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Transport in Plants
Chapter 36
• How does
water get to
the top of a
tree?
• How does the
leaf excess
energy
(sugars) get
down to the
roots?
• What
things
move –
• Into the
plant ?
• Out of the
plant?
• Inside the
plant?
Overview
•
•
•
•
Review Osmosis
Movement across membranes
Water Potential equation
Water transport in Xylem
– Into the root
– Up the stem
– Out the leaf
• Stomata functioning
• Phloem transport
Membranes Movement
• Only water (somewhat) and gasses can
move freely across membrane.
• Charged and larger molecules need
protein channels to pass membrane.
• May move with it gradient passively
• May use ATP to actively pump molecules
against their concentration /charge
gradients across membranes.
Types of cellular transport
across membranes
• Protons actively pumped out
• Cations (+) move across by
facilitated diffusion due to membrane
potential (charge gradient).
• Cotransport uses anion(-) gradient
potential to move cations or neutral
molecules across membrane.
Types of cellular transport
across membranes
Fig. 8.12
Fig. 8.11
Water Potential
• Two components
– Concentration (solutes present)
– Pressure (resistance to cell wall, gravity,
and physical pumping)
• Water Potential Ψ=Ψ (solute) + Ψ
(pressure)
• Osmosis
• Reverse osmosis
Water Potential
•
•
•
•
Ψ values measured in MPa
Ψ(total)=Ψ (solute) + Ψ (pressure)
Distilled water has a Ψ (solute)=0
Any concentration of a solute makes Ψ
(solute)=negative
• The higher the concentration, the more
negative Ψ (solute)
• Water moves to more negative Ψ (total)
• Ions/water can be stored in the tonoplast
Root pressure
Leaf
stem
Water Movement in xylem
• Xylem cells are dead and hollow.
• Three stages:
– How water gets in to roots
– How water moves up the stem
– How water leaves the leaf
• Water loss controlled by stomata
Cell to cell water
movement
• Symplast movement
never leaves
cytoplasm
• Apoplast moves from
cytoplasm to cell wall
• Cellulose is
hydrophilic
– Paper towels
Movement in root
• Cellulose wall or Lower Ψ (solute) of root
hair cytoplasm draws water in from soil
– mycorrhizae
• Moves to endodermis
– Casparian strip (waxy layer in cell wall) blocks
apoplast route.
– Water must cross a membrane- controlling Ψ
(solute)
• Cells in Stele actively maintain low Ψ
(solute) drawing water into xylem.
– Serpentine soils
• Rigid xylem walls build up pressure
forcing water up – Root pressure
Fig. 36.7
exodermis
root hair
epidermis
Newly forming
Vascular cylinder
cortex
Casparian
strip (gold)
within all the
abutting walls
of cells of the
endodermis
Guttation
• On dewy
mornings root
pressure may
exceed
evaporation
forcing water
up and out
leaves.
• Root pressure
usually only
loads xylem,
not the major
force in water
movement up
the stem
Movement up stem
• Water is pulled from above by
tension
– negative Ψ (pressure) in the leaves
• Water molecules stick togethercohesion
• Cell walls help support the weight of
the water column by adhesion
• Due to hydrogen bonding
Surface tension- Transpirational Pull
• Each water molecule hydrogen bonds to
four other molecules.
– Water resists stretching out
• Evaporation increase surface area of
water in leaf- increasing its resistance
• Transpiration pulls water up from belowcreating tension, a negative Ψ (pressure)
– Extends down the roots to interface with soil
• Drier soils create even more tension
• Can break water column- cavitation
– Vapor plugs xylem vessel
– New xylem need
– Tracheids in conifers make them more drought
tolerant
Fig. 36.10
Fig 36.11
• Water
always
moves to
the lower Ψ
(total)
• What has
the lowest
Ψ (total) ?
• Orientation of cellulose microfibrils
allows guard cell to move apart when
the uptake water- turgor pressure.
• Fig 36.13a
• Guard cells actively accumulate K+
ions in tonoplast to open stomata.
• Responds to tension in xylem; CO2;
Circadian rhythms; hormonal control
• Guard cells have chloroplasts
• Sugars loaded by cotransport in companion, or
transfer (bundle sheath) cells.
• Creates high concentration in sieve tube
members in source.
• Sugars loaded at
source by active
transport.
• Water moves in
by osmosis,
creates positive
Ψ (pressure)
• Moves by bulk
(pressure) flow
• Sugars unloaded
at sink, less
osmotic pull
water moves out
• Fig. 36.17
Aphids=Phloem miners!!