Transcript Transport of Water Across the Root

Transport of Water Across the Root
• Water is absorbed from the soil by osmosis
• Water moves down the  gradient
• Water only enters the root near the root tip
• Here there are root hairs which increase the surface area for osmosis
• Water potential is higher in the epidermal cells than in the central cells
• Water moves across the cortex down the  gradient to xylem vessels,
• Water can move via the symplast or apoplast routes
Transverse section of a root
endodermis
Casparian strip in
the walls of the
endodermal cells
xylem vessels
Subject to copyright clearance a suitable image showing
a transverse section of a root could be inserted here.
e.g. one similar to that found at:
stele
www.uri.edu/artsci/bio/plant_anatomy/31.html
Diagram of transverse root
section
epidermis with root hairs
cortex
endodermis
pericycle
xylem
phloem
Water is transported
across the root by two
routes
Apoplast
route
between the cells
via the cell walls
Symplast
route
cell cytoplasm to
cell cytoplasm
The Symplast Route
• Through the cytoplasm
• Water enters the root hair cells across the partially permeable
membrane by osmosis
• Water moves from higher  in the soil to the lower  in the cell
• Water moves across the root from cytoplasm to cytoplasm
down the  gradient
• It passes from one cell to the other via plasmadesmata
• Water moves into the xylem by osmosis
• The only way across the endodermis
• Normally the most important pathway
The Apoplast Route
• Water moves through the cellulose cell wall and intercellular
spaces
• The permeable fibres of cellulose do no resist water flow
• Water cannot pass the endodermis by this route
• Because the Casparian strip in the endodermis cell wall is
impermeable to water
• Due to the waterproof band of suberin
• So all water must pass the endodermis via the cytoplasm
• Therefore it is under cellular control
• Apoplast route is important when transpiration rates are high as
it is faster and requires no energy
The Casparian strip acts as an apoplast block
The Casparian strip is made
of suberin, which is
impermeable to water
Subject to copyright clearance a suitable image showing
the casparian strip could be inserted here.
Water is unable to pass
through the endodermis by
the apoplast route
e.g. one similar to that found at:
www.botany.uwc.ac.za/ecotree/root/rootA.htm
The endodermis actively
transports salts into the
root xylem
Lowering the  in the xylem,
so water moves in down the
 gradient by osmosis
Water moves up the stem
in the xylem vessels
Transverse Section of a Stem (Dicot)
Vascular bundles
Subject to copyright clearance a suitable image showing
a transverse section of a stem could be inserted here.
e.g. one similar to that found at:
http://www.skidmore.edu/academics/biology/plant_bio/
Vascular bundle from a
stem
Epidermis
Subject to copyright clearance a suitable image showing
a vascular bundle could be inserted here.
e.g. one similar to that found at:
http://www.skidmore.edu/academics/biology/plant_bio/
Phloem
Xylem vessels
Xylem vessels with different types of lignin strengthening the cell walls
Subject to copyright clearance a suitable image showing
xylem vessels with different strengthening could be
inserted here.
e.g. one similar to that found at:
http://www.skidmore.edu/academics/biology/plant_bio/
Xylem vessels
• no cell contents (dead)
•lignin fibres strengthen the cell walls
•form continuous tubes
•so do not collapse when pressure inside falls
Mechanisms for the Transport of Water up the
Xylem
1. Capillarity
2. Root Pressure
3. Cohesion-Tension
Capillarity
Water rises up narrow tubes due to the
adhesive forces between the water
molecules and the wall of the tube
Water rises
higher in
narrower tubes
Xylem vessels are
very narrow
Limitations
1.Water will only rise 50mm
2.The flow rate is slower than the
rate observed in xylem
Root Pressure
Water
Root pressure causes
the mercury to rise in
the manometer
Cut stump of a
well watered
plant
Mercury
Manometer
Root Pressure
• Water is pushed up the xylem by hydrostatic pressure
• Mineral salts are pumped into the xylem vessels in the root by the
endodermal cells
• Lowering the  in the xylem
• Water moves in from the surrounding cells by osmosis
• Raising the hydrostatic pressure so pushing water up the xylem
What would happen if the roots were deprived of O2?
The ‘pumping’ of the ions would stop as it requires ATP produced in
aerobic respiration. O2 required for aerobic respiration
Root Pressure: Evidence
1. Cut stumps of plants exude water from their cut ends
2. In certain conditions some leaves exude water from their
leaves = guttation
3. Pressures recorded by mercury manometers attached to the
cut stumps could push water in the xylem up to 30m
Guttation
Water droplets exude from
the leaves
Subject to copyright clearance a suitable image showing
guttation could be inserted here.
e.g. one similar to that found at:
http://grapes.msu.edu/guttation.htm
Limitations of the Root Pressure Hypothesis
• The pressure measured is not enough to get water to the top
of trees
• Only find root pressure in spring
• Relies on the use of the plant’s energy (ATP) for active
transport
Cohesion - Tension
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Water is pulled up the xylem by the water lost in transpiration
The sun provides the energy to ‘pull’ the water up by providing the
energy for evaporation
Water moves up the xylem by mass flow from the higher pressure
in roots to the lower pressure in the leaves
The column of water does not break because of the cohesive
forces between the water molecules
Hydrogen bonds between individual water molecules is the force of
attraction
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Evidence for the Cohesion Tension Hypothesis
1. Cut stems attached to a tube containing water over mercury can
pull the mercury up almost 1m
2. Dendrographs record that tree trunks have a narrower diameter
during the day when transpiration rate is high i.e. when most
tension is created.
3. Puncturing the xylem of the stem of a transpiring shoot under
water containing a dye causes the dye to move into the xylem
both ways.
• The dye must be pulled in so the xylem is under tension.
Variation in trunk diameter and transpiration rate over 24 hours
The diameter of the trunk decreases
as transpiration rate increases
Evaporation from the leaves draws
water from the xylem by osmosis,
water is pulled up the xylem creating
a tension.
2400
1200
2400
The tension pulls the xylem vessel
walls in, so the trunk diameter gets
smaller
The trunk has a larger diameter
when there is less transpiration
This supports the cohesion tension
hypothesis but not root pressure.
Water movement across the leaf
cuticle
upper epidermis
palisade mesophyll
xylem
water is pulled along the
xylem
water moves into cells down 
gradient by osmosis
spongy mesophyll
lower epidermis
water evaporates from the
spongy mesophyll cell
surface lowering cell 
cuticle
stoma
lowest  in the air
water vapour diffuses into the
air down  gradient
The Cohesion Tension Hypothesis for Movement of Water up
the Xylem Vessels
Transpiration
Lower pressure/tension
at top of xylem
Water evaporates from the
spongy mesophyll cells and
diffuses into the atmosphere
Lower  in the leaf cells
Water is pulled
up xylem vessels
Water moves from down the  gradient
Water moves across root
from soil down  gradient
Cohesive forces between water
molecules prevent water column
breaking
Via the apoplast and
symplast paths
Questions
1.
Explain, in terms of water potential how water moves from the
soil to the endodermis in a root
(5marks)
2.
Explain why, in summer, the diameter of a branch is smaller at
noon than at midnight.
(4 marks)
3. Explain the root pressure hypothesis for water movement in
the xylem.
(3 marks)
4. Give two limitations of this hypothesis,
click here to end
(2marks)
Click on the marks above to check
your answers
Answer Q1
• Water is absorbed from the soil by the root hairs
• By osmosis down the water potential gradient
• The water potential is higher in the epidermal cells than in the
xylem in the root centre
• Water moves from cell to cell through the cytoplasm down the
water potential gradient
• Water also moves through the fibres of the cell wall and
intercellular spaces
• But must go through the endodermal cells due to the Casparian
strip
Any 5 points
Back to question
Answer Q2
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Temperature higher at noon so transpiration rate higher
More water evaporates from the surface of the mesophyll cells
Reducing the the water potential
Water moves from the xylem in the leaves into the cells
Creating a tension pulling the water up the xylem
This pulls the xylem vessels in so reducing the diameter of the
trunk
Any four points
Back to question
Answer Q3
• Root pressure is a hydrostatic pressure pushing water up the
xylem
• Mineral ions are actively transported out of the endodermal
cells into the xylem vessels
• Lowering the water potential in the xylem
• So water moves in from the surrounding cells by osmosis /
down the water potential gradient
• Raising the hydrostatic pressure
Any three points
Back to question
Answer Q4
• The pressure measured is not enough to get water to the top of
trees
• Only find root pressure in spring
• Relies on the use of the plant’s energy (ATP) for active transport
Any two
Back to question
Now think of some synoptic links and make a list.
Try out this web site to review transport of water in plants
www.biologymad.com