Download Transport in Vascular Plants

Transport in Vascular
Plants
Chapter 36
p. 738-755
Proton Pumps in Plant Cells

Move H+ ions OUT of cell
(inside cell more negative)


“Membrane Potential”
Creates potential energy
that can be used to do
work:

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
1) Helps roots take up K+
ions into cell
2) Cotransport of ions (NO3-)
3) Cotransport of neutral
substances (sucrose)
Transport of H2O & Solutes
H2O will move from area of ↓ [solutes] to
area of ↑ [solutes]
 Plant cells have cell walls that create physical
pressure within a cell
 Water Potential (Ψ): created by [solutes] and
physical pressure



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Determines direction of H2O flow (in or out of cell)
H2O moves from ↑ Ψ to ↓ Ψ
Measured in Megapascals (MPa) – pure H2O is
0.0MPa
How Solutes & Pressure Affect Ψ
Ψ = Ψs + Ψp
 Ψs = Solute/Osmotic
Potential = amount of
dissolved solutes



By adding solutes, you ↓ ability
of H2O to do work, therefore Ψs is
ALWAYS negative
Ψp = Pressure Potential
= physical pressure on a
solution

Can be + or – compared to
atmos. pressure
Water Uptake & Loss in Plants

If cell (Ψ=-0.7MPa) is placed pure H2O, water
moves INTO cell


If cell (Ψ=-0.7MPa) is placed in sucrose solution,
water moves OUT of cell


Cell becomes turgid
Cell plasmolyzes & plant will wilt
Aquaporins: proteins through which H2O travels
across membranes

Control rate of H2O flow
Plant Cell Compartmentalization

Plant cells have 3 compartments, separated by
membranes:

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
1) Cell Wall
2) Cell Membrane
3) Vacuole



Surrounded by tonoplast membrane
Cell Sap: contents of vacuole
Proton pumps move H+ ions INTO vacuole, creating gradient
Short-Distance Transport

Symplast: continuum of cytosol btwn cells
connected by plasmodesmata


Apoplast: continuum of cell walls & extracellular
space btwn cells


Move cell to cell
Move location
Transmembrane Route: mvmnt of substances
across cell walls into neighboring cells
Long Distance Transport

Bulk Flow: mvmnt of fluids driven by
pressure in xylem & phloem


1) in Xylem: transpiration creates (–)
pressure in leaves, pulling xylem sap upwards
2) in Phloem: loading sugar into sieve tubes
creates (+) pressure at leaf end

Forces sap into rest of plant body
Functions of Transport in Plants

Plants must use intracellular, short
distance, and long distance transport to
perform:




1)
2)
3)
4)
H2O & mineral absorption in roots
Mvmnt of xylem sap upwards
Control of transpiration
Mvmnt of organic nutrients w/in phloem
H2O & Mineral Absorption in Plants

1) H2O & minerals move
through root epidermis &
cortex via apoplastic or
symplastic pathways


Some H2O & minerals
move from apoplastic to
symplastic as transported
inward
Mycorrhizae: symbiotic
structure of roots & fungi
that help ↑ H2O & mineral
absorption from soil
H2O & Mineral Absorption in Plants
(con’t)

2) At endodermis, Caspian Strip creates
waxy barrier that blocks transport of
materials outside symplastic pthwy


H2O & minerals not already in symplastic
pthwy can not cross into vascular cylinder
3) H2O & minerals move into xylem
vessels & upward throughout plant
Movement of Xylem Sap Upwards

Xylem sap is moved in 2 ways:

1) As H2O & minerals enter xylem vessels in
roots, creates (+) root pressure, pushing sap
upwards


Minor effect
2) As H2O transpires from leaves, creates (-)
pressure, pulling sap upwards


Major effect
Uses Transpiration-Cohesion-Tension Mechanism
Transpiration-Cohesion-Tension

1) Transpirational Pull: as H2O diffuses into air
pockets of mesophyll, it evaporates & is lost thru
stomata


Creates –Ψ in leaves, which keeps pulling H2O in
direction out stoma
2) Cohesion-Adhesion: cohesion of H2O
molecules to each other & adhesion to cell walls
pulls xylem sap ↑

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Only works if H2O chain is unbroken
If air bubble forms, vessel is useless & H2O must be redirected
...only youngest, outermost secondary xylem actively transports
H 2O
Control of Transpiration

# stomata & size of pores regulate H2O loss in
plants
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Guard cells change shape when take in K+
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Affected by both genetics & evolution (i.e.: CO2 levels in
atmosphere)
K+ in: guard cells bend, stomata open
K+ out: guard cells straighten, stomata close
Stomata close at night, open during day because:

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1) light stimulates accumulation of K+ in guard cells
2) ↓ CO2 in leaf air spaces
3) “Internal Clock” of guard cells
Stomata may close during day if under environmental stress (i.e.:
H2O deficiency)
Xerophyte Transpiraiton

Xerophyte: plant adapted to dry, arid
climates

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small, thick leaves
thick cuticle
reflective or hairy leaves that trap H2O
CAM Plants:

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Mesophyll incorporates CO2 into organic cmpds
at night (stomata open)
During day, org. cmpds release CO2 into plant
for photosynthesis (stomata close)
Translocation of Organic Nutrients in
Phloem

Phloem sap ALWAYS flows from sugar
source → sugar sink


Sugar Sink: plant organ that consumes
sugars (i.e.; root, stem, fruit, etc.)
Driven by (+) pressure created at source end
Sugars (sucrose) move from mesophyll →
sieve tube cells via symplastic & apoplastic
pthwys
 Sucrose cotransports with H+ ions into
sieve tube & companion cells
