Transcript Transport of Phloem Sap

Transport in Plants
Transport Occurs on Three Levels

Water loss and uptake from cells.
–

Transport of substances short distances.
–

Root cells absorb minerals form the soil.
Transport of sugar through sieve tube members.
Transport of substances long distances.
–
Transport of water and sugar up the stem.
Transport on the Cellular Level
• Relies on selective permeability of the plasma membrane.
– Transport proteins embedded in the membrane
– Selective channels for potassium but not for sodium
• Set up electrochemical gradients.
• Gated channels require stimulus to open and close gates.
• Proton pumps
• Require ATP (active transport)
– Uses chemiosmosis to couple proton gradient to transport across
the membrane.
– Sets up membrane potential.
– Facilitates potassium uptake by root cells.
– Cotransport of nitrates against their gradient when protons diffuse
with their gradient into the root cell.
– This is how sucrose gets into cells.
Water Potential Drives water up the
Stem.

Water moves by osmosis and pressure potential
from cells with a higher water potential to cells with a
lower water potential.
–
–
–
–
Pressure potential is pressure exerted by the plant cell wall.
Solute potential causes water from a hypotonic environment
to a hypertonic environment.
The combination of pressure and solute potential determine
water potential which is the tendency of water to leave one
place in favor of another.
Scientists measure water potential in megapascals.
Water Potential a Closer Look


Pure water in open air has a water potential of zero.
Solutes lower water potential.
–

Water has a tendency to move into the cell.
Physical pressure increases water potential.
–
–
Water has a tendency to move out of the cell.
Tension is negative pressure


Pulling water up with a plunger
Water will always move in the direction of lower or
negative water potential
Aquaporins






It is believed by scientist that water moves through gated
channels called aquaporins.
Affects the rate at which water travels across the membrane.
Tonoplast is the membrane bound vacuole which may take up
90% of a plant cell.
Plasmodermata are channels that connect the cytoplasm of
two neighboring plant cells.
Symplast is the continuous route through plant tissue due to
plasmodermata.
Apoplast is the continuous path between cells that resides
outside the plasma membrane but inside the cell wall.
Three Routes of Lateral Transport

From cell to cell through cell wall and plasma
membrane.
–


Must cross cell membrane twice.
Symplast – Only have to cross cell membrane once
and then can travel from cell to cell through
plasmodermata.
Apoplasts – never cross cell membrane or enter cell.
–
Efficient for traveling longer distances.
Bulk Flow Necessary for Longer
Distances

Hydrostatic pressure.
–
–

Transport of phloem sap
Guttation
Tension created by transpiration
–
–
–
Decreasing the diameter of the xylem increases tension.
Negative water potential of the cells surrounding the
stomata pull water from cells with a higher water potential
near the xylem and create tension.
Older Xylem filled with resin decreases the pathways for
water to move up the stem adds to adds to hydrostatic
pressure.
Absorption of Water and Minerals By
the Roots


Route – soil – epidermis- cortex – xylem
Most absorption occurs at the root tips.
–


Symbiotic fungi called mycorrhizae facilitate the
access of these minerals form the soil.
Dilute soil solution moves through the cortex
via apoplasts.
Transport proteins in the plasma membrane
accumulate K+.
From Cortex to Xylem

Water moves through apoplasts until it reaches the
endodermis.
–
–
–

The endodermis is an inner membrane that encompasses the
vascular bundle.
It is selectively permeable and actively pumps minerals into the
center of the root called the stele.
Water moves into the root because it is hypertonic to the soil
solution.
The Casparian strip is a belt of waxy substance called suberin
that forces water to cross the endodermis through symplasts.
–
Allows the endodermis to be selective about the minerals that can
enter the vascular system.
Pushy Xylem Sap

Because the rot cortex has a higher water
potential than the stele water moves into the
stele creating positive pressure
–


At night when the rate of transpiration is low
guttation occurs because the endodermal cells
continue to pump minerals into the stele.
Water cannot go very far using root pressure.
Most of the water is pulled by transpiration.
The Transpiration Pull



Evaporation of water that coats cells within the leaf
tissue keeps the relative humidity high in the airs
spaces.
As the water evaporates a film of water within these
spaces forms a meniscus with an increasing
curvature that creates tension.
Tension is a negative pressure and water is pulled
from areas where the hydrostatic pressure is greater.
–

Water is being pulled by adhesive and cohesive forces.
Water is replace by leaf xylem.
Ascent of the Sap




Cohesion allows water molecules from below
to be pulled from above.
The small diameter of the tracheids and
xylem vessels allows the much more of the
hydrophillic walls of the xylem to be exposed.
Adhesive forces facilitates for the climbing of
the water column.
Ascent of the sap is solar powered.
Transpiration Trade Off


Allows nutrients to be distributed throughout
all the tissues of the plant.
Provides evaporative cooling.
How Stomata Open and Close

Three triggers for opening:
–
Blue light receptors in guard cells.



–
–
Reduced CO2
Internal clock

–
promotes uptake of K= into guard cells
Water follows and the buckle open due to uneven thickness of
their cell walls.
Light also stimulates photosynthesis and produces ATP
necessary for the proton pumps.
Circadian rhythms
Stomata to close when:


temperatures increases
Increases CO2 within the air spaces of the leaves.
Xerophytes

Plants adapted to arid climates
–
–
–
–
Small thick leaves
Stomata located on shady part of the leaves or in
depressions.
Fleshy stems perform photosynthesis
CAM and C=4+ plants.
Transport of Phloem Sap



Translocation – transport of food(sugar)
within the plant.
Phloem sap is primarily sucrose.
Source
–
–
–
Where sugar is made by photosynthesis
Where starch is broken down.
Leaves mostly.
Transport of Sugar

Sink
–
–
–
–
Organ that consumes sugar.
Organ that stores sugar.
Growing roots, shoots, fruits and stems.
Storage organ can be a sink or a source.

–
Sink when storing nutrients to be consumed in the
winter.
The sinks use sources that are close by.
Phloem Loading and Unloading


Phloem is transported and loaded into sieve tube members.
Sucrose moves from mesophyll cells where it is made through
symplasts.
–
–
–



In some species sucrose moves through both symplasts and
apoplasts.
Companion cells help load sugar into sieve tube members.
Sucrose is cotransported across the membrane with protons.
Sucrose is accumulated in various parts of the plant and
requires active transport.
There is a sugar gradient that is created between the sink and
source and water moves in by osmosis.
This creates pressure between the sink and the source and
sugar solution is squeezed by hydrostatic pressure to the sink.