Transport in plant3

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
stele
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:
www.uri.edu/artsci/bio/plant_anatomy/31.html
Diagram of transverse root section
epidermis with root hairs
cortex
endodermis
xylem
phloem
pericycle
Water is transported across the root by two routes
Apoplast route
Symplast route
between the cells via the cell walls
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
Water is unable to pass through the endodermis by the apoplast route
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
Subject to copyright clearance a suitable image showing the casparian strip could be inserted here.
e.g. one similar to that found at:
www.botany.uwc.ac.za/ecotree/root/rootA.htm
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
Xylem vessels
Phloem
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/
Xylem vessels with different types of lignin strengthening the cell walls
Xylem vessels
form continuous tubes
lignin fibres strengthen the cell walls
so do not collapse when pressure inside falls
no cell contents (dead)
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/
Mechanisms for the Transport of Water up the Xylem
Capillarity
Root Pressure
Cohesion-Tension
Capillarity
Water rises up narrow tubes due to the adhesive forces between the water molecules and the wall of the tube
Xylem vessels are very narrow
Water rises higher in narrower tubes
1.Water will only rise 50mm
2.The flow rate is slower than the rate observed in xylem
Limitations
Root Pressure
Root pressure causes the mercury to rise in the manometer
Cut stump of a well watered plant
Water
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
Cut stumps of plants exude water from their cut ends

In certain conditions some leaves exude water from their leaves = guttation

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
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

Evidence for the Cohesion Tension Hypothesis
Cut stems attached to a tube containing water over mercury can pull the mercury up almost 1m
Dendrographs record that tree trunks have a narrower diameter during the day when transpiration rate is high i.e. when most tension is created.
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.
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
upper epidermis
palisade mesophyll
spongy mesophyll
lower epidermis
stoma
cuticle
cuticle
water vapour diffuses into the air down  gradient
xylem
lowest  in the air
water evaporates from the spongy mesophyll cell surface lowering cell 
water moves into cells down  gradient by osmosis
water is pulled along the xylem
The Cohesion Tension Hypothesis for Movement of Water up the Xylem Vessels
Water evaporates from the spongy mesophyll cells and diffuses into the atmosphere
Transpiration
Lower  in the leaf cells
Water moves from down the  gradient
Water is pulled up xylem vessels
Lower pressure/tension at top of xylem
Cohesive forces between water molecules prevent water column breaking
Water moves across root from soil down  gradient
Via the apoplast and symplast paths
Questions
Explain, in terms of water potential how water moves from the soil to the endodermis in a root (5marks)
Explain why, in summer, the diameter of a branch is smaller at noon than at midnight. (4 marks)
Explain the root pressure hypothesis for water movement in the xylem. (3 marks)
Give two limitations of this hypothesis, (2marks)
Click on the marks above to check
your answers
click here to end
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
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
Back to question
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
Now think of some synoptic links and make a list.
www.biologymad.com
Try out this web site to review transport of water in plants