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8/8/2019 Transportation in Plants-Mine
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Transportation in Plants
The mechanisms of the
movement of water and glucose
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Water Potential
It is water potentialcombined with adhesive
and cohesive forces that
allow water to move UP the
plant against the pull of
gravity
TRANSPIRATION is the
escape of water molecules
in the form of vapor that
contributes most totransporting water from the
roots to the shoots
Highest water
potential
Lowest waterpotential
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Water Potential
Pure water has a waterpotential of zero
The more solutes present,the more negative thepotential
Water moves from areas
of high water potential tolow water potential
Water moves in byosmosis from the roothairs.
Thus the soil has as higherwater potential than theroot hair
The outside airsurrounding the leaf hasthe least water potential ofall!
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Mechanics ofWaterMovement
Water moves by two different pathways toward the center ofthe root.
Apoplast--THROUGH cell walls without ever entering thecells!
Symplast--THROUGH the plasmodesmata between cells
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Mechanics ofWaterMovement
The pathway along apoplast involves the
movement of water through the nonliving
portions of cells
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Mechanics ofWaterMovement
The pathway through symplast involves the living
portion of the cells.
It moves from the cytoplasm of one cell to the cytoplasm
of another cell through PLASMODESMATA
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Mechanics ofWaterMovement
When water reaches the endodermis, it can
continue to the vascular cylinder ONLY thorough
the symplast pathway.
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Figure 36.6 Compartments of plant cells and tissues and routes for lateral transport
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Mechanism ofWaterMovement
1. OSMOSIS
Water moves into root hairs by osmosis from the
soil
The gradient is established by
Constant movement of water out of soil
Higher mineral concentration in the stele by selective
passage of ions through the endodermis
Root pressurethe movement of water into the
root by this concentration gradient forces water
UP the xylem
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Mechanism ofWaterMovement
This root pressure isevidenced by Guttation
Guttation is when tinydroplets of water and
minerals (sap) is foundat the ends of leaves ofgrasses and small herbsin the early morning.
Root pressure is NOTstrong enough to have amajor effect on watertransport in largerplants!
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Mechanism ofWaterMovement
2. CAPILLARY ACTION (capillarity)
The rise of liquids in narrow tubes
The height of the column depends on an equilibrium ofthe forces of adhesion, cohesion and gravity
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Mechanism ofWaterMovement
Cohesionattractive force
between like molecules
Adhesionattractive force
between unlike molecules
Although capillarity aids in
the movement of water UP
a plant, it is transpirationthat is the dominant
driving force for water
movement
Water exhibits hydrogen
bonding so it has a
strong cohesive force
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Mechanism ofWaterMovement
As water climbs due to adhesion, itbrings other water molecules along due
to its cohesive force as if the moleculeswere holding hands
Water does NOT form a meniscus inxylem which is WHY capillarity is not the
driving force for water moving up a plant
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Figure 36.0x Trees
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Mechanism ofWaterMovement
3. COHESION-TENSIONTheory
Root pressure andcapillarity make minor
contributions to watermovement.
TRANSPIRATIONtheevaporation of water fromplants through openstomata causing negativepressure or tension todevelop within leaves andxylem tissue.
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Mechanism ofWaterMovement
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Mechanism ofWaterMovement
The role of the stomata is to balance the intake
of CO2 and the loss of H2O
In other words, balance the rate of
photosynthesis with the rate of transpiration
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Mechanism ofWaterMovement
Stomata close when temperature are high
Reduces loss of water
C4 and CAM plants have adaptations to continue
photosynthesis when stomata close.
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Mechanism ofWaterMovement
Stomata open when CO2 concentrations are low
in the leaf
Increases loss of water
Allows photosynthesis to take place
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Mechanism ofWaterMovement
Stomatal opening is accompanied by diffusion of
K+ ions INTO the guard cells
Water follows the K+ ions into the cells making
them turgid
The filling of the
vacuole with first K+ions and then water
causes the guard
cells to widen the
stomatal opening.
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Mechanism ofWaterMovement
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Mechanism of SUGAR Movement
TRANSLOCATION isthe movement ofcarbohydrates
through phloem froma source [leaf] to asink [where the foodis utilized like a stemor root]
It is described by thepressure flowhypothesis
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Mechanism of SUGAR Movement
1. Sugars enter sieve-tubemembers
Soluble carbohydrates
(glucosenot starch)move from site ofphotosynthesis (palisadeor spongy mesophyll) byactive transport
This establishes aconcentration gradient inthe sieve-tube membersthat is HIGHER than thatat the sink (a root)
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Mechanism of SUGAR Movement
2. Water enters the sieve-
tube members as a
result of the solutes
moving in What happens to the
pressure within the
tube?
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Mechanism of SUGAR Movement
3. Pressure in the sieve-tube
members at the source moves
water and sugars to the sieve-
tube members at the sink
through sieve-tubes. In other
words.
When water floods the sieve-tube,
the pressure builds
So, water is forced to keepmoving since the cell wall is too
rigid to expand
There is nothing for water to do
but move through the nearest
escape hatch
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Mechanism of SUGAR Movement
4. Pressure is reduced in sieve-tube members at the sink assugars are removed forutilization by nearby cells.
As water and sugars move by
bulk flow from source
sink,pressure builds at the sink
But, a sink is an area wherecarbohydrates are being utilized
So, sugar is being removed byactive transport
This increases the waterconcentration within the sieve-tube members
Water then diffuses OUT of thecell into the xylem relieving thepressure
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Mechanism of SUGAR Movement
Any cell can act as a
sink IF it stores sugar
as starch since starch
is INSOLUBLE Any cell can act as a
source IF it breaks
down starch and
forms SOLUBLEsugars
Glucose