Transcript Chapter 38

CHAPTER 38
LECTURE
SLIDES
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Transport Mechanisms
• Water first enters the roots
• Then moves to the xylem
– Innermost vascular tissue
• Water rises through the xylem because of
a combination of factors
• Most of that water exits through the
stomata in the leaves
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• Water and dissolved minerals travel great
distances in xylem
– Some “pushing” comes from pressure of
water entering roots
– Most of the force is “pulling” created by
transpiration
• Evaporation from thin films of water in the stomata
• Occurs due to cohesion (water molecules stick to
each other) and adhesion (stick to walls)
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• Osmosis
– If a single plant cell is placed into water
• Concentration of solutes inside cell greater than
solution
• Water moves into cell by osmosis
• Cell expands and becomes turgid
– If cell placed in high concentration of sucrose
• Water leaves cell
• Cell shrinks – plasmolysis
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• Water potential has 2 components
1. Physical forces such as plant cell wall or
gravity
• Contribution of gravity usually not considered
• Turgor pressure resulting from pressure against
cell wall
• Concentration of solute in each solution
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• Once thought water
moved across plasma
membranes only by
osmosis through the
lipid bilayer
– Water moved more
rapidly than predicted
• Osmosis is enhanced
by membrane water
channels called
aquaporins
– Speed up osmosis but
do not change direction
of water movement
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• Most of the water absorbed by
the plant comes in through the
region of the root with root
hairs
– Surface area further increased
by mycorrhizal fungi
• Once absorbed through root
hairs, water and minerals
must move across cell layers
until they reach the vascular
tissues
• Water and dissolved ions then
enter the xylem and move
throughout the plant
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• Eventually on their journey inward,
molecules reach the endodermis
• Any further passage through the cell walls
is blocked by the Casparian strips
• Molecules must pass through the cell
membranes and protoplasts of the
endodermal cells to reach the xylem
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• Because the mineral ion concentration in
the soil water is usually much lower than it
is in the plant, an expenditure of energy
(ATP) is required for these ions to
accumulate in root cells
• Plasma membranes of endodermal cells
contain a variety of protein transport
channels, through which proton pumps
transport specific ions against even larger
concentration gradients
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Xylem Transport
• Root pressure is caused by the continuous
accumulation of ions in the roots
– When transpiration from leaves is low or absent – at
night
• Causes water to move into plant and up the
xylem despite the absence of transpiration
• Guttation (production of dew) is loss of water
from leaves when root pressure is high
• Root pressure alone, however, is insufficient to
explain xylem transport
– Transpiration provides the main force
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• Cavitation
– An air bubble can break the tensile strength of
a water column
– A gas-filled bubble can expand and block the
tracheid or vessel
– Damage can be minimized by anatomical
adaptations
• Presence of alternative pathways
• Pores smaller than air bubbles
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• Tracheids and vessels are essential for
the bulk transport of minerals
– Ultimately the minerals are relocated through
the xylem from the roots to other metabolically
active parts of the plant
– Phosphorus, potassium, nitrogen, and
sometimes iron
– Calcium
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Rate of Transpiration
• Over 90% of the water taken in by the
plant’s roots is ultimately lost to the
atmosphere
• At the same time, photosynthesis requires
a CO2 supply from the atmosphere
• Closing the stomata can control water loss
on a short-term basis
• However, the stomata must be open at
least part of the time to allow CO2 entry
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• Guard cells
– Only epidermal cells containing chloroplasts
– Have thicker cell walls on the inside and
thinner cell walls elsewhere
• Bulge and bow outward when they become turgid
• Causing the stomata to open
– Turgor in guard cells results from the active
uptake of potassium (K+), chloride (Cl)
– Water enters osmotically
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• Active pumping of sucrose out of guard
cells in the evening leads to loss of turgor
and closes the guard cell
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Rate of Transpiration
• Transpiration rates increase with
temperature and wind velocity because
water molecules evaporate more quickly
• Several pathways regulate stomatal
opening and closing
– Abscisic acid (ABA) initiates a signaling
pathway to close stomata in drought
• Opens K+, Cl– channels
• Water loss follows
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• Other pathways regulating stomata
– Close when CO2 concentrations are high
– Close when temperature exceeds 30º–34ºC
and water relations unfavorable
• Alternative photosynthetic pathways, such as
Crassulacean acid metabolism (CAM), reduce
transpiration
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Water Stress Responses
• Many morphological adaptations allow
plants to limit water loss in drought
conditions
– Dormancy
– Loss of leaves – deciduous plants
– Covering leaves with cuticle and wooly
trichomes
– Reducing the number of stomata
– Having stomata in pits on the leaf surface
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• Plants have adapted to flooding conditions
which deplete available oxygen
– Flooding may lead to abnormal growth
– Oxygen deprivation most significant problem
– Plants have also adapted to life in fresh water
• Form aerenchyma, which is loose parenchymal
tissue with large air spaces
• Collect oxygen and transport it to submerged parts
of the plant
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• Plants such as mangroves grow in areas
flooded with salt water
– Must supply oxygen to submerged roots and
control salt balance
– Pneumatophores – long, spongy, air-filled
roots that emerge above the mud
• Provide oxygen to submerged roots
– Succulent leaves contain large amount of
water to dilute salt
– May secrete salt or block salt uptake
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• Halophytes
– Plants that can tolerate soils with high salt
concentrations
– Some produce high concentrations of organic
molecules in their roots
• This decreases the water potential enhancing
water uptake from the soil
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Phloem Transport
• Most carbohydrates produced in leaves
are distributed through phloem to rest of
plant
• Translocation provides building blocks for
actively growing regions of the plant
• Also transports hormones, mRNA, and
other molecules
– Variety of sugars, amino acids, organic acids,
proteins, and ions
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phloem
fluid
Stylet
Phloem
a.
400 µm
b.
25 µm
a: © Andrew Syred/Photo Researchers, Inc.; b: © Bruce Iverson Photomicrography.
• Using aphids to obtain the critical samples
and radioactive tracers to mark them, plant
biologists have demonstrated that
substances in phloem can move
remarkably fast, as much as 50 to 100
cm/h
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• Phloem-loading occurs at the source
– Carbohydrates enter the sieve tubes in the
smallest veins at the source
– Sieve cells must be alive to use active
transport to load sucrose
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