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Vascular tissue
Syllabus Theme A
Plant Structure and Function
Transports
nutrients throughout a plant; such
transport may occur over long distances
A3: Transport in Vascular Plants
Campbell & Reece Chap. 36
pp. 738-755
TRANSPORT
CO2
O2
Sugars are produced by
photosynthesis in the leaves.
STOMATA
Light
H2O
Transport of water and solutes by individual
Sugar
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
Transpiration, creates a force within
leaves that pulls xylem sap upward.
Sugars are transported as
phloem sap to roots and other
parts of the plant.
Water and minerals are
transported upward from
roots to shoots as xylem sap.
Roots absorb water
and dissolved minerals
from the soil.
O2
H2O
Minerals
Selective Permeability of
Membranes: A Review
The selective permeability of a plant cell’s
plasma membrane
Controls
the movement of solutes into and out
of the cell
Specific transport proteins
Enable
plant cells to maintain an internal
environment different from their surroundings
CO2
Roots exchange gases
with the air spaces of soil,
taking in O2 and discharging
CO2. In cellular respiration,
O2 supports the breakdown
of sugars.
Effects of Differences in Water
Potential
To survive
Plants
must balance water uptake and loss
Osmosis
Determines
the net uptake or water loss by a
cell
Is affected by solute concentration and pressure
1
Effects of Differences in Water
Potential
Water potential
Is
a measurement that combines the effects of
solute concentration and pressure
Determines the direction of movement of water
Water
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
from regions of high water potential to
regions of low water potential
Pressure potential
Is the physical pressure on a solution
Quantitative Analysis of Water
Potential
Application of physical pressure
Flows
Reduces
water potential
(c)
(a)
The addition of solutes
Increases
(b)
0.1 M
solution
water potential
ψ = ψS + ψp
Pure
water
H2O
H2O
H2O
ψ = 0 MPa
Negative pressure
Decreases
water potential
(d)
ψP = 0
ψS = −0.23
ψ = −0.23 MPa
ψ = 0 MPa
ψP = 0.23
ψS = −0.23
ψ = 0 MPa
ψ = 0 MPa
ψP = 0.30
ψS = −0.23
ψ = 0.07 MPa
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
Initial flaccid cell:
ψP = 0
ψS = −0.7
0.4 M sucrose solution:
ψP = 0
ψS = −0.9
H2O
ψP = −0.30
ψS = 0
ψ = −0.30 MPa
ψP = 0
ψS = −0.23
ψ = −0.23 MPa
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
ψ = − 0.7 MPa
ψ = −0.9 MPa
ψP = 0
ψS = −0.9
ψ = −0.9 MPa
2
If the same flaccid cell is placed in a
solution with a lower solute concentration
The
Turgor loss in plants causes wilting
Which
can be reversed when the plant is
watered
cell will gain water and become turgid
Initial flaccid cell:
ψP = 0
ψS = −0.7
ψ = − 0.7 MPa
Distilled water:
ψP = 0
ψS = 0
ψ = 0 MPa
Turgid cell
at osmotic equilibrium
with its surroundings
ψP = 0.7
ψS = −0.7
ψ = − 0 MPa
Pathway of water and minerals
Key
Symplast
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 plus extracellular
spaces
Apoplast
Transmembrane route
Apoplast
The symplast is the
continuum of
cytosol connected
by plasmodesmata.
The apoplast is
the continuum
of cell walls and
extracellular
spaces.
Symplast
Symplastic route
Apoplastic route
Casparian strip
Functions of the Symplast and
Apoplast in Transport
Endodermal cell
Pathway along
apoplast
Pathway
through
symplast
Water and minerals can travel through a
plant by one of three routes
of one cell, across a cell wall, and into
another cell
Via the symplast
Along the apoplast
Casparian strip
Out
1
Plasma
membrane
Apoplastic
route
Vessels
(xylem)
2
Symplastic
route
Root
hair
Epidermis
Cortex
Endodermis
Vascular cylinder
Roots absorb water and minerals from the soil
3
The Roles of Root Hairs and
Cortical Cells
The Endodermis: A Selective
Sentry
The endodermis
Root hairs?
Innermost
Mycorrhizae - mutual beneficial relationship
with fungi, which facilitate the absorption of
water and minerals from the soil
2.5 mm
Water can cross the cortex
Via
layer of cells in the root cortex
the vascular cylinder
Last checkpoint for the selective passage
of minerals from the cortex into the
vascular tissue
Surrounds
the symplast or apoplast
Rise of water up the stem
The waxy Casparian strip of the endodermal
Plants lose an enormous amount
wall
Blocks
apoplastic transfer of minerals from the
cortex to the vascular cylinder
Root Pressure
of water through transpiration
Define transpiration!
The transpired water must be
replaced by water transported up
from the roots
Guttation
Root pressure
At night, when transpiration is very low
Root
cells continue pumping mineral ions into
the xylem of the vascular cylinder
Lowers the water potential
Water flows in from the root cortex
Generating
sometimes
results in
guttation
Exudation of
water droplets
root pressure
4
The Transpiration-CohesionTension Mechanism
Water is pulled upward by negative pressure in
the xylem
Water vapour 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
Ψ = –0.15 MPa
Ψ = –10.00 MPa
Cell wall
Air-water
interface
Airspace
Low rate of
transpiration
Cuticle
High rate of
transpiration
Upper
epidermis
Cytoplasm
Evaporation
Mesophyll
Airspace
Airspace
Cell wall
Lower
epidermis
Evaporation
Vacuole
Water film
Which exerts a pulling force on water in the xylem,
pulling water into the leaf
Cuticle
CO2
O2
Xylem
O2
Stoma
Water vapor
Water vapor
Cohesion and Adhesion
CO2
Xylem
sap
Outside air Ψ
= –100.0 MPa
Mesophyll
cells
Stoma
Leaf Ψ (air spaces)
= –7.0 MPa
The transpirational pull on xylem sap
Transpiration
Leaf Ψ (cell walls)
= –1.0 MPa
Atmosphere
Xylem
cells
Is
Trunk xylem Ψ
= – 0.8 MPa
Water potential gradient
transmitted all the way from the leaves
to the root tips and even into the soil
solution
Is facilitated by cohesion and adhesion
Water
molecule
Adhesion
Cohesion
and adhesion
in the xylem
Cell
wall
Cohesion,
by
hydrogen
bonding
Water
molecule
Root xylem Ψ
= – 0.6 MPa
Root
hair
Soil Ψ
= – 0.3 MPa
Soil
particle
Water uptake
from soil
The Control of Transpiration
Leaves generally have broad surface
Effects of Transpiration on
Wilting and Leaf Temperature
Plants lose a
areas
High
Water
large amount of
water by
transpiration
If the lost water
is not replaced
by absorption
through the
roots
surface-to-volume ratios
These characteristics
Increase photosynthesis
Increase water loss through stomata
20 µm
The plant will
lose water and
wilt
5
Effects of Transpiration on
Wilting and Leaf Temperature
Transpiration also results in evaporative
cooling
Why
Stomata: Major Pathways for
Water Loss
About 90% of the water a plant loses
Escapes
must the temperature be lowered?
through stomata
Each stoma is flanked by guard cells
Which
control the diameter of the stoma by
changing shape
Cells turgid/Stoma open
Cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Changes in turgor pressure that open
and close stomata
from the reversible uptake and loss
of K+ ions by the guard cells
Sugar Translocation
Result
H2O
H2O
H2O
H2O
H2O
K+
H2O
Translocation
Is the transport of organic nutrients in the plant
In the phloem tissue
Phloem sap
Is an aqueous solution that is mostly sucrose
Travels from a sugar source to a sugar sink
H2O
H2O
H2O
H2O
6
Sugar must be loaded into sieve-tube
A sugar source
Is
a plant organ that is a net producer of sugar,
such as mature leaves
members before being exposed to sinks
In many plant species, sugar moves by
symplastic and apoplastic pathways
A sugar sink
Mesophyll cell
Cell walls (apoplast)
Is
an organ that is a net consumer or storer of
sugar, such as a tuber or bulb
Plasmodesmata
Mesophyll cell
loading requires active transport
Proton pumping and cotransport of
sucrose and H+
Enable
the cells to accumulate sucrose
High H+ concentration
Cotransporter
H+
Proton
pump
S
Phloem
parenchyma cell
Pressure Flow: The Mechanism of
Translocation in Angiosperms
Vessel
(xylem)
In angiosperms
Sap moves
through a sieve
tube by bulk
flow driven by
positive
pressure
Sieve tube
(phloem)
H2 O
Source cell
(leaf)
Sucros e
1
H2 O
2
Pressure flow
Phloem
Bundlesheath cell
Transpiration stre am
In many plants
Sieve-tube
member
Companion
(transfer) cell
Plasma membrane
Sink cell
(storage
root)
Key
ATP
4
3
Sucros e
H+
Low H+ concentration
H+
Sucrose
S
Apoplast
H2 O
Symplast
7