Download Transport in Plants

Transport in Plants
Chapter 36
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• To get onto land, plants evolved way
to keep from drying out, to stand
upright.
• Transport nutrients and water both
over long distance and short
distances.
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• At cellular level - plasma membrane
allows for transport into cell
(transport proteins).
• Some transport proteins act as
selective channels - determine what
can go into/out of cell.
• Plant cell - proton pumps function in
pumping H+ ions out of cell.
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• Proton pump can aid in cotransport
- H+ is pumped out of cell aiding in
pumping in/out (against
concentration gradient) of another
substance (glucose)
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• Plants rely on osmosis to survive.
• Direction of water movement
depends on solute concentration
and physical pressure. (water
potential)
• Water moves from high water
potential to low water potential.
• Water potential measured in MPa abbreviated psi.
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• Applying pressure to water can
reverse movement of water.
• Using syringe (negative pressure)
can force water to move upwards.
• Combined effects of pressure and
solute concentrations on water
potential are incorporated into
equation: psi = psiP (pressure potential
+ psis (solute potential)
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• Flaccid cell, psip = 0.
• Placed in solution with lower psi,
water will leave cell.
• Cell will plasmolyze, shrinking and
pulling away from wall.
• As cell swells, it will push against
wall, producing turgor pressure.
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• Placed in pure water - cell will have
lower water potential due to solutes
and water will enter cell.
• Walled cell with greater solute
concentration than its surroundings
will be turgid or firm.
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• Aquaporins are specific transport
proteins - aid in passive movement
of water only.
• Cell wall gives plants shape, but not
passing of materials.
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• Membrane that bounds vacuole
(tonoplast) regulates molecular
traffic between cytosol and
contents of vacuole (cell sap)
• Plasmodesmata (connections
between cells) connect symplast
(cytoplasm stream)
• Cell walls of adjacent plant cells apoplast.
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• Because of distance water and nutrients
need to travel between roots and leaves,
simple diffusion not efficient enough.
• Water and solutes move through xylem
vessels and sieve tubes by bulk flow,
movement of fluid driven by pressure.
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• Tension allows for transport of
materials.
• Transpiration forces water to move
up plant in stream (negative
pressure) - allows materials to move
in bulk.
• Larger diameter of stem, faster
material can move.
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Absorption of water by roots
• Water, mineral salts from soil
enter plant through epidermis of
roots, cross root cortex, pass into
stele, then flow up xylem vessels to
shoot system.
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Fig. 36.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• Much of absorption of water and
minerals occurs near root tips epidermis is permeable to water
and where root hairs are located.
• Root hairs allow for maximum
uptake.
• Most plants form partnerships with
symbiotic fungi for absorbing water
and minerals from soil.
• Water, minerals in root cortex
cannot be transported to rest of
plant until they enter xylem of
stele.
• Endodermis, innermost layers of
root cortex, surrounds stele, is last
checkpoint for absorption into
vascular tissue.
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•
•
Transport of Xylem
Xylem sap flows into veins of leaf
providing them with water.
Plants lose water through
transpiration; water replaced
through water transport.
Xylem sap rises against gravity
through pumping system.
Accumulation of minerals in stele
lowers water potential; generates
positive pressure (root pressure)
forces fluid up xylem.
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•
•
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• Root pressure causes guttation exudation of water droplets (seen
in morning on tips of grass blades)
• Roots accumulate water during
night, transpiration is low, water
enters leaf at faster rate.
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• Xylem sap pulled through plant creating
stream of water - cannot be broken.
• Cavitation (formation of water vapor
pockets in xylem vessel) breaks chain.
• Occurs when xylem sap freezes in
water.
• Cannot be fixed in trees, but stream can
form around it.
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Control of transpiration
• Guard cells control amount of water
lost during day (through stomata).
• Transpiration also cools plant down.
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• When transpiration exceeds
delivery of water by xylem, (soil
begins to dry out) leaves begin to
wilt as cells lose turgor pressure.
• Guard cells control diameter of
stoma by changing shape, widening
or narrowing gap between 2 cells.
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• Potassium helps in regulation of
guard cells.
• Stomata open during day, closed at
night to minimize water loss when
too dark for photosynthesis.
• Regulated in 3 ways.
• 1st - blue-red wavelengths signal
plant to start photosynthesizing.
• 2nd - depletion of CO2.
• 3rd - internal clock in plant cues
plant to start photosynthesizing started at dawn.
• Opening and closing cycle of
stomata is an example of circadian
rhythm, cycles that have intervals
of approximately 24 hours.
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• Plants adapted to arid climates
(xerophytes) - leaf modifications
that reduce rate of transpiration.
• Some -smaller, thicker leaves.
• Some - shed leaves during
extremely dry months.
• Some - stomata concentrated on
lower (shady) leaf surface.
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Phloem sap
• Phloem transports organic products
of photosynthesis throughout plant
via translocation.
• Phloem sap - aqueous solution sugar (mostly disaccharide sucrose)
most abundant solute.
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• Xylem - unidirectional movement;
phloem movement - variable.
• Sieve tubes carry food from sugar
source to sugar sink.
• Sugar source - plant organ
(especially mature leaves) where
sugar is being produced by either
photosynthesis or the breakdown of
starch.
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• Sugar sink - organ (growing roots,
shoots, or fruit) - net consumer or
storer of sugar.
• Storage organ (like a tuber) can be
sink in summer (storing for winter)
but source during beginning of
spring.
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