmata can open or close in response to
turgor pressure, cytoplasmic calcium levels, or
cytoplasmic pH
 Plant viruses can cause plasmodesmata to dilate,
allowing viral RNA to pass between cells
© 2014 Pearson Education, Inc.
Figure 36.18
Plasmodesma
Cytoplasm
of cell 2
Virus
particles
Cytoplasm
of cell 1
© 2014 Pearson Education, Inc.
Cell walls
100 nm
Phloem: An Information Superhighway
 Phloem is a “superhighway” for systemic transport
of macromolecules and viruses
 Systemic communication through the phloem
helps integrate functions of the whole plant
© 2014 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)
© 2014 Pearson Education, Inc.
Figure 36.UN01
© 2014 Pearson Education, Inc.
Figure 36.UN04
H2O
CO2
O2
O2
Minerals
© 2014 Pearson Education, Inc.
CO2
H2O
Figure 36.UN05
© 2014 Pearson Education, Inc.