Download Chapter 36 Plant Structures

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.
What adaptations must have evolved
for plants to establish themselves on
land?
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.
Roots
• Soil is a resource mined by the root system
• Taproot systems anchor plants and are
characteristic of gymnosperms
• 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 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
• Root hairs increase the absorbing surface area
© 2011 Pearson Education, Inc.
Figure 36.5
Roots
Fungus
Stems
• 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.
• 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.
Leaves: Site of Photosynthesis
Flowers: Reproductive Organs
• Three major 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 36.7a
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.
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 ______
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 ______ 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
• What causes the pressure created by the xylem
sap?
© 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
© 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
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 36.13a
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
© 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
• 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.
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
• This results primarily from the reversible uptake
and loss of potassium ions (K) by the guard cells
© 2011 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
© 2011 Pearson Education, Inc.
• 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
© 2011 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
© 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.
• A storage organ (root) 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
• Unlike water phloem sap can run up or down the
plant
© 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.
• 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.
• 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.
Phloem: An Information Superhighway
• Phloem is a “superhighway” for systemic transport
of macromolecules and viruses
• Systemic communication helps integrate
functions of the whole plant
• 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.
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