Chapter 36: Transport in Vascular
Plants
Proton
pump- active transport creates H+ gradient  used to
pump sugar
H2O moves high Y to low Y if no barrier exists
Y=Ys + Yp
Ys pure H2O= 0; Ys negative value otherwise
 YpTurgid=+ ; Ypflaccid=Turgor pressure- membrane against wallhealthy plant cell
Flaccid- cells plasmolyze  plant wilts
Short distance transport options
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Molecule moves across cell membrane &
wall of one cell to cross into another cell
Molecule moves through symplast
(continuum of cytosol due to
plasmodesmata); molecule crosses only 1
membrane & wall
Molecule moves along cell walls &
extracellular spaces of apoplast
(continuum of cell wall) without entering
cytosol of cells it passes
Long distance transport/Bulk Flow
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Movement of fluid driven by pressure
H2O & solutes move through tracheids &
vessels of xylem & through sieve tubes of
phloem
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Phloem- loaded sugar increases pressure &
forces sap to the opposite end of the tube
Xylem- transpiration reduces pressure in leaf
xylem creating tension that pulls xylem sap up
Cells performing bulk flow have no
cytoplasm (dead cells)
Short distance transport in roots
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H2O & minerals enter root epidermis, cross root cortex,
pass into vascular cylinder, & flow up tracheids & vessels
to shoots
Root hairs & mycorrhizae (roots & fungi symbiosis)
increase surface area for uptake of H2O & minerals
Endodermis
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Innermost layer of root cortex cells that surround the vascular
cylinder
Act as last checkpoint for selective passage of materials into
vascular tissue
Materials must pass through a selective membrane to enter
cylinder
H2O & mineral transport: Roots to
shoots
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Transpiration
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Loss of H2O vapor from leaves & other aerial parts
occur because atmospheric Y is lower than leaf Y;
H2O must be replaced from root H2O or plant wilts
Root pressure
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Upward push of xylem sap results when minerals are
pumped into vascular cylinder lowering Y so H2O
flows in; contributes very little to transport of H2O up
to leaves
Guttation (water droplets on leaf) results when more
H2O into xylem than leaf transpires
Transpiration-Cohesion-Tension
Mechanism
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Responsible for pulling xylem sap up to leaves
Transpirational pull
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Negative pressure at air H2O interface moves further
down into xylem; H2O moves from high to low Y
(xylem to mesophyll)
Cohesion & adhesion
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Facilitates long distance transport of xylem sap from
root tips to leaves
Cohesion allows H2O from above to “tug” adjacent
H2O up; relayed down entire xylem
adhesion of H2O to xylem walls resists gravity so H2O
continues up
***Ascent of xylem sap is solar powered
Effects of transpiration
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Rate of transpiration increases in sunny, warm, dry,
windy conditions
Regulating stomate opening size minimizes loss of H2O
to prevent loss of turgor pressure & wilting risk
Evaporative cooling results from transpiration (lowers
leaf temp to prevent enzyme denaturization on hot days)
90% of H2O escapes through stomata
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Density of stomates is genetic & environmental
High light, low CO2 levels during leaf development increase
stomate density
Turgor pressure controls the opening & closing of guard cells
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Turgid guard cells buckle outward- stomata open
Flaccid guard cells become less bowed- stomata close
Changes in turgor pressure result from reversible uptake of
K+
Modifications to reduce transpiration
Xerophytes (arid climate plants)
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Small thick leaves
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Hairy, reflective leaves
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Stomata in depressions to shelter from wind
Shed leaves in dry months
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Trap boundary layer of H2O
Thick cuticle
Stomata concentrated on underside of leaf
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Limits H2O loss by reducing surface area to leaf
volume
Store H2O in fleshy leaves
CAM plants
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CO2 into organic acids at night; convert back to CO2
in daylight
Stomata remain closed during the day
Transport of Organic Nutrients in
the Phloem
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Translocation
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organic nutrients move from sugar source to
sugar sink through sieve tube members by
positive pressure
Leaves=sugar source
 Growing roots, buds, stems, fruits, storage organs
(tubers or bulbs) = sugar sinks
 Sinks receive sugar from nearest source
 Active transport often required to load sugar into
sieve tubes through proton pumps
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