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Biology 102 Week 5
Plant Structure, Growth, and
Development
Transport in Vascular Plants
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: No two Plants Are Alike
• To some people, the fanwort is an intrusive weed,
but to others it is an attractive aquarium plant
• This plant exhibits plasticity, the ability to alter
itself in response to its environment
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In addition to plasticity, plant species have by
natural selection accumulated characteristics of
morphology that vary little within the species
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 35.1: The plant body has a hierarchy of
organs, tissues, and cells
• Plants, like multicellular animals, have organs
composed of different tissues, which are in turn
are composed of cells
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The Three Basic Plant Organs: Roots, Stems, and
Leaves
• Basic morphology of vascular plants reflects their
evolution as organisms that draw nutrients from
below-ground and above-ground
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• Three basic organs evolved: roots, stems, and
leaves
• They are organized into a root system and a shoot
system
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-2
Reproductive shoot (flower)
Terminal bud
Node
Internode
Terminal
bud
Vegetable
shoot
Leaf
Shoot
system
Blade
Petiole
Axillary
bud
Stem
Taproot
Lateral roots
Root
system
Roots
• Functions of roots:
– Anchoring the plant
– Absorbing minerals and water
– Often storing organic nutrients
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• In most plants, absorption of water and minerals
occurs near the root tips, where vast numbers of
tiny root hairs increase the surface area
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Many plants have modified roots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-4a
Prop roots.
LE 35-4b
Storage roots.
LE 35-4c
“Strangling” aerial roots.
LE 35-4d
Buttress roots.
LE 35-4e
Pneumatophores.
Stems
• A stem is an organ consisting of
– An alternating system of nodes, the points at
which leaves are attached
– Internodes, the stem segments between nodes
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• An axillary bud is a structure that has the potential
to form a lateral shoot, or branch
• A terminal bud is located near the shoot tip and
causes elongation of a young shoot
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• Many plants have modified stems
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LE 35-5a
Stolons.
LE 35-5b
Storage leaves
Stem
Roots
Bulbs.
LE 35-5c
Tubers.
LE 35-5d
Rhizomes.
Node
Rhizome
Root
Leaves
• The leaf is the main photosynthetic organ of most
vascular plants
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• Leaves generally consist of
– A flattened blade and a stalk
– The petiole, which joins the leaf to a node of
the stem
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• Monocots and eudicots differ in the arrangement
of veins, the vascular tissue of leaves
• Most monocots have parallel veins
• Most eudicots have branching veins
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• In classifying angiosperms, taxonomists may use
leaf morphology as a criterion
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LE 35-6a
Simple leaf
Petiole
Axillary bud
LE 35-6b
Leaflet
Compound leaf
Petiole
Axillary bud
LE 35-6c
Doubly compound leaf
Leaflet
Petiole
Axillary bud
• Some plant species have evolved modified leaves
that serve various functions
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LE 35-7a
Tendrils.
LE 35-7b
Spines.
LE 35-7c
Storage leaves.
LE 35-7d
Bracts.
LE 35-7e
Reproductive
leaves.
The Three Tissue Systems: Dermal, Vascular, and
Ground
• Each plant organ has dermal, vascular, and
ground tissues
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LE 35-8
Dermal
tissue
Ground
tissue
Vascular
tissue
• In nonwoody plants, the dermal tissue system
consists of the epidermis
• In woody plants, protective tissues called periderm
replace the epidermis in older regions of stems
and roots
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• The vascular tissue system carries out longdistance transport of materials between roots and
shoots
• The two vascular tissues are xylem and phloem
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• Xylem conveys water and dissolved minerals
upward from roots into the shoots
• Phloem transports organic nutrients from where
they are made to where they are needed
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• The vascular tissue of a stem or root is collectively
called the stele
• In angiosperms the stele of the root is a solid
central vascular cylinder
• The stele of stems and leaves is divided into
vascular bundles, strands of xylem and phloem
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• Tissues that are neither dermal nor vascular are
the ground tissue system
• Ground tissue includes cells specialized for
storage, photosynthesis, and support
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Common Types of Plant Cells
• Like any multicellular organism, a plant is
characterized by cellular differentiation, the
specialization of cells in structure and function
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• Some major types of plant cells:
– Parenchyma
– Collenchyma
– Sclerenchyma
– Water-conducting cells of the xylem
– Sugar-conducting cells of the phloem
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-9
WATER-CONDUCTING CELLS OF THE XYLEM
PARENCHYMA CELLS
Vessel
Parenchyma cells in Elodea
leaf, with chloroplasts (LM)
Tracheids
100 µm
60 µm
Pits
COLLENCHYMA CELLS
80 µm
Cortical parenchyma cells
Tracheids and vessels
(colorized SEM)
Vessel
element
Vessel elements with
perforated end walls
Tracheids
SUGAR-CONDUCTING CELLS OF THE PHLOEM
Collenchyma cells (in cortex of Sambucus,
elderberry; cell walls stained red) (LM)
Sieve-tube members:
longitudinal view
(LM)
SCLERENCHYMA CELLS
5 µm
Companion
cell
Sclereid cells in pear (LM)
Sieve-tube
member
Plasmodesma
25 µm
Sieve
plate
Cell wall
Nucleus
Cytoplasm
Companion
cell
30 µm
15 µm
Fiber cells (transverse section from ash tree) (LM)
Sieve-tube members:
longitudinal view
Sieve plate with pores (LM)
Concept 35.2: Meristems generate cells for new
organs
• Apical meristems are located at the tips of roots
and in the buds of shoots
• Apical meristems elongate shoots and roots, a
process called primary growth
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• Lateral meristems add thickness to woody plants,
a process called secondary growth
• There are two lateral meristems: the vascular
cambium and the cork cambium
• The vascular cambium adds layers of vascular
tissue called secondary xylem (wood) and
secondary phloem
• The cork cambium replaces the epidermis with
periderm, which is thicker and tougher
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-10
Primary growth in stems
Shoot apical
meristems
(in buds)
Epidermis
Cortex
Primary phloem
Primary xylem
Vascular
cambium Lateral
meristems
Cork
cambium
Pith
Secondary growth in stems
Periderm
Cork
cambium
Pith
Cortex
Primary
phloem
Primary
xylem
Root apical
meristems
Secondary
xylem
Secondary
phloem
Vascular cambium
• In woody plants, primary and secondary growth
occur simultaneously but in different locations
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LE 35-11
Terminal bud
Bud scale
Axillary buds
Leaf scar
This year’s growth
(one year old)
Node
Stem
Internode
One-year-old side
branch formed
from axillary bud
near shoot apex
Leaf scar
Last year’s growth
(two years old)
Scars left by terminal
bud scales of previous
winters
Growth of two
years ago (three
years old)
Leaf scar
Concept 35.3: Primary growth lengthens roots and
shoots
• Primary growth produces the primary plant body,
the parts of the root and shoot systems produced
by apical meristems
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Primary Growth of Roots
• The root tip is covered by a root cap, which
protects the apical meristem as the root pushes
through soil
• Growth occurs just behind the root tip, in three
zones of cells:
– Zone of cell division
– Zone of elongation
– Zone of maturation
Video: Root Growth in a Radish Seed (time lapse)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-12
Cortex
Vascular cylinder
Epidermis
Key
Root hair
Dermal
Zone of
maturation
Ground
Vascular
Zone of
elongation
Apical
meristem
Root cap
100 µm
Zone of cell
division
• The primary growth of roots produces the
epidermis, ground tissue, and vascular tissue
• In most roots, the stele is a vascular cylinder
• The ground tissue fills the cortex, the region
between the vascular cylinder and epidermis
• The innermost layer of the cortex is called the
endodermis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-13
Epidermis
Cortex
Vascular
cylinder
Endodermis
Pericycle
Core of
parenchyma
cells
Xylem
100 µm
Phloem
100 µm
Transverse section of a typical root. In the
roots of typical gymnosperms and eudicots,
as well as some monocots, the stele is a
vascular cylinder consisting of a lobed core
of xylem with phloem between the lobes.
Endodermis
Pericycle
Transverse section of a root with parenchyma in
the center. The stele of many monocot roots is a
vascular cylinder with a core of parenchyma
surrounded by a ring of alternating xylem and
phloem.
Key
Dermal
Ground
Vascular
Xylem
Phloem
50 µm
• Lateral roots arise from within the pericycle, the
outermost cell layer in the vascular cylinder
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LE 35-14
100 µm
Emerging
lateral
root
Cortex
Vascular
cylinder
Epidermis
Lateral root
Primary Growth of Shoots
• A shoot apical meristem is a dome-shaped mass
of dividing cells at the tip of the terminal bud
• It gives rise to a repetition of internodes and leafbearing nodes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-15
Apical meristem
Leaf primordia
Developing
vascular
strand
Axillary bud
meristems
0.25 mm
Tissue Organization of Stems
• In gymnosperms and most eudicots, the vascular
tissue consists of vascular bundles that are
arranged in a ring
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• In most monocot stems, the vascular bundles are
scattered throughout the ground tissue, rather
than forming a ring
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-16
Phloem
Xylem
Sclerenchyma
(fiber cells)
Ground
tissue
Ground tissue
connecting
pith to cortex
Pith
Epidermis
Key
Cortex
Epidermis
Vascular
bundles
Dermal
Vascular
bundles
Ground
1 mm
A eudicot (sunflower) stem. Vascular bundles form
a ring. Ground tissue toward the inside is called
pith, and ground tissue toward the outside is called
cortex. (LM of transverse section)
Vascular
1 mm
A monocot (maize) stem. Vascular bundles are scattered
throughout the ground tissue. In such an arrangement,
ground tissue is not partitioned into pith and cortex. (LM
of transverse section)
Tissue Organization of Leaves
• The epidermis in leaves is interrupted by stomata,
which allow CO2 exchange between the air and
the photosynthetic cells in a leaf
• The ground tissue in a leaf is sandwiched between
the upper and lower epidermis
• The vascular tissue of each leaf is continuous with
the vascular tissue of the stem
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-17
Key
to labels
Guard
cells
Dermal
Stomatal pore
Ground
Vascular
Cuticle
Sclerenchyma
fibers
Epidermal
cells
50 µm
Surface view of a spiderwort
(Tradescantia) leaf (LM)
Stoma
Upper
epidermis
Palisade
mesophyll
Bundlesheath
cell
Spongy
mesophyll
Lower
epidermis
Guard
cells
Cuticle
Vein
Xylem
Phloem
Cutaway drawing of leaf tissues
Guard
cells
Vein
Air spaces
Guard cells
100 µm
Transverse section of a lilac
(Syringa) leaf (LM)
Concept 35.4: Secondary growth adds girth to
stems and roots in woody plants
• Secondary growth occurs in stems and roots of
woody plants but rarely in leaves
• The secondary plant body consists of the tissues
produced by the vascular cambium and cork
cambium
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-18a
Primary and secondary growth
in a two-year-old stem
Epidermis
Cortex
Primary
phloem
Vascular
cambium
Primary
xylem
Pith
Pith
Primary xylem
Vascular cambium
Primary phloem
Cortex
Epidermis
Phloem ray
Xylem
ray
Primary
xylem
Secondary xylem
Vascular cambium
Secondary phloem
Primary phloem
First cork cambium
Cork
Periderm
(mainly cork
cambia
and cork)
Primary
phloem
Secondary
phloem
Vascular
cambium
Secondary
xylem
Primary
xylem
Pith
Secondary
xylem (two
years of
production)
Vascular cambium
Secondary phloem
Bark
Most recent
cork cambium
Cork
Layers of
periderm
LE 35-18b
Secondary phloem
Vascular cambium
Secondary
xylem
Cork
cambium
Late wood
Early wood
Periderm
Cork
Transverse section
of a three-yearold Tilia (linden)
stem (LM)
Xylem ray
Bark
0.5 mm
0.5 mm
The Vascular Cambium and Secondary Vascular
Tissue
• The vascular cambium is a cylinder of
meristematic cells one cell thick
• It develops from undifferentiated cells and
parenchyma cells that regain the capacity of divide
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In transverse section, the vascular cambium
appears as a ring, with regions of dividing cells
called fusiform initials and ray initials
• The initials increase the vascular cambium’s
circumference and add secondary xylem to the
inside and secondary phloem to the outside
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-19
Vascular
cambium
Types of cell division
Accumulation of secondary growth
• As a tree or woody shrub ages, the older layers of
secondary xylem, the heartwood, no longer
transport water and minerals
• The outer layers, known as sapwood, still
transport materials through the xylem
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-20
Growth ring
Vascular
ray
Heartwood
Secondary
xylem
Sapwood
Vascular cambium
Secondary phloem
Bark
Layers of periderm
Cork Cambia and the Production of Periderm
• The cork cambium gives rise to the secondary
plant body’s protective covering, or periderm
• Periderm consists of the cork cambium plus the
layers of cork cells it produces
• Bark consists of all the tissues external to the
vascular cambium, including secondary phloem
and periderm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 35.5: Growth, morphogenesis, and
differentiation produce the plant body
• The three developmental processes of growth,
morphogenesis, and cellular differentiation act in
concert to transform the fertilized egg into a plant
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Molecular Biology: Revolutionizing the Study of Plants
• New techniques and model systems are
catalyzing explosive progress in our
understanding of plants
• Arabidopsis is the first plant to have its entire
genome sequenced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-21
Cell organization and biogenesis (1.7%)
DNA metabolism (1.8%)
Carbohydrate metabolism (2.4%)
Unknown
(36.6%)
Signal transduction (2.6%)
Protein biosynthesis (2.7%)
Electron transport
(3%)
Protein
modification (3.7%)
Protein
metabolism (5.7%)
Transcription (6.1%)
Other biological
processes (18.6%)
Other metabolism (6.6%)
Transport (8.5%)
Growth: Cell Division and Cell Expansion
• By increasing cell number, cell division in
meristems increases the potential for growth
• Cell expansion accounts for the actual increase in
plant size
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The Plane and Symmetry of Cell Division
• The plane (direction) and symmetry of cell division
are immensely important in determining plant form
• If the planes of division are parallel to the plane of
the first division, a single file of cells is produced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-22a
Division in
same plane
Single file of cells forms
Plane of
cell division
Division in
three planes
Cube forms
Nucleus
Cell divisions in the same plane produce a single file of cells, whereas cell divisions in three planes give rise to a cube.
• If the planes of division vary randomly,
asymmetrical cell division occurs
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-22b
Developing
guard cells
Asymmetrical
cell division
Unspecialized
epidermal cell
Unspecialized Guard cell
epidermal cell “mother cell”
Unspecialized
epidermal cell
An asymmetrical cell division precedes the development of epidermal guard cells, the cells that border stomata (see Figure 35.17).
• The plane in which a cell divides is determined
during late interphase
• Microtubules become concentrated into a ring
called the preprophase band
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-23
Preprophase bands
of microtubules
Nuclei
Cell plates
10 µm
Orientation of Cell Expansion
• Plant cells rarely expand equally in all directions
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• Orientation of the cytoskeleton affects the direction
of cell elongation by controlling orientation of
cellulose microfibrils within the cell wall
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LE 35-24
Cellulose
microfibrils
Vacuoles
Nucleus
5 µm
Microtubules and Plant Growth
• Studies of fass mutants of Arabidopsis have
confirmed the importance of cytoplasmic
microtubules in cell division and expansion
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LE 35-25
fass seeding
Wild-type seeding
Mass fass mutant
Morphogenesis and Pattern Formation
• Pattern formation is the development of specific
structures in specific locations
• It is determined by positional information in the
form of signals indicating to each cell its location
• Polarity is one type of positional information
• In the gnom mutant of Arabidopsis, the
establishment of polarity is defective
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Morphogenesis in plants, as in other multicellular
organisms, is often controlled by homeotic genes
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gene Expression and Control of Cellular
Differentiation
• In cellular differentiation, cells of a developing
organism synthesize different proteins and diverge
in structure and function even though they have a
common genome
• Cellular differentiation to a large extent depends
on positional information and is affected by
homeotic genes
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LE 35-28
Cortical
cells
20 µm
Location and a Cell’s Developmental Fate
• A cell’s position in a developing organ determines
its pathway of differentiation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Shifts in Development: Phase Changes
• Plants pass through developmental phases, called
phase changes, developing from a juvenile phase
to an adult phase
• The most obvious morphological changes typically
occur in leaf size and shape
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-29
Leaves produced
by adult phase
of apical meristem
Leaves produced
by juvenile phase
of apical meristem
Genetic Control of Flowering
• Flower formation involves a phase change from
vegetative growth to reproductive growth
• It is triggered by a combination of environmental
cues and internal signals
• Transition from vegetative growth to flowering is
associated with the switching-on of floral meristem
identity genes
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• Plant biologists have identified several organ
identity genes that regulate the development of
floral pattern
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-30
Pe
Ca
St
Se
Pe
Se
Normal Arabidopsis flower. Arabidopsis
normally has four whorls of flower parts: sepals
(Se), petals (Pe), stamens (St), and carpels (Ca).
Pe
Pe
Se
Abnormal Arabidopsis flower. This flower has
an extra set of petals in place of stamens and
an internal flower where normal plants have
carpels.
• The ABC model of flower formation identifies how
floral organ identity genes direct the formation of
the four types of floral organs
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-31a
Sepals
Petals
Stamens
A
B
Carpels
C
B+C
A+B
gene
gene
activity
activity
A gene
activity
A schematic diagram of the ABC
hypothesis
C gene
activity
• An understanding of mutants of the organ identity
genes depicts how this model accounts for floral
phenotypes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 35-31b
Active
genes:
Whorls:
Carpel
Stamen
Petal
Sepal
Wild type
Mutant lacking A
Side view of organ identity mutant flowers
Mutant lacking B
Mutant lacking C
Chapter 36
Transport in Vascular Plants
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Pathways for Survival
• For vascular plants, the evolutionary journey onto
land involved differentiation into roots and shoots
• Vascular tissue transports nutrients in a plant;
such transport may occur over long distances
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 36.1: Physical forces drive the transport
of materials in plants over a range of distances
• Transport in vascular plants occurs on three
scales:
– Transport of water and solutes by individual
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• A variety of physical processes are involved in the
different types of transport
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-2_4
CO2
O2
Light
H2O
Sugar
O2
H2O
Minerals
CO2
Selective Permeability of Membranes: A Review
• The selective permeability of the plasma
membrane controls movement of solutes into and
out of the cell
• Specific transport proteins enable plant cells to
maintain an internal environment different from
their surroundings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Central Role of Proton Pumps
• Proton pumps in plant cells create a hydrogen ion
gradient that is a form of potential energy that can
be harnessed to do work
• They contribute to a voltage known as a
membrane potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-3
CYTOPLASM
EXTRACELLULAR FLUID
ATP
Proton pump
generates membrane potential
and
gradient.
• Plant cells use energy stored in the proton
gradient and membrane potential to drive the
transport of many different solutes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-4a
CYTOPLASM
EXTRACELLULAR FLUID
Cations (
, for
example) are
driven into the cell
by the membrane
potential.
Transport protein
Membrane potential and cation uptake
• In the mechanism called cotransport, a transport
protein couples the passage of one solute to the
passage of another
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-4b
Cell accumulates
anions (
,
for example) by
coupling their
transport to; the
inward diffusion
of
through a
cotransporter.
Cotransport of anions
• The “coattail” effect of cotransport is also
responsible for the uptake of the sugar sucrose by
plant cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-4c
Plant cells can
also accumulate
a neutral solute,
such as sucrose
(
), by
cotransporting
down the
steep proton
gradient.
Cotransport of a neutral solute
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
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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
• Pressure potential is the physical pressure on a
solution
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Quantitative Analysis of Water Potential
• The addition of solutes reduces water potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-5a
Addition of
solutes
0.1 M
solution
Pure
water
H2O
 = 0 MPa
P = 0
S = –0.23
P = –0.23 MPa
• Physical pressure increases water potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-5b
Applying
physical
pressure
H2O
 = 0 MPa
P = 0
S = –0.23
P = –0 MPa
LE 36-5c
Applying
physical
pressure
H2O
 = 0 MPa
P = 0.30
S = –0.23
P = –0.07 MPa
• Negative pressure decreases water potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-5d
Negative
pressure
H2O
P = –0.30
S = –0.23
P = –0.30 MPa
P = 0.30
S = –0.23
P = –0.23 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
Video: Plasmolysis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-6
Plasmolyzed cell
at osmotic
equilibrium

0.4 M sucrose solution:
P = 0
S = –0.9
P = –0.9 MPa
P = 0
S = –0.9
P = –0.9 MPa
 conditions: cellular > environmental
Initial flaccid cell:
P = 0
 S = –0.7
 P = –0.7 MPa
Distilled water:
P = 0
S = 0
 P = 0 MPa
Turgid cell
at osmotic
equilibrium

udings
P = 0.7
 S = –0.7
 P = 0 MPa
• If the same flaccid cell is placed in a solution with
a lower solute concentration, the cell will gain
water and become turgid
Video: Turgid Elodea
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Turgor loss in plants causes wilting, which can be
reversed when the plant is watered
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Aquaporin Proteins and Water Transport
• Aquaporins are transport proteins in the cell
membrane that allow the passage of water
• Aquaporins do not affect water potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Three Major Compartments of Vacuolated Plant
Cells
• Transport is also regulated by the compartmental
structure of plant cells
• The plasma membrane directly controls the traffic
of molecules into and out of the protoplast
• The plasma membrane is a barrier between two
major compartments, the cell wall and the cytosol
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The third major compartment in most mature plant
cells is the vacuole, a large organelle that
occupies as much as 90% or more of the
protoplast’s volume
• The vacuolar membrane regulates transport
between the cytosol and the vacuole
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-8a
Cell wall
Cytosol
Vacuole
Plasmodesma
Key
Symplast
Apoplast
Vacuolar membrane
(tonoplast)
Plasma membrane
Cell compartments
• 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 and
extracellular spaces
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-8b
Key
Symplast
Apoplast
Transmembrane route
Apoplast
Symplast
Symplastic route
Apoplastic route
Transport routes between cells
Functions of the Symplast and Apoplast in Transport
• Water and minerals can travel through a plant by
three routes:
– Transmembrane route: out of one cell, across
a cell wall, and into another cell
– Symplastic route: via the continuum of cytosol
– Apoplastic route: via the the cell walls and
extracellular spaces
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Bulk Flow in Long-Distance Transport
• In bulk flow, movement of fluid in the xylem and
phloem is driven by pressure differences at
opposite ends of the xylem vessels and sieve
tubes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 36.2: Roots absorb water and minerals
from the soil
• Water and mineral salts from the soil enter the
plant through the epidermis of roots and ultimately
flow to the shoot system
Animation: Transport in Roots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-9
Casparian strip
Pathway along
apoplast
Endodermal cell
Pathway
through
symplast
Casparian strip
Plasma
membrane
Apoplastic
route
Vessels
(xylem)
Symplastic
route
Root
hair
Epidermis
Endodermis Vascular cylinder
Cortex
The Roles of Root Hairs, Mycorrhizae, and
Cortical Cells
• Much of the absorption of water and minerals
occurs near root tips, where the epidermis is
permeable to water and root hairs are located
• Root hairs account for much of the surface area of
roots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Most plants form mutually beneficial relationships
with fungi, which facilitate absorption of water and
minerals from the soil
• Roots and fungi form mycorrhizae, symbiotic
structures consisting of plant roots united with
fungal hyphae
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-10
2.5 mm
• After soil solution enters the roots, the extensive
surface area of cortical cell membranes enhances
uptake of water and selected minerals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Endodermis: A Selective Sentry
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 36.3: Water and minerals ascend from
roots to shoots through the xylem
• Plants lose an enormous amount of water through
transpiration, the loss of water vapor from leaves
and other aerial parts of the plant
• The transpired water must be replaced by water
transported up from the roots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Factors Affecting the Ascent of Xylem Sap
• Xylem sap rises to heights of more than 100 m in
the tallest plants
• Is the sap pushed upward from the roots, or is it
pulled upward by the leaves?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pushing Xylem Sap: Root Pressure
• At night, when transpiration is very low, 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Root pressure sometimes results in guttation, the
exudation of water droplets on tips of grass blades
or the leaf margins of some small, herbaceous
eudicots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pulling Xylem Sap: The Transpiration-Cohesion
Tension Mechanism
• Water is pulled upward by negative pressure in the
xylem
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transpirational Pull
• Water vapor 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, which exerts a pulling force
on water in the xylem, pulling water into the leaf
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-12
Y = –0.15 MPa
Y = –10.00 MPa
Cell wall
Air-water
interface
Airspace
Low rate of
transpiration
Cuticle
Upper
epidermis
High rate of
transpiration
Cytoplasm
Evaporation
Mesophyll
Airspace
Air
space
Cell wall
Evaporation
Water film
Lower
epidermis
Cuticle
CO2
O2
CO2
Xylem
O2
Stoma
Vacuole
Cohesion and Adhesion in the Ascent of Xylem Sap
• The transpirational pull on xylem sap is
transmitted all the way from the leaves to the root
tips and even into the soil solution
• Transpirational pull is facilitated by cohesion and
adhesion
Animation: Transpiration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-13
Xylem
sap
Outside air 
= –100.0 MPa
Mesophyll
cells
Stoma
Leaf  (air spaces)
= –7.0 MPa
Water
molecule
Transpiration
Atmosphere
Trunk xylem 
= –0.8 Mpa
Water potential gradient
Leaf  (cell walls)
= –1.0 MPa
Xylem
cells
Adhesion Cell
wall
Cohesion,
Cohesion and by
adhesion in
hydrogen
the xylem
bonding
Water
molecule
Root
hair
Root xylem 
= –0.6 MPa
Soil 
= –0.3 MPa
Soil
particle
Water
Water uptake
from soil
Xylem Sap Ascent by Bulk Flow: A Review
• The movement of xylem sap against gravity is
maintained by the transpiration-cohesion-tension
mechanism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 36.4: Stomata help regulate the rate of
transpiration
• Leaves generally have broad surface areas and
high surface-to-volume ratios
• These characteristics increase photosynthesis and
increase water loss through stomata
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-14
20 µm
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 absorption
through the roots, the plant will lose water and wilt
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Stomata: Major Pathways for Water Loss
• About 90% 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-15a
Cells turgid/Stoma open
Cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Changes in guard cell shape and stomatal opening and closing
(surface view)
• Changes in turgor pressure that open and close
stomata result primarily from the reversible uptake
and loss of potassium ions by the guard cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-15b
Cells turgid/Stoma open
H 2O
Cells flaccid/Stoma closed
H 2O
H 2O
H 2O
K+
H 2O
H 2O
H 2O
H 2O
H 2O
Role of potassium in stomatal opening and closing
H 2O
Xerophyte Adaptations That Reduce Transpiration
• Xerophytes are plants adapted to arid climates
• They have leaf modifications that reduce the rate
of transpiration
• Their stomata are concentrated on the lower leaf
surface, often in depressions that provide shelter
from dry wind
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-16
Cuticle
Upper epidermal tissue
Lower epidermal Trichomes Stomata
tissue
(“hairs”)
100 µm
Concept 36.5: Organic nutrients are translocated
through the phloem
• Translocation is the transport of organic nutrients
in a plant
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Movement from Sugar Sources to Sugar Sinks
• Phloem sap is an aqueous solution that is mostly
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Sugar must be loaded into sieve-tube members
before being exposed to sinks
• In many plant species, sugar moves by symplastic
and apoplastic pathways
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-17
Key
Apoplast
Symplast
Companion
(transfer) cell
Mesophyll cell
Cell walls (apoplast)
Sieve-tube
member
High H+ concentration
Cotransporter
Proton
pump
Plasma membrane
Plasmodesmata
Sucrose
Mesophyll cell
Bundlesheath cell
Phloem
parenchyma cell
Low H+ concentration
• In many plants, phloem loading requires active
transport
• Proton pumping and cotransport of sucrose and
H+ enable the cells to accumulate sucrose
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pressure Flow: The Mechanism of Translocation in
Angiosperms
• In studying angiosperms, researchers have
concluded that sap moves through a sieve tube by
bulk flow driven by positive pressure
Animation: Translocation of Phloem Sap in Summer
Animation: Translocation of Phloem Sap in Spring
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-18
Sieve tube
(phloem)
Vessel
(xylem)
H2O
Source cell
(leaf)
Sucrose
H2O
Sink cell
(storage
root)
Sucrose
H2O
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 36-19
25 µm
Sievetube
member
Sap
droplet
Aphid feeding
Stylet
Stylet in sieve-tube
member (LM)
Sap droplet
Severed stylet
exuding sap