Transport of Water Across the Root• Water is absorbed from the soil by osmosisosmosis

• Water moves down thedown the gradient gradient

• Water only enters the root near the root tip

• Here there are root hairsroot hairs which increase the surface areaincrease the surface area for osmosis

• Water potential is higher in the epidermal cells than in the central cells

• Water moves across the cortex down thedown the gradient gradient to xylem vessels,

• Water can move via the symplastsymplast or apoplastapoplast routes

Transverse section of a root

endodermisendodermis

Casparian strip Casparian strip in the walls of in the walls of the endodermal the endodermal cellscells

xylem vesselsxylem vessels

stelestele

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 Apoplast routeroute

Symplast Symplast routeroute

between the between the cells via the cell cells via the cell wallswalls

cell cytoplasm cell cytoplasm to cell to cell cytoplasmcytoplasm

The Symplast RouteThe Symplast Route

• Through the cytoplasmcytoplasm • 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 plasmadesmataplasmadesmata• Water moves into the xylem by osmosis• The only way across the endodermisThe only way across the endodermis• Normally the most important pathway

The Apoplast RouteThe Apoplast Route

• Water moves through the cellulose cell wallcellulose cell wall and intercellularintercellular spacesspaces

• 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 So all water must pass the endodermis via the

cytoplasmcytoplasm• 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 blockThe Casparian strip is made of suberin, which is impermeable to waterWater 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

1. Capillarity2. Root Pressure3. 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 Limitations

Root Pressure Root pressure causes the mercury to rise in the manometer

Cut stump of a well watered plant

Water

Mercury Manometer

Root PressureRoot Pressure

• Water is pushedpushed up the xylem by hydrostatic pressure

• Mineral salts are pumpedpumped into the xylem vessels in the root by the endodermal cells

• Lowering the Lowering the in the xylem

• Water moves in from the surrounding cells by osmosisosmosis

• Raising the hydrostatic pressureRaising 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: EvidenceRoot 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 - TensionCohesion - Tension

• Water is pulledpulled up the xylem by the water lost in transpirationtranspiration

• 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 cohesivecohesive forces forces between the water molecules

• Hydrogen bondsHydrogen bonds between individual water molecules is the force of attraction

+

+

- +

+

-

Evidence for the Cohesion Tension Evidence for the Cohesion Tension HypothesisHypothesis

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 leavesEvaporation from the leaves draws water from the xylem by osmosis, water is pulledpulled 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

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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 water vapour diffuses into the air down the air down gradient gradient

xylem

lowest lowest in the air in the air

water evaporates from water evaporates from the spongy mesophyll the spongy mesophyll cell surface lowering cell cell surface lowering cell

water moves into cells down water moves into cells down gradient by osmosis gradient by osmosis

water is pulled along water is pulled along the xylem the xylem

The Cohesion Tension Hypothesis for Movement of Water up the Xylem Vessels

Water evaporates from Water evaporates from the spongy mesophyll the spongy mesophyll cells and diffuses into cells and diffuses into the atmosphere the atmosphere

Transpiration

Lower in the leaf cellsWater 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 gradientVia 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, (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.

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Try out this web site to review transport of water in plants