Post on 07-May-2018
Plant Nutrition and Transport
Chapter 29
Impacts, Issues
Leafy Cleanup Crews
The EPA is using hybrid plants to remove some
dangerous toxins from highly contaminated sites
– a process known as phytoremediation
29.1 Plant Nutrients
and Availability in Soil
Nutrients
• Elements or molecules essential for an
organism’s growth and survival
Plants require sixteen elemental nutrients
available from soil, water, and air
• Nine macronutrients, required in large amounts
• Seven micronutrients, required in trace amounts
Plant Nutrients
and Deficiency Symptoms
Properties of Soil
Soil consists of mineral particles mixed with
decomposing organic material (humus)
• Water and air in spaces between particles
Mineral particles in soil differ in size (sand, silt,
and clay) which affects compaction
• Clay particles are negatively charged, and can
hold positively charged ions dissolved in water
Soils and Plant Growth
Different soil types affect growth of different plants
• Most plants grow best in soils containing 10 to 20
percent humus
• Soils with equal proportions of sand, silt, and
humus (loams) have the best oxygen and water
penetration
• Swamps and bogs have too much organic matter
How Soils Develop
Soils develop over thousands of years
Most form in layers (horizons) with distinct
properties (soil profiles)
Topsoil (the A horizon) contains the most
organic material
Soil Horizons
Fig. 29-2, p. 495
O HORIZON Fallen leaves and
other organic material littering
the surface of mineral soil
A HORIZON Topsoil, with decomposed
organic material; variably deep [only a few
centimeters in deserts, elsewhere extending
as far as 30 centimeters (1 foot) below the
soil surface]
B HORIZON Compared with A horizon, larger
soil particles, not much organic material,
more minerals; extends 30 to 60 centimeters
(1 to 2 feet) below soil surface
C HORIZON No organic material, but partially
weathered fragments and grains of rock from
which soil forms; extends to underlying
bedrock
BEDROCK
Leaching and Erosion
Leaching
• Process by which water removes soil nutrients
and carries them away
• Fastest in sandy soils
Soil erosion
• A loss of soil under the force of wind and water
• Increases with sparse vegetation and poor
farming practices
Erosion Due to Poor Farming Practices
29.1 Key Concepts
Plant Nutrients and Soil
Many plant structures are adaptations to limited
amounts of water and essential nutrients
The amount of water and nutrients available for
plants to take up depends on the composition of
soil
Soil is vulnerable to leaching and erosion
29.2 How Do Roots
Absorb Water and Nutrients?
Root specializations such as root hairs,
mycorrhizae, and nodules help the plant absorb
water and nutrients
Root Hairs
Root hairs
• Thin extensions of root epidermal cells that
enormously increase surface area available for
absorbing water and dissolved mineral ions
• New root hairs constantly form just behind the
root tip
Mycorrhizae
Mycorrhizae
• Forms of mutualism between root and fungi in
which both species benefit
• Fungal hyphae share minerals absorbed from soil
• Root cells provide fungus with food
Root Nodules
Root nodules
• Masses of root cells infected with bacteria that fix
atmospheric nitrogen into a form usable by plants
(nitrogen fixation)
• A mutualism between certain types of soil
bacteria and legumes
Root Specializations
How Roots Control Water Uptake
Osmosis drives water from soil into the walls of
parenchyma cells of the root cortex
Water enters cell cytoplasm by diffusion or
through aquaporins; active transporters pump
dissolved mineral ions into cells
Water and ions move from cell to cell through
plasmodesmata
The Casparian Strip
Endodermis between the cortex and vascular
cylinder secretes a waxy substance which forms
a waterproof band (Casparian strip) between
plasma membranes of endodermal cells
The Casparian strip forces water and ions to
enter the vascular cylinder through
plasmodesmata or through endodermal cell
membranes (controlled by transport proteins)
Exodermis
Exodermis
• A layer of cells just below the root surface that
can deposit a Casparian strip that functions like
the one next to the vascular cylinder
Control of Water and Ion Uptake
by Transport Proteins
Fig. 29-5a, p. 497
Fig. 29-5a, p. 497
vascular cylinder
epidermis
endodermis
primary
phloem
primary
xylem
cortex
A In roots, the
vascular cylinder’s
outer layer is a sheet
of endodermis, one
cell thick.
Fig. 29-5b, p. 497
Fig. 29-5b, p. 497
vascular cylinder
tracheids
and vessels
in xylem
sieve tubes
in phloem
B Parenchyma cells that
make up the layer secrete a
waxy substance into their
walls wherever they touch.
The secretions form a
Casparian strip, which
prevents water from
seeping around the cells
into the vascular cylinder.endodermal cell
Casparian strip
Fig. 29-5c, p. 497
Fig. 29-5c, p. 497
C Water and ions can
only enter the
vascular cylinder by
moving through cells
of the endodermis.
They enter the cells
via plasmodesmata or
via transport proteins
in the cells’ plasma
membranes.
Vascular
cylinder
Casparian
strip
water and nutrients
Cortex
29.3 How Does Water
Move Through Plants?
The upward movement of water through xylem,
from roots to leaves, is driven by two properties
of water: evaporation and cohesion
Tracheids and vessel members
• Water conducting tubes of xylem
• Cells are dead at maturity
• Lignin-impregnated walls remain
Tracheids and Vessel Members
Fig. 29-6a, p. 498
Fig. 29-6a, p. 498
perforation
in the side
wall of
tracheid
a Tracheids have
tapered, unperforated
end walls. Perforations
in the side walls of
adjoining tracheids
match up.
Fig. 29-6b, p. 498
Fig. 29-6b, p. 498
vessel member
b Three adjoining vessel
members. The thick, finely
perforated end walls of dead cells
connect to make long tubes that
conduct water through xylem.
Fig. 29-6c, p. 498
Fig. 29-6c, p. 498
perforation
plate
c Perforation plate at the end
wall of one type of vessel
member. The perforated ends
allow water to flow freely
through the tube.
Cohesion-Tension Theory
Continuous negative pressure (tension) created
by evaporation of water from leaves and stems
(transpiration) pulls water upward through xylem
Hydrogen bonds among water molecules
(cohesion) in continuous columns inside xylem
tubes keep water from breaking into droplets
Cohesion-Tension Theory
Fig. 29-7a, p. 499
Fig. 29-7a, p. 499
A The driving force of
transpiration
Evaporation of water
molecules from above
ground plant parts puts
water in xylem into a
state of tension that
extends from roots to
leaves. For clarity,
tissues inside the vein
are not shown.
mesophyll
(photosynthetic cells) upper epidermisvein
stoma
Fig. 29-7b, p. 499
Fig. 29-7b, p. 499
B Cohesion of water
inside xylem tubes
Even though long
columns of water that
fill narrow xylem tubes
are under continuous
tension, they resist
breaking apart. The
collective strength of
many hydrogen bonds
keeps individual water
molecules together.
phloemxylem
vascular
cambium
Fig. 29-7c, p. 499
Fig. 29-7c, p. 499
C Ongoing water
uptake at roots
Water molecules
lost from the plant
are being
continually
replaced by water
molecules taken
up from soil.
Tissues in the vein
not shown.
cortex
vascular
cylinder endodermis
water
molecule
root hair
cell
29.2-29.3 Key Concepts: Water Uptake
and Movement Through Plants
Certain specializations help roots of vascular
plants take up water and nutrients
Xylem distributes absorbed water and solutes
from roots to leaves
29.4 How Do Stems
and Leaves Conserve Water?
Water is an essential resource for all land plants
Water-conserving structures (cuticle and
stomata) and processes are key to the survival
of land plants
The Water-Conserving Cuticle
Cuticle
• A translucent, water-impermeable layer coating
the walls of all plant cells exposed to air
• Consists of epidermal cell secretions: waxes,
pectin, and cellulose fibers embedded in cutin
Controlling Water Loss at Stomata
Stomata
• Openings through the plant epidermis that
regulate water vapor loss and gas exchange
• Formed by two guard cells
Guard cells open or close the stoma depending
on the amount of water in their cytoplasm
• Swollen cells open stoma
• Collapsed cells close stoma
Controlling Water Loss at Stomata
Environmental cues open or close stomata
• Water availability (abscisic acid released by root
cells)
• Carbon dioxide levels in leaf (aerobic respiration)
• Light intensity (triggers potassium pumps)
• Air pollution (prevents photosynthesis)
Stomata and Industrial Smog
29.4 Key Concepts
Water Loss Versus Gas Exchange
A cuticle and stomata help plants conserve
water, a limited resource in most land habitats
Closed stomata stop water loss but also stop
gas exchange
Some plant adaptations are trade-offs between
water conservation and gas exchange
29.5 How Do Organic Compounds
Move Through Plants?
Phloem distributes the organic products of
photosynthesis through plants
Concentration and pressure gradients in the
sieve-tube system of phloem force organic
compounds to flow to different parts of the plant
Phloem:
Sieve-Tube Members and Sieve Plates
Fig. 29-10a, p. 502
one of a series of
living cells that abut,
end to end, and form
a sieve tube
companion cell (in
the background,
pressed tightly
against sieve tube)
perforated end plate
of sieve-tube cell, of
the sort shown in (b)
Organic Products of Photosynthesis
Plants store carbohydrates as starch, and
distribute them as sucrose and other small,
water-soluble molecules
Pressure-Flow Theory
Translocation
• Gradients set up by companion cells move
organic molecules into sieve tubes at sources,
and unload them at sinks
Pressure-flow theory
• Internal pressure (turgor) builds up in sieve tubes
at a source, pushing solute-rich fluid to a sink,
where sucrose is removed from the phloem
Translocation of Organic Compounds:
Sources and Sinks
Fig. 29-12a, p. 503
Fig. 29-12a, p. 503
Translocation
inter-
connected
sieve tubesSOURCE (e.g.,mature leaf cells)
A Solutes move into a sieve tube against their concentration gradients by active transport.
WATER
B As a result of increased solute concentration, the fluid in the sieve tube becomes hypertonic.
C The pressure difference pushes the fluid from the source to the sink. Water moves into and out of the sieve tube along the way.
flow
D Both pressure and solute concentrations gradually decrease as the fluid moves from source to sink.
E Solutes are unloaded into sink cells, which then become hypertonic with respect to the sieve tube. Water moves from the sieve tube into sink cells.
SINK (e.g., developing root cells)
Fig. 29-12b, p. 503
Fig. 29-12b, p. 503
upper leaf epidermis
photosynthetic cell
sieve tube in leaf
vein
companion cell next
to sieve tube
lower leaf epidermis
Typical source region
Photosynthetic tissue in a leaf
Fig. 29-12c, p. 503
Fig. 29-12c, p. 503
sieve
tube
Typical sink region
Actively growing cells in a young root
29.5 Key Concepts
Sugar Distribution Through Plants
Phloem distributes sucrose and other organic
compounds from photosynthetic cells in leaves
to living cells throughout the plant
Organic compounds are actively loaded into
conducting cells, then unloaded in growing
tissues or storage tissues
Summary:
Processes that Sustain Plant Growth
Fig. 29-13, p. 504
ATP formation
by roots
absorption of
minerals and
water by roots
transport of minerals
and water to leaves
respiration of
sucrose by rootstransport of
sucrose to rootsphotosynthesis