Transport of Water in Plants[1]
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Transcript of Transport of Water in Plants[1]
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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
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Transverse section of a root
endodermisendodermis
Casparian strip inCasparian strip inthe walls of thethe walls of theendodermal cellsendodermal cells
xylem vesselsxylem vessels
stelestele
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!iagram of transverse rootsection
epidermis with root hairs
cortex
endodermis
xylem
phloem
pericycle
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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
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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 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
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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
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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
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Transverse $ection of a $tem (!icot%
&asclar bndles
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&asclar bndle from astem
'ylem vessels
Phloem
pidermis
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'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%
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)echanisms for the Transport of Water p the'ylem
*+ "apillarity
+ Root Pressre
-+ "ohesion.Tension
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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
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Root PressreRoot pressre cases
the mercry to rise inthe manometer
"t stmp of awell wateredplant
Water
)ercry)anometer
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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
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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
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6ttation
Water droplets exude fromthe leaves
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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
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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
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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+
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&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+
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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 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
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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
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: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
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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
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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
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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
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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