transport of water in plants

8/2/2019 Transport of Water in Plants

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Transport Of Water in

PlantsRevision Exercise


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

Water moves down thedown the gradientgradient

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

gradientgradient to xylem vessels,

Water can move via the symplastsymplast or apoplastapoplast routes


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Casparian strip inCasparian strip inthe walls of thethe walls of theendodermal cellsendodermal cells

Transverse section of a root


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Diagram of transverse rootsection

epidermis with root hairs

cortex

endodermis

xylem

phloem

pericycle


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Water is transportedacross the root by two

routes

ApoplastApoplastrouteroute

SymplastSymplastrouteroute

between the cellsbetween the cellsvia the cell wallsvia the cell walls

cell cytoplasm tocell cytoplasm tocell cytoplasmcell cytoplasm


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The Symplast RouteThe Symplast Route

Through the cytoplasmcytoplasm

Water enters the root hair cells across the partially permeablemembrane 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


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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 isimpermeable to water

Due to the waterproof band of suberin

So all water must pass the endodermis via the cytoplasmSo 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


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The Casparian strip acts as an apoplast blockThe Casparian strip is madeof suberin, which is

impermeable to water

Water is unable to passthrough the endodermis bythe apoplast route

The endodermis activelytransports salts into theroot xylem

Lowering the in the xylem,so water moves in down the gradient by osmosis

Water moves up the stem in

the xylem vessels


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Transverse Section of a Stem (Dicot)

Vascular bundles


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Vascular bundle from a stem


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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)


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Mechanisms for the Transport of Water up theXylem

1. Capillarity

2. Root Pressure

3. Cohesion-Tension


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Capillarity Water rises up narrow tubes due to theadhesive forces between the watermolecules and the wall of the tube

Xylem vessels arevery narrow

Water riseshigher in narrowertubes

1.Water will only rise 50mm 2.The flow rate is slower than therate observed in xylem

LimitationsLimitations


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Root PressureRoot pressure causes

the mercury to rise inthe manometer

Cut stump of awell wateredplant

Water

MercuryManometer


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Root PressureRoot Pressure

Water is pushedpushed up the xylem by hydrostatic pressure

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

Lowering theLowering 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 inaerobic respiration. O2 required for aerobic respiration


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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 theirleaves = guttation

3. Pressures recorded by mercury manometers attached to thecut stumps could push water in the xylem up to 30m


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Guttation

Water droplets exude fromthe leaves


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Limitations of the Root Pressure Hypothesis

The pressure measured is not enough to get water to the topof trees

Only find root pressure in spring

Relies on the use of the plants energy (ATP) for activetransport


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Cohesion - TensionCohesion - Tension

Water ispulledpulledup the xylem by the water lost in transpirationtranspiration

The sun provides the energy to pull the water up by providing theenergy for evaporation

Water moves up the xylem by mass flow from the higher pressurein roots to the lower pressure in the leaves

The column of water does not break because of the cohesivecohesiveforcesforces between the water molecules

Hydrogen bondsHydrogen bonds between individual water molecules is the force ofattraction

+

+

- +

+

-


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Evidence for the Cohesion Tension HypothesisEvidence for the Cohesion Tension Hypothesis

1. Cut stems attached to a tube containing water over mercury canpull the mercury up almost 1m

2. Dendrographs record that tree trunks have a narrower diameterduring the day when transpiration rate is high i.e. when mosttension is created.

3. Puncturing the xylem of the stem of a transpiring shoot underwater containing a dye causes the dye to move into the xylemboth ways.

The dye must be pulled in so the xylem is under tension.


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Variation in trunk diameter and transpiration rate over 24 hours

The diameter of the trunk decreasesas transpiration rate increases

Evaporation from the leavesEvaporation from the leaves drawswater from the xylem by osmosis,water is pulledpulled up the xylem creatinga tension.

The tension pulls the xylem vesselwalls in, so the trunk diameter getssmaller

The trunk has a larger diameter

when there is less transpiration

2400

Tra

ns

pirat

ion

rate

Br

anc h

dia

m ete

r

12002400

This supports the cohesion tensionhypothesis but not root pressure.


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Water movement across the leaf

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

stoma

cuticle

cuticle

water vapour diffuses into thewater vapour diffuses into theair downair down gradientgradient

xylem

lowestlowest in the airin the air

water evaporates from thewater evaporates from thespongy mesophyll cellspongy mesophyll cellsurface lowering cellsurface lowering cell

water moves into cells downwater moves into cells down

gradient by osmosisgradient by osmosis

water is pulled along thewater is pulled along thexylemxylem

Th C h i T i H th i f M t f W t


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The Cohesion Tension Hypothesis for Movement of Water upthe Xylem Vessels

Water evaporates from theWater evaporates from the

spongy mesophyll cells andspongy mesophyll cells anddiffuses into the atmospherediffuses into the atmosphere

Transpiration

Lower in the leaf cellsWater moves from down the gradientWater is pulled

up xylem vessels

Lower pressure/tensionat top of xylem

Cohesive forces between watermolecules prevent water columnbreaking

Water moves across rootfrom soil down gradient

Via the apoplast andsymplast paths


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Questions

1. Explain, in terms of water potential how water moves from thesoil to the endodermis in a root (5marks)

2. Explain why, in summer, the diameter of a branch is smaller atnoon than at midnight. (4 marks)

3. Explain the root pressure hypothesis for water movement in thexylem. (3 marks)

4. Give two limitations of this hypothesis, (2marks)

Click on the marks above to checkyour answers

click here to end


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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 thexylem in the root centre

Water moves from cell to cell through the cytoplasm down thewater potential gradient

Water also moves through the fibres of the cell wall andintercellular spaces

But must go through the endodermal cells due to the Casparianstrip

Any 5 points

Back to question


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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


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Answer Q3

Root pressure is a hydrostatic pressure pushing water up thexylem

Mineral ions are actively transported out of the endodermalcells into the xylem vessels

Lowering the water potential in the xylem

So water moves in from the surrounding cells by osmosis / downthe water potential gradient

Raising the hydrostatic pressure

Any three points

Back to question


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Answer Q4

Back to question

The pressure measured is not enough to get water to the top oftrees

Only find root pressure in spring Relies on the use of the plants energy (ATP) for active transport

Any two


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