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Cecie Starr
Christine Evers
Lisa Starr
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Chapter 26
Plant Nutrition and Transport
(Sections 26.5 - 26.8)
Albia Dugger • Miami Dade College
26.5 Water-Conserving Adaptations
of Stems and Leaves
• Plants must conserve water for photosynthesis, growth,
membrane functions, and other processes
• A cuticle and stomata restrict the amount of water vapor that
diffuses out of the plant’s surfaces – but also restrict access to
CO2 for photosynthesis, and oxygen for aerobic respiration
Cuticle
• Cuticle prevents water loss from evaporation
• It consists of epidermal cell secretions: waxes, pectin, and
cellulose fibers embedded in cutin, an insoluble lipid polymer
• The cuticle is translucent, so it does not prevent light from
reaching photosynthetic tissues
Stomata
• Two specialized cells (guard cells) define each stoma
• When guard cells swell with water, a gap (stoma) forms
between them
• When guard cells lose water, the gap closes
• guard cell
• One of a pair of cells that define a stoma across the
epidermis of a leaf or stem
Stomata in Action
Stomata in Action
Fig. 26.8a, p. 422
Stomata in Action
stoma
A Cuticle (gold) and stoma
on a leaf. The stoma is an
opening between two
specialized epidermal cells
called guard cells.
Fig. 26.8a, p. 422
Stomata in Action
Fig. 26.8b, p. 422
Stomata in Action
B This stoma is open. When
the guard cells swell with
water, they bend so that a gap
opens between them.
The gap allows the plant to
exchange gases with air. The
exchange is necessary to keep
metabolic reactions running.
guard cells
Fig. 26.8b, p. 422
Stomata in Action
Fig. 26.8c, p. 422
Stomata in Action
C This stoma is closed. The
guard cells, which are not
plump with water, are collapsed
against each other so there is
no gap between them.
A closed stoma limits water
loss, but it also limits gas
exchange, so photosynthesis
and respiration reactions slow.
Fig. 26.8c, p. 422
Stomata in Action
Fig. 26.8d,e, p. 422
solutes water
Stomata in Action
D How do stomata open and
close? When a stoma is open, the
guard cells are maintaining a
relatively high concentration of
solutes by pumping solutes into
their cytoplasm. Water diffusing
into the hypertonic cytoplasm
keeps the cells plump.
ABA signal
solutes
water
E When water is scarce, a hormone
(ABA) activates a pathway that
lowers the concentrations of
solutes in guard cell cytoplasm.
Water follows its gradient and
diffuses out of the cells, and the
stoma closes.
Fig. 26.8d,e, p. 422
Factors Affecting Stomata
• Water availability, the level of carbon dioxide inside the leaf,
and light intensity affect whether stomata open or close
• Examples:
• Light causes guard cells to pump potassium ions into their
cytoplasm; the stoma opens to begin photosynthesis
• Root cells release abscisic acid (ABA) when soil water
becomes scarce; binding in guard cells closes stoma
Smog and Stomata
• Stomata close in response to some chemicals in polluted air
• Closure protects the plant from chemical damage, but also
prevents uptake of carbon dioxide for photosynthesis, and so
inhibits growth
Smog and Stomata
Smog and Stomata
Fig. 26.9a, p. 423
Smog and Stomata
Fig. 26.9b, p. 423
Smog and Stomata
Fig. 26.9c, p. 423
Key Concepts
• Water Loss Versus Gas Exchange
• A cuticle and stomata help plants conserve water
• Closed stomata stop water loss but also stop gas
exchange
• Some plant adaptations are trade-offs between water
conservation and gas exchange
ANIMATION: Stomata
26.6 Movement of
Organic Compounds in Plants
• Phloem distributes the organic products of photosynthesis
through plants
• Phloem is a vascular tissue with organized arrays of
conducting tubes, fibers, and strands of parenchyma cells
• Sieve tubes that conduct dissolved organic compounds in
phloem consist of living cells
Sieve Plates
• Sieve-tube cells are
positioned side by side
and end to end
• Their abutting end walls
(sieve plates) are
porous
Sieve Tubes in Phloem
Sugar Transport
• Companion cells actively transport the organic products of
photosynthesis (sugars) into sieve tubes
• Sugars travel through sieve tubes to all other parts of the
plant, where they are broken down for energy, remodeled into
other compounds, or stored for later use
• Sucrose is the main carbohydrate transported in phloem
Translocation
• Organic compounds in phloem flow from a source (region
where companion cells load molecules into sieve tubes) to a
sink (region where molecules are being used or stored)
• Translocation
• Process that moves organic molecules through phloem
Pressure Flow Theory
• A pressure gradient drives the movement of fluid in phloem
• pressure flow theory
• Explanation of how flow of fluid through phloem is driven
by differences in pressure and sugar concentration
between a source and a sink
Steps in Pressure Flow Theory
1. Companion cells load sugars into sieve-tube members by
active transport
2. Solute concentration in sieve tubes increases, so water
moves in by osmosis – increased fluid volume increases
internal pressure (turgor)
3. High pressure pushes fluid toward sink regions
4. Pressure and solute concentrations decrease as fluid moves
from source to sink
5. Sugars are unloaded at sink regions; water follows by
osmosis
Translocation by Pressure Flow
Translocation by
Pressure
Flow
interconnected
sieve tubes
SOURCE
(e.g., mature
leaf cells)
1 Solutes move into a
WATER
sieve tube against their
concentration gradients
by active transport.
3 The pressure difference
pushes the fluid from the
source to the sink. Water
moves into and out of the
sieve tube along the way.
2
increase in solute
concentration, the fluid in
the sieve tube becomes
hypertonic. Water moves
in from the surrounding
xylem, increasing phloem
turgor.
flow
4
5 Solutes are unloaded
from the tube into sink
cells, which become
hypertonic with respect
to fluid in the tube. Water
moves from the sieve
tube into sink cells.
2 As a result of the
SINK
(e.g., developing
root cells)
4 Both pressure
and solute
concentrations gradually
decrease as the fluid
moves from source to
sink.
Fig. 26.12, p. 425
Translocation by
Pressure
Flow
interconnected
sieve tubes
SOURCE
(e.g., mature
leaf cells)
1 Solutes move into a
WATER
sieve tube against their
concentration gradients
by active transport.
3 The pressure difference
pushes the fluid from the
source to the sink. Water
moves into and out of the
sieve tube along the way.
5 Solutes are unloaded
from the tube into sink
cells, which become
hypertonic with respect
to fluid in the tube. Water
moves from the sieve
tube into sink cells.
2 As a result of the increase
flow
in solute concentration, the
fluid in the sieve tube
becomes hypertonic. Water
moves in from the
surrounding xylem,
increasing phloem turgor.
4 Both pressure
and solute concentrations
gradually decrease as the
fluid moves from source
to sink.
SINK
(e.g., developing
root cells)
Stepped Art
Fig. 26.12, p. 425
ANIMATION: Pressure Flow Hypothesis
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Sieve-Tube Pressure
• Honeydew exuding from
an aphid after the
insect’s mouthparts
penetrated a sieve tube
Key Concepts
• Distribution of Organic Molecules Through Plants
• Phloem distributes organic products of photosynthesis
from leaves to living cells throughout the plant
• Organic compounds are actively loaded into conducting
tubes at sources, then unloaded in sinks
Mean Green Cleaning Machines (revisited)
• With elemental pollutants such as lead or mercury, the best
phytoremediation strategies use plants that take up toxins and
store them in aboveground tissues
• With organic toxins such as TCE, the best phytoremediation
strategies use plants with biochemical pathways that break
down the compounds to less toxic molecules
Animation: Translocation in Phloem