Download Plant Nutrition and Transport

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
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
Fig. 29-2, p. 495
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
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. 29-5a, p. 497
Fig. 29-5b, p. 497
vascular cylinder
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.
tracheids
and vessels
in xylem
sieve tubes
in phloem
endodermal cell
Casparian strip
Fig. 29-5b, 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.
Casparian
strip
Vascular
cylinder
water and nutrients
Cortex
Fig. 29-5c, p. 497
Animation: Root functioning
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
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. 29-6a, 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. 29-6b, 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.
Fig. 29-6c, p. 498
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
mesophyll
(photosynthetic cells)
vein
upper epidermis
A The driving force of
transpiration
stoma
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.
Fig. 29-7a, p. 499
Fig. 29-7b, p. 499
xylem
vascular
cambium
phloem
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.
Fig. 29-7b, p. 499
Fig. 29-7c, p. 499
vascular
cylinder
endodermis
water
cortex molecule
root hair
cell
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.
Fig. 29-7c, p. 499
Animation: Transpiration
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
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)
Fig. 29-10a, p. 502
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
Translocation
SOURCE
(e.g.,mature
leaf cells)
interconnected
sieve tubes
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
C The pressure
flow
becomes
difference
hypertonic.
pushes the fluid
from the source
D Both pressure
to the sink. Water
and solute
moves into and
concentrations
out of the sieve
gradually
tube along the
decrease as the
way.
fluid moves from
source to sink.
E Solutes are
SINK (e.g.,
unloaded into
developing
sink cells, which
root cells)
then become
hypertonic with
respect to the
sieve tube. Water
moves from the
sieve tube into
sink cells.
Fig. 29-12a, 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. 29-12b, p. 503
Fig. 29-12c, p. 503
sieve
tube
Typical sink region
Actively growing cells in a young root
Fig. 29-12c, p. 503
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
ATP formation
by roots
respiration of
sucrose by roots
absorption of
minerals and
water by roots
transport of
sucrose to roots
transport of minerals
and water to leaves
photosynthesis
Fig. 29-13, p. 504
Animation: Cohesion-tension theory (or
Water transport)
Animation: Interdependent processes
Animation: Soil profile
Animation: Stomata
Animation: Translocation in phloem
Animation: Uptake of nutrients by plants
Animation: Water absorption
Video: Leafy clean-up crews
Video: Sequoias