Ch:36
Transport in Vascular Plants
By:
Stephanie Tuminello
Patrick Singer
Esther Urena
Melissa Giammarino
Solange Beckles
Transport in Vascular Plants
Occurs at 3 Levels
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1-Transport of water and solutes in
individual cells
2- short distance transport at organ and
tissue level from cell to cell
3-long distance transport in xylem and
phloem at the level of the whole plant
Proton Pumps
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uses ATP to pump H+ across cell membrane through
active transport
Contributes to voltage called membrane potential
Membrane potential is separation of charges across a
membrane
Plants use membrane potential and energy in the
gradient to transport solutes
Cotransport couples passage of one solute to the
passage of another in the opposite direction
In plants sucrose is cotransported with H+
Water potential
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Osmosis is passive transport of water across a membrane
Water potential(Ψ) is the combination of solute
concentration and pressure
Water moves from regions of higher water potential to
lower water potential
Solute potential is proportional to the amount of dissolved
solute particles
Solute particles bind to water molecules and reduce the
amount of free water
Pressure potential is the amount of pressure exerted on
the solution
Water Potentials Effect on
Cells
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Cells that have the same water potential as the
surrounding solution are flaccid
If the solution has a lower water potential than the cell
water will leave the cell and the cell will plasmolyze
When plasmolyed, the protoplast will shrink and pull
away from the cell wall
If the solution has a higher water potential than the cell
water will enter the cell and it will become turgid, the
ideal state for plants
Water often crosses membranes through proteins called
aquaporins
The Three Compartments of Cells
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The plasma membrane is selectively permeable barrier between
the cell wall and the cytosol
Most plant cells also have vacuoles
The tonoplast, which is the vacuolar membrane, regules
molecular traffic between the cytosol and cell sap
The tonoplasts H+ gradient is used to move ions across the
vascular membrane
Plasmodesmata connect the cytosol of neighboring cells the
continuum of cytosol of neighboring cell is the symplast
The continuum of cell walls and the extercelluar space is the
apoplast
3 Routes of Short-Distance
Transport
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In the transmembrane route is when
substances move out of one cell and across
the cell wall to enter the neighboring cell
In the symplastic route solutes and water
move from cell to cell via the plasmodesmata
after entering one cell
In the apoplastic route solutes and water
move along the byways provided by the
continuum of cell walls
Long-Distance Transport
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Bulk Flow, how long distance transport
occurs, is the movement of a fluid driven by
pressure
Bulk Flow occurs through the tracheids and
vessels of the xylem, driven by negative
pressure
Transpiration is the evaporation of water from
a leaf which reduces pressure and creates
tension pulling sap from the roots
Bulk flow occurs in the sieve tubes of the
phloem
Roots Absorb Water and
Minerals From Soil
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Most absorption occurs at root tips where the epidermis is
permeable to water.
Root hairs are extensions of the epidermal cells.
Soil particles coated with water and minerals attach to
root hairs.
This solution then flows along the apoplast into the root
cortex.
Here water and certain solutes are taken up into the
symplast.
Roots and fungi form mycorrhizae, symbiotic structures
consisting of plant roots united with fungal hyphae.
The hyphae absorb minerals and transfers them to the
host plant.
The Endodermis
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The endodermis is the innermost layer of cells in the root
cortex and surrounds the vascular cylinder
Last selective barrier for minerals going to the vascular
tissue
Casparian strips line the transvverse and radial walls of
the vascular cylinder, making it impervious to water
Due to casparian strips minerals must pass through
passively selective plasma membranes
The last part of minerals path from the soil to xylem
pathway is the trachieds and vessels of the xylem, which
lack protoplasts and thereofre are part of the apoplast
The Ascent of Xylem Sap
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At night when transpiration is really low, root
cells continue to pump mineral ions into the
xylem of vascular plants.
Because of the accumulation of minerals due
to the epidermis, there is a lower water
potential within the vascular cylinder.
Root pressure is the upward push of xylem
sap.
When more water enters the leaf than is
transcribed the result is guttation.
Water and Minerals Ascend From
Roots to Shoots Through the Xylem
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Xylem sap flows upward starting at the roots,
then travel throughout the shoot system, into
veins that branch out into each leaf.
Transpiration is the loss of water vapor from
leaves and other aerial parts.
Plants wilt if the water last through
transpiration is not replaced by water
traveling up from the roots.
Xylem Sap Ascent By Bulk Flow
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The movement of fluid in bulk flow is
driven by a water potential difference at
opposite ends of a conduit.
No energy is expended in lifting xylem
sap by bulk flow.
Absorption of sunlight causes water to
evaporate from mesophyll cells, driving
transpiration, lowering water potential
Accent of Xylem Sap
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Cohesion and adhesion facillitate the long-distance
transport of xylem sap from the leaves to the roots into
the soil solution
The cohesion of water due to hydrogen bonding makes it
possible to pull a column of sap from above without the
water molecules separating
The strong adhesion of water molecules to the hydrophilic
walls of xylem cells aids in offsetting the downward pull of
gravity
Transpirational pull can extend down to the roots only
through an unbroken chain of water molecules.
Cavitation is the formation of a water vapor pocket in a
vessel and can cause the chain to break.
Transpirational Pull
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The air in the airspaces are used to express the mesophyll to the
carbon dioxide it needs to perform photosynthesis is saturated with
water vapor because it is in contact with the most walls of the cells
Since the air inside the leaf has a lower water potential then the air
outside the leaf, water vapor in the air enters the space of leaf diffuses
down its water potential gradient and exits the leaf through the
stomata. This is called transpiration
The leading hypothesis as to how loss of water vapor from a leaf
translates into the pulling force for upward movement of water through
a plant is that negative pressure that causes water to move up through
the xylem develops at the air water interface in the mesophyl wall
Transpirational pull depends on some of the special properties that
water possesses, such as adhesion, cohesion, and surface tension
Negative pressure lowers water potential the negative water potential
of leaves provides the pull in transpirational pull
Stomata help regulate the rate
of transpiration
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Leaves have very large surface areas to
increase the rate of photosynthesis
Leaves have about 20 to 30 times more
internal surface area than outside
surface area due to the cells irregular
shape
Effects Of Transpiration On
Wilting And Leaf Temperature
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Usually transpiration draws up water from the
roots quickly enough to replace the water lost
by evaporation
In some extended periods of drought water
cannot be drawn up as fast as it evaporates,
when this happens wilting occurs
Transpiration also causes evaporative cooling
which lowers the temperature of the leaf as
much as 10 or 15 degrees compared with the
surrounding air to prevent enzyme
denaturation
Stomata
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The amount of water lost by a leaf depends
both on the number of stomata and average
size of the pores
The stomatal density is under both
environmental and genetic controlling factors
Guard cells buckle outward when turgid,
increasing the size of the pores between
them, thus more water is lost
Stomatal Opening and Closing
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The active transport of H +out of guard cells creates voltage
that drives k + into guard cells
When k + accumulates stomata open, and close when k + leaves
the cell
A depletion of CO2 in the air spaces of the leaf occurs during the
beginning of photosynthesis in the mesophyll will result in the
stomata opening
The stomata will also open due to the internal clock of the
guard cells
Cycles with intervals of 24 hours are circadian rhythms
Environmental stresses, such as water deficiency, can cause the
stomata to close even during the day
Guard cells regulate photosynthesis and transpiration on a
moment-to-moment basis on many different stimuli
Xerophyte Adaptations that
Reduce Transpiration
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Xerophytes are plants adapted to dry climates
Several leaf modifications help reduce the rate of transpiration:
Small, thick leaves reduce surface area relative to leaf volume
limiting water loss.
Hairy leaves trap a boundary layer of water.
Stomata are located on the underside of the leaf in clusters
protecting them from the wind.
They use CAM, crassulacean acid metabolism, to attain CO2.
mesophyll cells transform CO2 into organic acids during the
night allowing the stomata to close during the day when its
hotter.
During the day, sugars are synthesized by the C3 photosynthetic
pathway.
Movement of Sugar
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Translocation is the movement of organic nutrients in
the plants
In angiosperms sieve tube membranes are the
conduits for translocation
Phloem sap is much different form xylem sap and
contains large amounts of sugar
A sugar source is a plant organ that is a net producer
of sugar
A sugar sink is a net consumer of sugar, most notably
mature leaves
Movement of Sugar (cont’d)
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Direction of flow in sugar tubes depends on the surrounding
plant parts and external factors such as season
Sometimes sugar travels through cells through the symplastic or
apoplastic pathways
Special cells called transfer cells have special ingrown walls that
improve solute transfer
Phloem lets sucrose out at the sink end of the seive tube
The concentration of free sugar is always lower in the sink then
in the tube because sugar in the sink is either absorbed or
converted into insoluble polymers
Pressure Flow
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In angiosperms sugar moves through
sieve tubes through bulk flow called
pressure flow
Pressure increases at the source end
and decreases at the sink end which
causes water to flow from the source to
the sink, carrying sugar with it