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© Cengage Learning 2015
Biology Concepts and Applications | 9e
Starr | Evers | Starr
© Cengage Learning 2015
Chapter 26
Plant Nutrition and Transport
© Cengage Learning 2015
26.1 Where Do Plants Get the Nutrients
They Require?
• A plant needs sixteen elements to survive
and grow
– Macronutrients: carbon, oxygen, hydrogen,
nitrogen, phosphorous, sulfur, potassium,
calcium and magnesium
– Micronutrients: chlorine, iron, boron,
manganese, zinc, copper, and molybednum
© Cengage Learning 2015
Properties of Soil
• Carbon, oxygen, and hydrogen atoms are
abundantly available in carbon dioxide and
water
• Plants get the other elements they need
when their roots take up minerals
dissolved in soil water
© Cengage Learning 2015
Properties of Soil (cont’d.)
• Soil consists mainly of mineral particles:
– Clay particles: attracts positively charged
mineral ions in soil water
– Sand and silt: intervene between tiny particles
of clay; allows roots to access oxygen
© Cengage Learning 2015
Properties of Soil (cont’d.)
• Soils with the best oxygen and water
penetration are loams
– Have roughly equal proportions of sand, silt,
and clay
© Cengage Learning 2015
Properties of Soil (cont’d.)
• Most plants do best in loams that contain
between 10 and 20 percent humus
– Decomposing organic material (e.g., fallen
leaves, feces) that releases nutrients and
traps minerals
© Cengage Learning 2015
How Soils Change
• Soils develop over thousands of years
– Most form in layers, or horizons, that are
distinct in color and other properties
– Topsoil: uppermost soil layer; most organic
matter and nutrients for plant growth
• Grasslands typically have a deep layer of topsoil;
tropical forests do not
© Cengage Learning 2015
How Soils Change (cont’d.)
O Horizon
A Horizon
B Horizon
C Horizon
Bedrock
© Cengage Learning 2015
How Soils Change (cont’d.)
• Minerals, salts, and other molecules
dissolve in water as it filters through soil
– Leaching: process by which water removes
soil nutrients and carries them away
– Soil erosion: loss of soil under the force of
wind and water
© Cengage Learning 2015
How Soils Change (cont’d.)
© Cengage Learning 2015
26.2 How Do Plant Roots Absorb Water
and Nutrients?
• Water moves from soil, through a root’s
epidermis and cortex, to the vascular
cylinder
– Osmosis drives this movement; fluid in the
plant typically contains more solutes than soil
water
• Xylem distributes water and mineral ions
to the rest of the plant
© Cengage Learning 2015
ANIMATION: Root organization
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© Cengage Learning 2015
How Do Plant Roots Absorb Water and
Nutrients? (cont’d.)
• Soil water enters a root and moves
through cell walls
– Soil water diffuses from epidermis, through
the cortex, reaching the vascular cylinder
– A Casparian strip (waterproof band of plasma
membranes) forces water to enter a vascular
cylinder by passing through endodermal cells
– Water diffuses from cell to cell through
plasmodesmata until it enters xylem
© Cengage Learning 2015
ANIMATION: Root functioning
To play movie you must be in Slide Show Mode
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© Cengage Learning 2015
How Do Plant Roots Absorb Water and
Nutrients? (cont’d.)
• Ions cannot cross the plasma membrane
– Mineral ions dissolved in soil water enter a
cell’s cytoplasm through transport proteins
• Once actively transported into cells,
mineral ions diffuse from cell to cell
through plasmodesmata until they enter
xylem
© Cengage Learning 2015
26.3 What Mutualisms Affect Root
Function?
• Mycorrhiza: mutually beneficial interaction
between a root and a fungus
– Filaments of the fungus (hyphae) form a
velvety cloak around roots or penetrate their
cells
– Root cells get some scarce minerals that the
fungus is better able to absorb
© Cengage Learning 2015
What Mutualisms Affect Root Function?
(cont’d.)
• Mutualism with nitrogen-fixing Rhizobium
– Roots release certain compounds into the soil
that are recognized by compatible Rhizobium
– Roots encapsulate Rhizobium bacteria inside
swellings called root nodules
– Rhizobium in root nodules fix nitrogen gas to
ammonia for plant use
– Plant provides oxygen-free environment and
sugars to the anaerobic Rhizobium bacteria
© Cengage Learning 2015
What Mutualisms Affect Root Function?
(cont’d.)
a root nodule
© Cengage Learning 2015
26.4 How Does Water Move Through
Xylem?
• Water that enters a root travels to the rest
of the plant inside tubes of xylem
– Xylem cells deposit secondary wall material
– Just before the cells die, they digest away
most of their primary wall
– Holes/pits remain in secondary walls where
plasmodesmata once connected living cells
– In mature xylem tubes, water flows laterally
through the pits, between adjacent cells
© Cengage Learning 2015
How Does Water Move Through Xylem?
(cont’d.)
xylem
tube
pit
water
© Cengage Learning 2015
How Does Water Move Through Xylem?
(cont’d.)
• Vessel elements:
– Cells that form in stacks in xylem and die
when mature
– Their pitted walls remain to form water-
conducting tubes
– Each tube consists of a stack of vessel
elements that meet end to end at perforation
plate
© Cengage Learning 2015
How Does Water Move Through Xylem?
(cont’d.)
vessel element
perforation plate
© Cengage Learning 2015
How Does Water Move Through Xylem?
(cont’d.)
• Tracheids:
– Tapered cells of xylem that die when mature
– Their interconnected, pitted walls remain and
form water-conducting tubes
© Cengage Learning 2015
How Does Water Move Through Xylem?
(cont’d.)
tracheid
© Cengage Learning 2015
Cohesion–Tension Theory
• Tracheids and vessel elements that
compose xylem tubes are dead and
cannot pump water upward against gravity
• The movement of water in vascular plants
is driven by two features of water:
– Evaporation and cohesion
© Cengage Learning 2015
Cohesion–Tension Theory (cont’d.)
• Cohesion–tension theory: water in xylem
is pulled upward by air’s drying power,
which creates a continuous negative
pressure called tension
© Cengage Learning 2015
Cohesion–Tension Theory (cont’d.)
• Transpiration: evaporation of water from
aboveground plant parts
– Creates tension that pulls a cohesive column
of water upward through xylem
© Cengage Learning 2015
ANIMATION: Transpiration
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© Cengage Learning 2015
Cohesion–Tension Theory (cont’d.)
• Pits in the sides of vessels and tracheids
are often bordered by pectin-containing
secondary walls
– Pectins shrink when dry, and swell when wet
© Cengage Learning 2015
Cohesion–Tension Theory (cont’d.)
• When mineral-rich water flows through a
pectin-bordered pit, the border swells and
eventually plugs the hole
– Water tends to flow toward the thirstiest
regions of the plant, where pectin-bordered
pits are dry and open
© Cengage Learning 2015
26.5 How Do Land Plants Conserve
Water?
• In land plants, at least 90 percent of water
taken up by roots is lost by evaporation
• A waterproof cuticle helps a land plant
conserve water
– The cuticle also restricts gas exchanges with
air
© Cengage Learning 2015
How Do Land Plants Conserve Water?
(cont’d.)
• Stoma: an opening between a pair of
guard cells (specialized cells of epidermis)
• When guard cells swell with water, they
bend slightly forming a gap (the stoma)
• Open stomata allow gases and water
vapor to cross the epidermis
• When guard cells lose water, they collapse
against one another, closing the stoma
© Cengage Learning 2015
How Do Land Plants Conserve Water? (cont’d.)
guard cells
closed stoma open stoma
© Cengage Learning 2015
How Do Land Plants Conserve Water?
(cont’d.)
cuticle
epidermis stoma
guard cells
© Cengage Learning 2015
How Do Land Plants Conserve Water?
(cont’d.)
• Stomata open or close based on
environmental cues
– Example: light from the sun causes guard
cells to begin pumping potassium ions
• Water follows the ions by osmosis, plumping the
guard cells, opening the stomata
• Carbon dioxide diffuses through the open stomata
into the plant’s tissues, and photosynthesis begins
© Cengage Learning 2015
26.6 How Do Sugars Move Through
Phloem?
• Sieve tubes: sugar-conducting tube of
phloem; consists of stacked sieve
elements
– Sieve elements are living cells that meet end
to end at sieve plates
– Companion cells: parenchyma cells that
provide metabolic support to its partnered
sieve element
© Cengage Learning 2015
How Do Sugars Move Through Phloem?
(cont’d.)
phloem
© Cengage Learning 2015
Pressure Flow Theory
• Sugars are transported via sieve tubes
– Movement of sugars through phloem is called
translocation
© Cengage Learning 2015
Pressure Flow Theory (cont’d.)
• Inside sieve tubes, fluid rich in sugars flow
from a source to a sink because of a
pressure gradient
– Source: region where sugars are produced or
released
– Sink: region where sugars are being broken
down for energy
© Cengage Learning 2015
Pressure Flow Theory (cont’d.)
• Pressure flow theory:
– A difference in turgor between sieve elements
in source and sink regions pushes sugar-rich
fluid through a sieve tube
© Cengage Learning 2015
Pressure Flow Theory (cont’d.)
© Cengage Learning 2015
26.7 Application: Leafy Cleanup
• J-Field, Aberdeen Proving Ground:
– From World War I until the 1970s, the United
States Army tested and disposed of weapons
at this site
– Chemicals, chemical weapons, explosives,
plastics, and solvents were burned in pits
– Lead, arsenic, mercury, and TCE
contaminated the soil and groundwater
© Cengage Learning 2015
Application: Leafy Cleanup (cont’d.)
• To clean up the soil and protect nearby
Chesapeake Bay, the Army and the
Environmental Protection Agency turned to
phytoremediation
– The use of plants to take up and concentrate
or degrade environmental contaminants
© Cengage Learning 2015
Application: Leafy Cleanup (cont’d.)
• Ford Motor Company’s Rouge Center:
decades of steelmaking left soil
contaminated with highly carcinogenic
compounds
– Researchers developed a phytoremediation
system based on native plants
© Cengage Learning 2015
Application: Leafy Cleanup (cont’d.)