Transcript Document

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.
Figure 36.2-3
CO2
H2O
O2
Light
Sugar
O2
H2O
and
minerals
CO2
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
© 2011 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
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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.
Figure 36.6
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Plasma membrane
Apoplast
Symplast
• 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
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• 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
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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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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
Video: Plasmolysis
© 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 
Figure 36.9a



Plasmolyzed
cell at osmotic
equilibrium with
its surroundings
P  0
S  0.9
  0.9 MPa
(a) Initial conditions: cellular   environmental 



0.4 M sucrose solution:
P
0
S 0.9
0.9 MPa

Initial flaccid cell:
P
0
S 0.7
0.7 MPa

Figure 36.9b
Initial flaccid cell:
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 
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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• 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)
Figure 36.10a
Plasma
membrane
Casparian strip
Apoplastic
route
Symplastic
route
Vessels
(xylem)
Root
hair
Epidermis
Endodermis
Cortex
Vascular cylinder
(stele)
Figure 36.10b
Casparian strip
Pathway along
apoplast
Pathway
through
symplast
Endodermal cell
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