transport of water in plants[1]

8/10/2019 Transport of Water in Plants[1]

1/27

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

Water moves down thedown thePotential () gradient) gradient

Water only enters the root near the root tip

Here there are root hairsroot hairswhich increase the surface areaincrease the surface areafor osmosis

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

Water moves across the cortex down thedown the Potential gradientgradienttoxylem vessels,

Water can move via the symplastsymplastor apoplastapoplastrotes


2/27

Transverse section of a root

endodermisendodermis

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

xylem vesselsxylem vessels

stelestele


3/27

!iagram of transverse rootsection

epidermis with root hairs

cortex

endodermis

xylem

phloem

pericycle


4/27

Water is transportedacross the root by two

rotes

ApoplastApoplastrouteroute

SymplastSymplastrouteroute

between the cellsbetween the cellsvia the cell wallsvia the cell walls

cell cytoplasm tocell cytoplasm tocell cytoplasmcell cytoplasm


5/27

The Symplast RouteThe Symplast Route

Through the cytoplasmcytoplasm

Water enters the root hair cells across the partially permeablemembrane by osmosis

Water moves from higher Potential ()) in the soil to the lower

in the cell

Water moves across the root from cytoplasm to cytoplasmdown the Potential ()) 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

ormally the most important pathway


6/27

The Apoplast RouteThe Apoplast Route

Water moves through the cellulose cell wallcellulose cell walland intercellularintercellular

spacesspaces The permeable fibres of cellulose do no resist water flow

Water cannot pass the endodermis by this route

!ecause the Casparian strip in the endodermis cell wall isimpermeable to water

"ue 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 re#uires no energy


7/27

The "asparian strip acts as an apoplast bloc#The 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

%owering the Potential ()) inthe xylem$ so water movesin down the gradient byosmosis

Water moves up the stem

in the xylem vessels


8/27

Transverse $ection of a $tem (!icot%

&asclar bndles


9/27

&asclar bndle from astem

'ylem vessels

Phloem

pidermis


10/27

'ylem vessels with different types of lignin strengthening the cell walls

'ylem vessels

form continos tbes

lignin fibres strengthen the cell walls

so do not collapse when pressre inside falls

no cell contents (dead%


11/27

)echanisms for the Transport of Water p the'ylem

*+ "apillarity

+ Root Pressre

-+ "ohesion.Tension


12/27

Capillarity Water rises p narrow tbes de to theadhesive forces between the watermolecles and the wall of the tbe

&ylem vessels arevery narrow

Water riseshigher innarrower tubes

*+Water will only rise /0mm +The flow rate is slower than therate observed in xylem

%imitations%imitations


13/27

Root PressreRoot pressre cases

the mercry to rise inthe manometer

"t stmp of awell wateredplant

Water

)ercry)anometer


14/27

Root 'ressureRoot 'ressure

Water is pushedpushedp the xylem by hydrostatic pressre

)ineral salts are pumpedpumpedinto the xylem vessels in the root by theendodermal cells

%owering the%owering the Potential (

))in the xylem

Water moves in from the srronding cells by osmosisosmosis

Raising the hydrostatic pressureRaising the hydrostatic pressureso pshing water p the xylem

What wold happen if the roots were deprived of 1,2

The 3pmping4 of the ions wold stop as it re5ires ATP prodced inaerobic respiration+ 1re5ired for aerobic respiration


15/27

Root 'ressure( videnceRoot 'ressure( vidence

*+ Cut stumps of plants exude water from their cut ends

,+ In certain conditions some leaves exude water from theirleaves - guttation

.+ 'ressures recorded by mercury manometers attached to thecut stumps could push water in the xylem up to ./m


16/27

6ttation

Water droplets exude fromthe leaves


17/27

7imitations of the Root Pressre Hypothesis

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

0nly find root pressure in spring

Relies on the use of the plant1s energy 2AT') for activetransport


18/27

Cohesion 3 TensionCohesion 3 Tension

Water ispulledpulledp the xylem by the water lost in transpirationtranspiration

The sn provides the energy to 3pll4 the water p by providing theenergy for evaporation

Water moves p the xylem by mass flow from the higher pressrein roots to the lower pressre in the leaves

The colmn of water does not brea# becase of the cohesivecohesiveforcesforces between the water molecles

4ydrogen bonds4ydrogen bondsbetween individal water molecles is the force ofattraction

5

5

3 5

5

3


19/27

vidence for the Cohesion Tension 4ypothesisvidence for the Cohesion Tension 4ypothesis

*+ Cut stems attached to a tube containing water over mercury canpull the mercury up almost *m

,+ "endrographs record that tree trun6s have a narrower diameterduring the day when transpiration rate is high i+e+ when mosttension is created+

.+ 'uncturing 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+


20/27

&ariation in trn# diameter and transpiration rate over 8 hors

The diameter of the trn# decreasesas transpiration rate increases

vaporation from the leavesvaporation from the leavesdrawswater from the xylem by osmosis,water is pulledpulledp the xylem creatinga tension+

The tension plls the xylem vesselwalls in, so the trn# diameter getssmaller

The trn# has a larger diameter

when there is less transpiration

800

Tra

ns

pirat

ion

rate

9ra

nc

h

dia

m et

er

*00800

This spports the cohesion tensionhypothesis bt not root pressre+


21/27

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 downgradient by osmosisgradient by osmosis

water is pulled along thewater is pulled along thexylemxylem

Th C h i T i 4 th i f 7 t f W t


22/27

The Cohesion Tension 4ypothesis for 7ovement of Water upthe &ylem 8essels

Water evaporates from theWater evaporates from the

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

Transpiration

%ower in the leaf cells

Water moves from down the gradientWater is pulled

up xylem vessels

%ower pressure9tensionat top of xylem

Cohesive forces between watermolecules prevent water columnbrea6ing

Water moves across rootfrom soil down gradient

8ia the apoplast andsymplast paths


23/27

:estions

*+ xplain, in terms of water potential how water moves from thesoil to the endodermis in a root (/mar#s%

+ xplain why, in smmer, the diameter of a branch is smaller atnoon than at midnight+ (8 mar#s%

-+ xplain the root pressre hypothesis for water movement inthe xylem+ (- mar#s%

8+ 6ive two limitations of this hypothesis, (mar#s%

Clic6 on the mar6s above to chec6your answers

clic6 here to end


24/27

Answer :*

Water is absorbed from the soil by the root hairs

9y 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 throgh the cytoplasm down thewater potential gradient

Water also moves throgh the fibres of the cell wall andintercelllar spaces

9t mst go throgh the endodermal cells de to the "asparianstrip

Any / points

!ac6 to #uestion


25/27

Answer :

Temperatre higher at noon so transpiration rate higher

)ore water evaporates from the srface of the mesophyll cells

Redcing the the water potential

Water moves from the xylem in the leaves into the cells

"reating a tension plling the water p the xylem

This plls the xylem vessels in so redcing the diameter of the

trn#

Any for points

!ac6 to #uestion


26/27

Answer :-

Root pressre is a hydrostatic pressre pshing water p thexylem

)ineral ions are actively transported ot of the endodermalcells into the xylem vessels

7owering the water potential in the xylem

$o water moves in from the srronding cells by osmosis ;down the water potential gradient

Raising the hydrostatic pressre

Any three points

!ac6 to #uestion


27/27

Answer :8

!ac6 to #uestion

The pressre measred is not enogh to get water to the top oftrees

1nly find root pressre in spring Relies on the se of the plant4s energy (ATP% for active transport

Any two

transport of water in plants[1]

Documents