Transcript H +

Chapter 29- Resource Acquisition,
Nutrition, and Transport in
Vascular Plants
Following the Big Ideas
 Energy and Homeostasis- Acquisition
of water and nutrients by plants involve
specialized mechanisms and
structures.
Overview: Underground Plants
 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
 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
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
Transport in plants-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
 H2O & minerals


transport in xylem
transpiration
 evaporation, adhesion & cohesion
 negative pressure (tension)
 Sugars


transport in phloem
bulk flow
 Calvin cycle in leaves loads sucrose into phloem
 positive pressure
 Gas exchange

photosynthesis
 CO2 in; O2 out
 stomates

respiration
 O2 in; CO2 out
 roots exchange gases within air spaces in soil
Why does
over-watering
kill a plant?
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
42
16
34
21
19
27
6
14
Shoot
apical
meristem
26
5
18
31
11
40
3
8
13
Buds
32
24
29
10
23
28
7
20
22
9
4
2
15
1
17
12
25
1 mm
Emerging phyllotaxy of Norway
spruce
 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, selfpruning, 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
Concept 29.2: Different mechanisms
transport substances over short or long
distances- 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
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
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 sodiumpotassium pumps
Figure 29.5
Solute transport across plant cell plasma membranes
EXTRACELLULAR
FLUID
H
Hydrogen
ion
H
H
CYTOPLASM
H
H
H
Proton
pump
H
H
S
H
H
H

H
H
H

H
H
H
H
H
H/sucrose
cotransporter
(a) H and membrane potential
Sucrose
(neutral solute)
(b) H and cotransport of neutral solutes
H
H


H
H
H
H/NO3−
cotransporter
H
H
Nitrate
H
H
Potassium ion

K
K

H
(c) H and cotransport of ions
K
H
K
K
K
K
Ion channel
(d) Ion channels
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
© 2014 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
 Water potential is abbreviated as  and
measured in a unit of pressure called the
megapascal (MPa)
   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:
  S  P
 The solute potential (S) of a solution is
directly proportional to its molarity
 Solute potential is also called osmotic
potential
 Pressure potential (P) 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
Figure 29.6
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  −0.7
  −0.7 MPa
Pure water:
P  0
S  0
  0 MPa
P  0
S  −0.9
  −0.9 MPa
(a) Initial conditions:
cellular   environmental 
Turgid cell
at osmotic
equilibrium
with its
surroundings
P  0.7
S  −0.7
 
0 MPa
(b) Initial conditions:
cellular   environmental 
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
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
 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
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
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–, SO42–)
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
Figure 29.10
Soil particle
K
K
2
Ca
K
Mg
2
Ca2
H
H2O  CO2
H2CO3
HCO3−  H
Root hair
Cell wall
 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 soildwelling microorganisms
 Secretions from living roots support a wide
variety of microbes in the near-root
environment
 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
N2
SOIL
N2
N2
ATMOSPHERE
Proteins from humus
(dead organic material)
SOIL
Nitrogen-fixing
bacteria
NH3
(ammonia)
H
(from soil)
Microbial
decomposition
Amino acids
Denitrifying
bacteria
Ammonifying
NH4
(ammonium)
bacteria
NO2−
Nitrifying (nitrite) Nitrifying
bacteria
bacteria
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
NH4
NO3−
(nitrate)
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
Bacterial root nodules
Found on roots of legumessymbiotic relationship!
 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
Mycorrhizae increase absorption
 The hyphae of mycorrhizal fungi extend into soil, where
their large surface area and efficient absorption enable
them to obtain mineral nutrients, even if these are in short
supply or are relatively immobile. Mycorrhizal fungi seem
to be particularly important for absorption of phosphorus,
a poorly mobile element, and a proportion of the
phosphate that they absorb has been shown to be passed
to the plant.
Mycorrhizae
 Symbiotic relationship between fungi &
plant



symbiotic fungi
greatly increases
surface area for
absorption of
water &
minerals
increases
volume of soil
reached by plant
increases
transport to
host plant
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

 Epiphytes grow on other plants and obtain
water and minerals from rain, rather than
tapping their hosts for sustenance
 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
 Carnivorous plants are photosynthetic but
obtain nitrogen by killing and digesting
mostly insects

Figure 29.15a
Staghorn fern, an epiphyte
Figure 29.15b
Parasitic plants
Mistletoe, a photosynthetic
parasite
Dodder, a nonphotosynthetic parasite
(orange)
Indian pipe, a nonphotosynthetic parasite of
mycorrhizae
Figure 29.15c
Carnivorous plants
Sundew
Pitcher plants
Venus flytraps
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
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
Water flow through root
 Porous cell wall
water can flow through cell wall route &
not enter cells
 plant needs to force water into cells

Casparian strip
Controlling the route of water in root
 Endodermis



cell layer surrounding vascular cylinder of root
lined with impermeable Casparian strip
forces fluid through selective cell membrane
 filtered & forced into xylem cells
Water & mineral absorption
 Water absorption from soil
osmosis
 aquaporins

 Mineral absorption
active transport
 proton pumps

 active transport of
H+
aquaporin
root hair
proton pumps
H2O
Mineral absorption
 Proton pumps

active transport of H+ ions out of cell
 chemiosmosis
 H+ gradient

creates membrane
potential
 difference in charge
 drives cation uptake

creates gradient
 cotransport of other
solutes against their
gradient
Transport in
Plants Transpiration
pull-cohesion
tension theory
states that for
every
molecule of
water
evaporated
from the leaf,
another
molecule is
drawn in at
the root
2006-2007
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
Ascent of xylem fluid
Transpiration
pull generated
by evaporation
from the leaf
 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
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
Control of Stomates
 Uptake of K+ ions by
guard cells



Guard cell
proton pumps
create a membrane
potential that drives
the uptake of K+
ions from epidermal
cell surrounding the
guard cells
water enters by
osmosis
guard cells become
turgid
 Loss of K+ ions by
guard cells


Epidermal cell
water leaves by
osmosis
guard cells become
flaccid
H2O
K+
H2O
K+
Nucleus
Chloroplasts
H2O
K+
H2O
K+
K+
H2O
K+
H2O
K+
H2O
K+
H2O
Thickened inner
cell wall (rigid)
H2O
K+
H2O
K+
H2O
K+
H2O
K+
Stoma open
Stoma closed
water moves
into guard cells
water moves out
of guard cells
Blue light causes stomates to open
Control of transpiration
 Balancing stomate function

always a compromise between
photosynthesis & transpiration
 leaf may transpire more than its weight in
water in a day…this loss must be balanced
with plant’s need for CO2 for photosynthesis
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
 Blue 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
Osmoregulation in Hydrophytes
(Aquatic plants)
 poorly developed root systems and
supportive xylem tissues
 no stomata (for submerged leaves)
 thin & finely divided leaves
 no cuticle
Adaptations That Reduce Evaporative Water
Loss
 Xerophytes are plants adapted to arid
climates



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
© 2014 Pearson Education, Inc.
Figure 29.20
Oleander (Nerium oleander)
Thick cuticle
Upper epidermal tissue
100 m
Ocotillo
(Fouquieria
splendens)
Trichomes Crypt Stoma Lower epidermal
tissue
(“hairs”)
Old man cactus
(Cephalocereus
senilis)
Osmoregulation- Plant Responses
to Water Limitations
 Wilting or curling leaves reduces sunlight exposure and water








evaporation
Stomata are on the underside of the leaf, thick waxy cuticle is on
the top
Adaptations like needles and fat photosynthesizing stems that
store water
Deep tap roots or shallow fibrous roots
Hairs or scales on leaves that reduce wind evaporation
Pits on underside of leaf where stomata are to reduce water loss
by raising water potential around stoma area
Thick fleshy leaves that store water
Small leaves
Stomata that only open at night
 Multiple layered epidermis and small intercellular spaces
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
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
 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
 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
Transport of sugars in phloem
 Loading of sucrose into phloem
flow through cells via plasmodesmata
 proton pumps

 cotransport of sucrose into cells down proton gradient
Pressure flow in phloem
 Bulk flow hypothesis

“source to sink” flow
 direction of transport in phloem is
dependent on plant’s needs

phloem loading
 active transport of sucrose
into phloem
 increased sucrose concentration
decreases H2O potential

water flows in from xylem cells
 increase in pressure due to increase in
H2O causes flow
Vessel
(xylem)
Sieve Source cell
tube
(leaf)
1 Loading of sugar
(phloem)
H2O
Bulk flow by negative pressure
Pressure Flow: The
Mechanism of
Translocation in
Angiosperms
Phloem sap flows
from source to sink
at rates as great as
1 m/hr, much too
fast to be accounted
for by either
diffusion or
cytoplasmic
streaming. In
studying
angiosperms,
researchers have
concluded that sap
moves through a
sieve tube by bulk
flow driven by
positive pressure
(thus the synonym
pressure flow.
The building of
pressure at the
source end and
reduction of that
pressure at the sink
end cause water to
flow from source to
sink, carrying the
sugar along. Xylem
recycles the water
from sink to source.
The pressure flow
hypothesis explains
why phloem sap
always flows from
source to sink.
Sucrose
H2O
1
2
Bulk flow by positive pressure
Figure 29.22
2 Uptake of water
3 Unloading of sugar
Sink cell
(storage
root)
3
4
H2O
4 Recycling of water
Sucrose
can
flow
1m/hr
Experimentation
 Testing pressure flow
hypothesis

using aphids to measure sap
flow & sugar concentration
along plant stem
Maple
sugaring
Connecting the Concepts With the Big Ideas
 Energy and Homeostasis




Plants depend on water’s special properties of
adhesion and cohesion to accomplish movement of
water via transpiration pull when stomata are open.
Plants employ stomata and root hairs to facilitate
gas exchange.
Negative feedback allows plants to regulate their
transpiration based on water availability
Related methods of osmoregulation are used
throughout the plant kingdom, showing their
common ancestry.
Important cooperative relationships that assist
plants in their acquisition of nutrients include
mycorrhizae and bacterial root nodules.