29lecturepresentation-160429123119 2.pdf

CAMPBELL BIOLOGY IN FOCUS
© 2014 Pearson Education, Inc.
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
29
Resource Acquisition,
Nutrition, and Transport
in Vascular Plants

Overview: Underground Plants
 Stone plants (Lithops) are adapted to life in the
desert
 Two succulent leaf tips are exposed above ground;
the rest of the plant lives below ground

Figure 29.1

 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

Concept 29.1: Adaptations for acquiring
resources were key steps in the evolution of
vascular plants
 The evolution of adaptations enabling plants to
acquire resources from both above and below
ground sources allowed for the successful
colonization of land by vascular plants

 The algal ancestors of land plants absorbed water,
minerals, and CO2 directly from 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

 The evolution of xylem and phloem in land plants
made possible the development of extensive root
and shoot systems that carry out long-distance
transport
 Xylem transports water and minerals from roots to
shoots
 Phloem transports photosynthetic products from
sources to sinks

Figure 29.2-1
H2O
and
minerals
H2O

Figure 29.2-2
H2O
and
minerals
H2O
O2
CO2
O2
CO2

Figure 29.2-3
Light
H2O
and
minerals
H2O Sugar
O2
CO2
O2
CO2

 Adaptations in each species represent
compromises between enhancing photosynthesis
and minimizing water loss

Shoot Architecture and Light Capture
 Stems serve as conduits for water and nutrients and
as supporting structures for leaves
 Shoot height and branching pattern affect light
capture
 There is a trade-off between growing tall and
branching

 Phyllotaxy, the arrangement of leaves on a stem, is
specific to each species
 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

Figure 29.3
Shoot
apical
meristem
Buds
34
21
42 29
16
11 19
27
32
24
40
28
15
10
23
31
18
13
26
22
12
17
25
20
1
2
3
4
5
6
7
8
9
14
1 mm

Figure 29.3a

Figure 29.3b
Shoot
apical
meristem
34
21
42 29
16
11 19
27
32
24
40
28
15
10
23
31
18
13
26
22
12
17
25
20
1
2
3
4
5
6
7
8
9
14
1 mm

 The productivity of each plant is affected by the
depth of the canopy, the leafy portion of all the
plants in the community
 Shedding of lower shaded leaves when they respire
more than photosynthesize, self-pruning, occurs
when the canopy is too thick

 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

Root Architecture and Acquisition of Water
and Minerals
 Soil is a resource mined by the root system
 Root growth can adjust to local conditions
 For example, roots branch more in a pocket of high
nitrate than in a pocket of low nitrate
 Roots are less competitive with other roots from the
same plant than with roots from different plants

 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

Concept 29.2: Different mechanisms transport
substances over short or long distances
 There are two major transport pathways through
plants
 The apoplast
 The symplast

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

 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

Figure 29.4
Apoplastic route
Cell wall
Symplastic route
Transmembrane route
Cytosol
Key
Plasmodesma
Plasma membrane Apoplast
Symplast

Short-Distance Transport of Solutes Across
Plasma Membranes
 Plasma membrane permeability controls short-
distance 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

Figure 29.5
CYTOPLASM EXTRACELLULAR
FLUID
Proton
pump
Hydrogen
ion
Sucrose
(neutral solute)
Potassium ion
Ion channel
Nitrate
(d) Ion channels
(c) H+
and cotransport of ions
(b) H+
and cotransport of neutral solutes
(a) H+
and membrane potential
H+
/sucrose
cotransporter
H+
/NO3
−
cotransporter
S
NO3
−
K+
K+
K+
K+
K+
K+
K+
H+ H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
S
S
S
S
S
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
NO3
−
NO3
−
NO3
−
NO3
−
NO3
−

Figure 29.5a
CYTOPLASM EXTRACELLULAR
FLUID
Proton
pump
Hydrogen
ion
(a) H+
and membrane potential
H+
H+
H+
H+
H+
H+
H+
H+

Figure 29.5b
Sucrose
(neutral solute)
(b) H+
and cotransport of neutral solutes
H+
/sucrose
cotransporter
S H+ H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
S
S
S
S
S

Figure 29.5c
Nitrate
(c) H+
and cotransport of ions
H+
/NO3
−
cotransporter
NO3
−
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
NO3
−
NO3
−
NO3
−
NO3
−
NO3
−

Figure 29.5d
Potassium ion
Ion channel
(d) Ion channels
K+
K+
K+
K+
K+
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

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

 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

 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

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

 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

 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
Water Movement Across Plant Cell Membranes

Figure 29.6
Plasmolyzed
cell at osmotic
equilibrium
with its
surroundings
Turgid cell
at osmotic
equilibrium
with its
surroundings
0.4 M sucrose
solution:
(a) Initial conditions:
cellular ψ > environmental ψ
−0.9 MPa
=
ψ 0 MPa
=
ψ
Initial flaccid cell:
0 MPa
=
ψ
0
0
=
=
ψP
ψS
Pure water:
(b) Initial conditions:
cellular ψ < environmental ψ
−0.7 MPa
=
ψ
−0.7
=
=
ψP
ψS
−0.9
=
=
ψP
ψS
0
−0.9 MPa
=
ψ
−0.9
=
=
ψP
ψS
0
0
0.7
=
=
ψP
ψS
−0.7

−0.9
Figure 29.6a
Plasmolyzed
cell at osmotic
equilibrium
with its
surroundings
0.4 M sucrose
solution:
(a) Initial conditions:
cellular ψ > environmental ψ
−0.9 MPa
=
ψ
−0.9
=
=
ψP
ψS
−0.9 MPa
=
ψ
=
=
ψP
ψS
−0.7 MPa
=
ψ
−0.7
=
=
ψP
ψS
0
0
0

−0.7
Figure 29.6b
−0.7 MPa
=
ψ
=
=
ψP
ψS
Turgid cell
at osmotic
equilibrium
with its
surroundings
Pure water:
(b) Initial conditions:
cellular ψ < environmental ψ
0 MPa
=
ψ
0
0
=
=
ψP
ψS
0 MPa
=
ψ
0.7
=
=
ψP
ψS
0
−0.7

 If a flaccid cell is placed in a solution with a lower
solute concentration, the cell will gain water and
become turgid (firm)
 Turgor loss in plants causes wilting, which can be
reversed when the plant is watered
Video: Turgid Elodea

Figure 29.7
Turgid
Wilted

Figure 29.7a
Wilted

Figure 29.7b
Turgid

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

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

Concept 29.3: Plants roots absorb essential
elements from the soil
 Water, air, and soil minerals contribute to plant
growth
 80–90% of a plant’s fresh mass is water
 96% of plant’s dry mass consists of carbohydrates
from the CO2 assimilated during photosynthesis
 4% of a plant’s dry mass is inorganic substances
from soil

Macronutrients and Micronutrients
 More than 50 chemical elements have been
identified among the inorganic substances in plants,
but not all of these are essential to plants
 There are 17 essential elements, chemical
elements required for a plant to complete its life
cycle
 Researchers use hydroponic culture, the growth of
plants in mineral solutions, to determine which
chemical elements are essential

Figure 29.8
Technique
Control: Solution
containing all minerals
Experimental: Solution
without potassium

Table 29.1

 Nine of the essential elements are called
macronutrients because plants require them in
relatively large amounts
 The macronutrients are carbon, oxygen, hydrogen,
nitrogen, phosphorus, sulfur, potassium, calcium,
and magnesium

 The remaining eight are called micronutrients
because plants need them in very small amounts
 The micronutrients are chlorine, iron, manganese,
boron, zinc, copper, nickel, and molybdenum
 Plants with C4 and CAM photosynthetic pathways
also need sodium
 Micronutrients function as cofactors, nonprotein
helpers in enzymatic reactions

Symptoms of Mineral Deficiency
 Symptoms of mineral deficiency depend on the
nutrient’s function and mobility within the plant
 Deficiency of a mobile nutrient usually affects older
organs more than young ones
 Deficiency of a less mobile nutrient usually affects
younger organs more than older ones
 The most common deficiencies are those of
nitrogen, potassium, and phosphorus

Figure 29.9
Nitrogen-deficient
Potassium-deficient
Phosphate-deficient
Healthy

Soil Management
 Ancient farmers recognized that crop yields would
decrease on a particular plot over the years
 Soil management, by fertilization and other practices,
allowed for agriculture and cities

Fertilization
 In natural ecosystems, nutrients are recycled
through decomposition of feces and humus, dead
organic material
 Soils can become depleted of nutrients as plants
and the nutrients they contain are harvested
 Fertilization replaces mineral nutrients that have
been lost from the soil

 Commercial fertilizers are enriched in nitrogen (N),
phosphorus (P), and potassium (K)
 Excess minerals are often leached from the soil and
can cause algal blooms in lakes
 Organic fertilizers are composed of manure,
fishmeal, or compost
 They release N, P, and K as they decompose

Adjusting Soil pH
 Soil pH affects cation exchange and the chemical
form of minerals
 Cations are more available in slightly acidic soil, as
H+
ions displace mineral cations from clay particles
 The availability of different minerals varies with pH
 For example, at pH 8 plants can absorb calcium but
not iron

 At present, 30% of the world’s farmland has reduced
productivity because of soil mismanagement

The Living, Complex Ecosystem of Soil
 Plants obtain most of their mineral nutrients from
the topsoil
 The basic physical properties of soil are
 Texture
 Composition

Soil Texture
 Soil particles are classified by size; from largest to
smallest they are called sand, silt, and clay
 Topsoil is formed when mineral particles released
from weathered rock mix with living organisms and
humus

 Soil solution consists of water and dissolved
minerals in the pores between soil particles
 After a heavy rainfall, water drains from the larger
spaces in the soil, but smaller spaces retain water
because of its attraction to clay and other particles
 Loams are the most fertile topsoils and contain
equal amounts of sand, silt, and clay

Topsoil Composition
 A soil’s composition refers to its inorganic (mineral)
and organic chemical components

 Inorganic components of the soil include positively
charged ions (cations) and negatively charged ions
(anions)
 Most soil particles are negatively charged
 Anions (for example, NO3
–
, H2PO4
–
, SO4
2–
) do not bind
with negatively charged soil particles and can be lost
from the soil by leaching

 Cations (for example, K+
, Ca2+
, Mg2+
) adhere to
negatively charged soil particles; this prevents them
from leaching out of the soil through percolating
groundwater
 During cation exchange, cations are displaced
from soil particles by other cations
 Displaced cations enter the soil solution and can be
taken up by plant roots
Animation: Minerals from Soil

Figure 29.10
Soil particle
K+
Root hair
Cell wall
Mg2+
K+
H+
H+
K+
Ca2+ Ca2+
H2O + CO2 H2CO3 HCO3
−
+

 Organic components of the soil include
decomposed leaves, feces, dead organisms, and
other organic matter, which are collectively named
humus
 Humus forms a crumbly soil that retains water but is
still porous
 It also increases the soil’s capacity to exchange
cations and serves as a reservoir of mineral nutrients

 Living components of topsoil include bacteria,
fungi, algae and other protists, insects, earthworms,
nematodes, and plant roots
 These organisms help to decompose organic
material and mix the soil

Concept 29.4: Plant nutrition often involves
relationships with other organisms
 Plants and soil microbes have a mutualistic
relationship
 Dead plants provide energy needed by soil-dwelling
microorganisms
 Secretions from living roots support a wide variety of
microbes in the near-root environment

Soil Bacteria and Plant Nutrition
 Soil bacteria exchange chemicals with plant roots,
enhance decomposition, and increase nutrient
availability

Rhizobacteria
 The soil layer surrounding the plant’s roots is the
rhizosphere
 Rhizobacteria thrive in the rhizosphere, and some
can enter roots
 The rhizosphere has high microbial activity because
of sugars, amino acids, and organic acids secreted
by roots

 Rhizobacteria known as plant-growth-promoting
rhizobacteria can play several roles
 Produce hormones that stimulate plant growth
 Produce antibiotics that protect roots from disease
 Absorb toxic metals or make nutrients more available
to roots

Bacteria in the Nitrogen Cycle
 Nitrogen can be an important limiting nutrient for
plant growth
 The nitrogen cycle transforms atmospheric
nitrogen and nitrogen-containing compounds
 Plants can only absorb nitrogen as either NO3
–
or
NH4
+
 Most usable soil nitrogen comes from actions of soil
bacteria

Figure 29.11
ATMOSPHERE
ATMOSPHERE
SOIL
SOIL
Nitrogen-fixing
bacteria
N2
N2
N2
NH3
(ammonia)
H+
(from soil)
Ammonifying
bacteria
Amino acids
NH4
+
(ammonium)
NO2
−
(nitrite)
Nitrifying
bacteria
NO3
−
(nitrate)
Nitrifying
bacteria
Denitrifying
bacteria
Microbial
decomposition
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
NH4
+
Root
Proteins from humus
(dead organic material)

Figure 29.11a
ATMOSPHERE
SOIL
Nitrogen-fixing
bacteria
N2
N2
NH3
(ammonia)
H+
(from soil)
Ammonifying
bacteria
Amino acids
NH4
+
(ammonium)
NO2
−
(nitrite)
Nitrifying
bacteria
NO3
−
(nitrate)
Nitrifying
bacteria
Denitrifying
bacteria
Microbial
decomposition
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
NH4
+
Root
Proteins from humus

Figure 29.11b
ATMOSPHERE
SOIL
Nitrogen-fixing
bacteria
N2
NH3
(ammonia)
H+
(from soil)
Ammonifying
bacteria
Amino acids
NH4
+
(ammonium)
NO2
−
(nitrite)
Nitrifying
bacteria
Microbial
decomposition
Proteins from humus

Figure 29.11c
N2
NO2
−
(nitrite)
NO3
−
(nitrate)
Nitrifying
bacteria
Denitrifying
bacteria
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
NH4
+
Root

 Conversion to NH4
+
 Ammonifying bacteria break down organic
compounds and release ammonium (NH4
+
)
 Nitrogen-fixing bacteria convert N2 gas into NH3
 NH3 is converted to NH4
+
 Conversion to NO3
–
 Nitrifying bacteria oxidize NH4
+
to nitrite (NO2
–
) then
nitrite to nitrate (NO3
–
)
 Different nitrifying bacteria mediate each step

 Nitrogen is lost to the atmosphere when denitrifying
bacteria convert NO3
–
to N2

Nitrogen-Fixing Bacteria: A Closer Look
 Nitrogen is abundant in the atmosphere but
unavailable to plants due to the triple bond between
atoms in N2
 Nitrogen fixation is the conversion of nitrogen from
N2 to NH3:
 Some nitrogen-fixing bacteria are free-living; others
form intimate associations with plant roots
N2 + 8 e–
+ 8 H+
+ 16 ATP → 2 NH3 + H2 + 16 ADP + 16

 Symbiotic relationships with nitrogen-fixing
Rhizobium bacteria provide some legumes with a
source of fixed nitrogen
 Along a legume’s roots are swellings called
nodules, composed of plant cells “infected” by
nitrogen-fixing Rhizobium bacteria

Figure 29.12
Roots
Nodules

 Inside the root nodule, Rhizobium bacteria assume
a form called bacteroids, which are contained within
vesicles formed by the root cell
 The plant obtains fixed nitrogen from Rhizobium,
and Rhizobium obtains sugar and an anaerobic
environment
 Each legume species is associated with a particular
strain of Rhizobium

Fungi and Plant Nutrition
 Mycorrhizae are mutualistic associations of fungi
and roots
 The fungus benefits from a steady supply of sugar
from the host plant
 The host plant benefits because the fungus
increases the surface area for water uptake and
mineral absorption
 Mycorrhizal fungi also secrete growth factors that
stimulate root growth and branching

The Two Main Types of Mycorrhizae
 Mycorrhizal associations consist of two major types
 Ectomycorrhizae
 Arbuscular mycorrhizae

Figure 29.13
Epidermis Cortex
Epidermal
cell
Endodermis
Mantle (fungal sheath)
Mantle
(fungal sheath)
Fungal
hyphae
between
cortical
cells (LM)
50 µm
1.5 mm
(Colorized
SEM)
Epidermis Cortex
Fungal
vesicle
Endodermis
Cortical cell
Casparian
strip
Plasma
membrane
Arbuscules
(LM)
10
µm
Root
hair
Fungal
hyphae
(a) Ectomycorrhizae
(b) Arbuscular mycorrhizae (endomycorrhizae)

Figure 29.13a
Epidermis Cortex
Epidermal
cell
Endodermis
Mantle (fungal sheath)
Mantle
(fungal sheath)
Fungal
hyphae
between
cortical
cells (LM)
50 µm
1.5 mm
(Colorized
SEM)
(a) Ectomycorrhizae

Figure 29.13aa
Mantle
(fungal sheath)
1.5 mm
(Colorized
SEM)

Figure 29.13ab
Epidermal
cell
(LM)
50 µm
Fungal
hyphae
between
cortical
cells

Figure 29.13b
Epidermis Cortex
Fungal
vesicle
Endodermis
Cortical cell
Casparian
strip
Plasma
membrane
Arbuscules
(LM)
10
µm
Root
hair
Fungal
hyphae
(b) Arbuscular mycorrhizae (endomycorrhizae)

Figure 29.13ba
Cortical cell
Arbuscules
(LM)
10
µm

 In ectomycorrhizae, the mycelium of the fungus
forms a dense sheath over the surface of the root
 These hyphae form a network in the apoplast but do
not penetrate the root cells
 Ectomycorrhizae occur in about 10% of plant
families, most of which are woody (for example,
pine, birch, and eucalyptus)

 In arbuscular mycorrhizae, microscopic fungal
hyphae extend into the root
 These mycorrhizae penetrate the cell wall but not the
plasma membrane to form branched arbuscules
within root cells
 The arbuscules are important sites of nutrient
transfer
 Arbuscular mycorrhizae occur in about 85% of plant
species, including most crops

Agricultural and Ecological Importance of
Mycorrhizae
 Seeds can be inoculated with fungal spores to
promote formation of mycorrhizae
 Some invasive exotic plants disrupt interactions
between native plants and their mycorrhizal fungi
 For example, garlic mustard slows growth of other
plants by preventing the growth of mycorrhizal fungi

Figure 29.14
Experiment Results
Invaded Uninvaded Sterilized
invaded
Sterilized
uninvaded
Soil type
Invaded Uninvaded
Soil type
White ash
Sugar maple
Seedlings
Red maple
Mycorrhizal
colonization
(%)
Increase
in
plant
biomass
(%)
300
200
100
0
30
20
10
0
40

Figure 29.14a
Experiment

Figure 29.14b
Results
Invaded Uninvaded Sterilized
invaded
Sterilized
uninvaded
Soil type
Invaded Uninvaded
Soil type
White ash
Sugar maple
Seedlings
Red maple
Mycorrhizal
colonization
(%)
Increase
in
plant
biomass
(%)
300
200
100
0
30
20
10
0
40

Epiphytes, Parasitic Plants, and Carnivorous
Plants
 Some plants have nutritional adaptations that use
other organisms in nonmutualistic ways
 Three unusual adaptations are
 Epiphytes
 Parasitic plants
 Carnivorous plants
Video: Sundew Traps Prey

 Epiphytes grow on other plants and obtain water
and minerals from rain, rather than tapping their
hosts for sustenance

Figure 29.15a
Staghorn fern, an epiphyte

 Parasitic plants absorb water, sugars, and
minerals from their living host plant
 Some species also photosynthesize, but others rely
entirely on the host plant for sustenance
 Some species parasitize the mycorrhizal hyphae of
other plants

Figure 29.15b
Parasitic plants
Mistletoe, a photosynthetic
parasite
Dodder, a nonphoto-
synthetic parasite
(orange)
Indian pipe, a nonphoto-
synthetic parasite of
mycorrhizae

Figure 29.15ba
Mistletoe, a photosynthetic
parasite

Figure 29.15bb
Dodder, a nonphoto-
synthetic parasite
(orange)

Figure 29.15bc
Indian pipe, a nonphoto-
synthetic parasite of
mycorrhizae

 Carnivorous plants are photosynthetic but obtain
nitrogen by killing and digesting mostly insects

Figure 29.15c
Carnivorous plants
Pitcher plants
Venus flytraps
Sundew

Figure 29.15ca
Pitcher plants

Figure 29.15cb
Pitcher plants

Figure 29.15cc
Sundew

Figure 29.15cd
Venus flytrap

Figure 29.15ce
Venus flytrap

Concept 29.5: 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

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 absorption of
water by roots
 After soil solution enters the roots, the extensive
surface area of cortical cell membranes enhances
uptake of water and selected minerals

 The concentration of essential minerals is greater in
the roots than in the soil because of active transport

 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
 Water can cross the cortex via the symplast or
apoplast
Transport of Water and Minerals into the Xylem
Animation: Transport in Roots

Figure 29.16
Apoplastic
route
Symplastic
route
Transmembrane
route
The endodermis: controlled entry
to the vascular cylinder (stele)
Apoplastic
route
Symplastic
route
Root
hair
Plasma
membrane
Epidermis
Casparian strip
Vascular
cylinder
(stele)
Vessels
(xylem)
Cortex
Endodermis
Pathway along
apoplast
Transport in the xylem
Casparian strip
Pathway
through
symplast
Endodermal
cell
1
1
2
2
4
5
3
3
4
5
5
4

Figure 29.16a
Apoplastic
route
Symplastic
route
Transmembrane
route
Apoplastic
route
Symplastic
route
Root
hair
Plasma
membrane
Epidermis
Casparian strip
Vascular
cylinder
(stele)
Vessels
(xylem)
Cortex
Endodermis
1
1
2
2
3
4
3
4
5
5

Figure 29.16b
Pathway along
apoplast
Casparian strip
Pathway
through
symplast
Endodermal
cell
5
4
5
4

 Water and minerals can travel to the vascular
cylinder through the cortex via
 The apoplastic route, along cell walls and
extracellular spaces
 The symplastic route, in the cytoplasm, moving
between cells through plasmodesmata
 The transmembrane route, moving from cell to cell by
crossing cell membranes and cell walls

 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

 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

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 loss of water vapor from a plant’s surface
 Transpired water is replaced as water travels up
from the roots

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

 Transpirational pull is generated when water vapor
in the air spaces of a leaf diffuses down its water
potential gradient and exits the leaf via stomata
 As water evaporates, the air-water interface retreats
farther into the mesophyll cell walls and becomes
more curved
 Due to the high surface tension of water, the
curvature of the interface creates a negative pressure
potential

 This negative pressure pulls water in the xylem into
the leaf
 The pulling effect results from the cohesive binding
between water molecules
 The transpirational pull on xylem sap is transmitted
from leaves to roots
Animation: Transpiration
Animation: Water Transport in Plants
Animation: Water Transport

Figure 29.17
Xylem
Microfibrils in
cell wall of
mesophyll cell
Microfibril
(cross section)
Air-water
interface
Water
film
Cuticle
Mesophyll
Cuticle Stoma
Air
space
Upper
epidermis
Lower
epidermis

Figure 29.17a
Xylem
Microfibrils in
cell wall of
mesophyll cell
Cuticle
Mesophyll
Cuticle Stoma
Air
space
Upper
epidermis
Lower
epidermis

Figure 29.17b
Microfibrils in
cell wall of
mesophyll cell
Microfibril
(cross section)
Air-water
interface
Water
film

Figure 29.18
Cohesion
by hydrogen
bonding
Water molecule
Cohesion
and adhesion
in the xylem
Water uptake from soil
Water
Soil particle
Root hair
Cell wall
Transpiration
Xylem
cells
Water molecule
Adhesion by hydrogen
bonding
Xylem sap
Mesophyll cells
Stoma
Atmosphere
Outside air ψ
Water
potential
gradient
Leaf ψ (air spaces)
Trunk xylem ψ
Leaf ψ (cell walls)
Trunk xylem ψ
Soil ψ
−7.0 MPa
−100.0 MPa
=
=
−1.0 MPa
=
−0.8 MPa
=
−0.6 MPa
=
−0.3 MPa
=

Figure 29.18a
Water molecule
Water uptake from soil
Water
Soil particle
Root hair

Figure 29.18b
Cohesion
by hydrogen
bonding
Cohesion
and adhesion
in the xylem
Cell wall
Xylem
cells
Adhesion by hydrogen
bonding

Figure 29.18c
Transpiration
Water molecule
Xylem sap
Mesophyll cells
Stoma
Atmosphere

 Cohesion and adhesion in the ascent of xylem
sap: Water molecules are attracted to each other
through cohesion
 Cohesion makes it possible to pull a column of
xylem sap
 Water molecules are attracted to hydrophilic walls of
xylem cell walls through adhesion
 Adhesion of water molecules to xylem cell walls
helps offset the force of gravity

 Thick secondary walls prevent vessel elements and
tracheids from collapsing under negative pressure
 Drought stress or freezing can cause cavitation, the
formation of a water vapor pocket by a break in the
chain of water molecules

Xylem Sap Ascent by Bulk Flow: A Review
 Bulk flow is driven by a water potential difference at
opposite ends of xylem tissue
 Bulk flow is driven by evaporation and does not
require energy from the plant; like photosynthesis, it
is solar powered

 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

Concept 29.6: 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

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

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

Figure 29.19
Guard cells turgid/Stoma open
K+
Guard cells flaccid/Stoma closed
H2O
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
(a) Changes in guard cell shape and stomatal opening and
closing (surface view)
Guard cell
(b) Role of potassium ions (K+
) in stomatal opening and closing
H2O H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O

Figure 29.19a
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
(a) Changes in guard cell shape and stomatal opening and
closing (surface view)
Guard cell

Figure 29.19b
K+
H2O
(b) Role of potassium ions (K+
) in stomatal opening and closing
H2O H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

 Changes in turgor pressure result primarily from the
reversible uptake and loss of potassium ions (K+
) by
the guard cells

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

 Drought stress 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

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 protein denaturation

Adaptations That Reduce Evaporative Water
Loss
 Xerophytes are plants adapted to arid climates

Figure 29.20
Old man cactus
(Cephalocereus
senilis)
Ocotillo
(Fouquieria
splendens)
Oleander (Nerium oleander)
Thick cuticle Upper epidermal tissue
Stoma Lower epidermal
tissue
Crypt
Trichomes
(“hairs”)
100
µm

Figure 29.20a
Ocotillo (leafless)

Figure 29.20b
Ocotillo after heavy rain

Figure 29.20c
Ocotillo leaves

Figure 29.20d
Oleander leaf cross section
Thick cuticle Upper epidermal tissue
Stoma Lower
epidermal
tissue
Crypt
Trichomes
(“hairs”)
100
µm

Figure 29.20e
Oleander flowers

Figure 29.20f
Old man cactus

 Some desert plants complete their life cycle during
the rainy season
 Others have 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

Concept 29.7: Sugars are transported from sources to
sinks via the phloem
 The products of photosynthesis are transported
through phloem by the process of translocation

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

 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

Figure 29.21
Apoplast
Symplast
Cell walls (apoplast)
Plasmodesmata
Mesophyll cell
Plasma
membrane
Companion
(transfer) cell
Sieve-tube
element
Mesophyll
cell
Bundle-
sheath cell
Phloem
parenchyma cell
(a) Sucrose manufactured in mesophyll cells
can travel via the symplast (blue arrows)
to sieve-tube elements.
(b) A chemiosmotic mechanism is
responsible for the active transport of
sucrose.
High H+
concentration
Low H+
concentration
Proton
pump
Cotransporter
Sucrose
S
S
H+
H+
H+

Figure 29.21a
Apoplast
Symplast
Cell walls (apoplast)
Plasmodesmata
Mesophyll cell
Plasma
membrane
Companion
(transfer) cell
Sieve-tube
element
Mesophyll
cell
Bundle-
sheath cell
Phloem
parenchyma cell
(a) Sucrose manufactured in mesophyll cells
can travel via the symplast (blue arrows)
to sieve-tube elements.

Figure 29.21b
(b) A chemiosmotic mechanism is
responsible for the active transport of
sucrose.
High H+
concentration
Low H+
concentration
Proton
pump
Cotransporter
Sucrose
S
S
H+
H+
H+

 In most 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

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
Animation: Phloem Translocation Spring
Animation: Phloem Translocation Summer

Figure 29.22
Loading of sugar
H2O
Vessel
(xylem)
Source cell
(leaf)
Sieve
tube
(phloem)
H2O
Sucrose
Uptake of water
Unloading of sugar
Recycling of water
Sink cell
(storage
root)
H2O
Sucrose
Bulk
flow
by
negative
pressure
Bulk
flow
by
positive
pressure
1
1
2
2
3
4 3
4

 Sometimes there are more sinks than can be
supported by sources
 Self-thinning is the dropping of sugar sinks such as
flowers, seeds, or fruits

Figure 29.UN01a

Figure 29.UN01b

Figure 29.UN02
Minerals H2O
O2
CO2
H2O
O2
CO2

Figure 29.UN03

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