1. 11
Principles of plant physiologyPrinciples of plant physiology
Chapter TwoChapter Two
Plant-Water RelationsPlant-Water Relations
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Water is absolutely essential for all livingWater is absolutely essential for all living
organisms.organisms.
Why is water importantWhy is water important ??
–Living cells are composed of 70-95 % waterLiving cells are composed of 70-95 % water
–Life absolutely depends on the properties ofLife absolutely depends on the properties of
water.water.
–Life probably evolved in water.Life probably evolved in water.
–water covers ¾ of the earthwater covers ¾ of the earth
–When organisms go dormant, they loose most ofWhen organisms go dormant, they loose most of
their watertheir water
–Limiting resource for crop productivityLimiting resource for crop productivity
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Tom Robbins, “Tom Robbins, “ Even Cowgirls Get the Blues”Even Cowgirls Get the Blues”
• Water-the ace of elementsWater-the ace of elements
– Water dives from the clouds withoutWater dives from the clouds without parachute, wings orparachute, wings or
safety netsafety net
– Water runs over the steepest precipiceWater runs over the steepest precipice and blinks not aand blinks not a
lashlash
– Water isWater is buried and rises againburied and rises again ; water; water walks on firewalks on fire and fireand fire
gets the blistersgets the blisters
– Stylishly composed in any situation - solid, gas or liquidStylishly composed in any situation - solid, gas or liquid
– speaking in penetrating dialects understood by all things -speaking in penetrating dialects understood by all things -
animal, vegetable or mineralanimal, vegetable or mineral
– water travels intrepidly through three dimensions:water travels intrepidly through three dimensions:
• sustaining,sustaining,
• destroying, anddestroying, andBeira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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1.1 Properties of water important to1.1 Properties of water important to
plantplant
Life is absolutely dependent on the properties of water
Pure water: Colourless and has no smell and taste
The hidden qualities of water make it a most interesting:
Physicochemical properties of water
I. Water is polar
Water's unique properties due to its simple composition and
structure
hydrogen atoms are "attached" to one side of the oxygenhydrogen atoms are "attached" to one side of the oxygen
atomatom
Positive and negative chargePositive and negative charge
The separation between negative and positive chargesThe separation between negative and positive charges
creates a polar moleculecreates a polar molecule
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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II. Hydrogen bondsII. Hydrogen bonds
A weak bond that forms between a hydrogen atomA weak bond that forms between a hydrogen atom
and electronegative atomand electronegative atom
Water can bond both to itself and to other moleculesWater can bond both to itself and to other molecules
responsible for theresponsible for the thermal properties of waterthermal properties of water
A.A. Water's high specific heat capacityWater's high specific heat capacity
the amount of heat energy that must bethe amount of heat energy that must be added oradded or
lostlost to change the temperature of one gram of waterto change the temperature of one gram of water
by 1°Cby 1°C
water can absorb a lot of heat before waterwater can absorb a lot of heat before water
molecules can move faster or begin to get hotmolecules can move faster or begin to get hot
much more energy is needed disrupt the hydrogenmuch more energy is needed disrupt the hydrogen
bonds in liquid waterbonds in liquid water
• water resists rapid temperature fluctuationswater resists rapid temperature fluctuations
• adding stability to earth's environmentsadding stability to earth's environmentsBeira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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B.B. high heat of vaporizationhigh heat of vaporization
the energy needed to change the phase of a liquid to a
gas (44 kJ/mole)
it takes a great deal of energy to break a molecule free
of its liquid partners
– water resists evaporation
– causes a cooling effect on plants
C. High latent heat of fusionC. High latent heat of fusion
takes lots of energy to convert from solid to a liquidtakes lots of energy to convert from solid to a liquid
statestate
lot of energy must be released by water to freezelot of energy must be released by water to freeze
Thus water resists freezingThus water resists freezing
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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D. Capillary actionD. Capillary action
i.i. CohesionCohesion
hydrogen bonds between water molecules makehydrogen bonds between water molecules make
liquid water self-stickyliquid water self-sticky
The hydrogens of one water molecule are attractedThe hydrogens of one water molecule are attracted
to the oxygen from other water moleculesto the oxygen from other water molecules
makes water bead up more on a surfacemakes water bead up more on a surface
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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ii. Adhesionii. Adhesion
the attraction of water molecules to non-waterthe attraction of water molecules to non-water
hydrophilic substanceshydrophilic substances
This property of water gives it the ability to climbThis property of water gives it the ability to climb
the wall of any container it is inthe wall of any container it is in
The top of the water column assumes aThe top of the water column assumes a uu-shape-shape
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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iii. Surface tensioniii. Surface tension
Water has a very high surface tensionWater has a very high surface tension
the cohesion of water molecules to eachthe cohesion of water molecules to each
other and to the water molecules below at theother and to the water molecules below at the
surface of a body of watersurface of a body of water
It is a measure of how difficult it is to breakIt is a measure of how difficult it is to break
the surface of a liquidthe surface of a liquid
Thus water acts as thought it has a skinThus water acts as thought it has a skin
because of cohesionbecause of cohesion
Plants take advantage of capillary action toPlants take advantage of capillary action to
pull water from the root into themselvespull water from the root into themselves
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20082008
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III. Universal solventIII. Universal solvent
dissolves more different kinds ofdissolves more different kinds of
substances than any other solventsubstances than any other solvent
Hydrogen bonds make water anHydrogen bonds make water an
excellent solventexcellent solvent
wherever water goes, either through thewherever water goes, either through the
ground or through plant body, it takesground or through plant body, it takes
along valuable chemicals, minerals, andalong valuable chemicals, minerals, and
nutrients.nutrients.
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Water is a versatile solvent owing to its polarityWater is a versatile solvent owing to its polarity
Ionic compounds dissolve in water.Ionic compounds dissolve in water.
• Charged regions of polar water moleculesCharged regions of polar water molecules
have an electrical attraction to charged ionshave an electrical attraction to charged ions
• Water surrounds individual ions, separatingWater surrounds individual ions, separating
and shielding them from one anotherand shielding them from one another
Polar compounds in general, are water-soluble.Polar compounds in general, are water-soluble.
Charged regions of polar water molecules have anCharged regions of polar water molecules have an
affinity for opposite charged regions of other polaraffinity for opposite charged regions of other polar
moleculesmolecules
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IV. Water is a liquid at physiologicalIV. Water is a liquid at physiological
temperaturetemperature
high boiling and melting point when compared tohigh boiling and melting point when compared to
other similar sized moleculesother similar sized molecules
Life exists between 0 and 100Life exists between 0 and 100oo
CC
<0<0oo
C too low to permit significant chemistry forC too low to permit significant chemistry for
metabolismmetabolism
>> 100 oC tends to disrupt bonds100 oC tends to disrupt bonds
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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VV. Transparent to Light. Transparent to Light
Chloroplasts inside a cell areChloroplasts inside a cell are
surrounded by watersurrounded by water
Photosynthesis if water werePhotosynthesis if water were
opaque ????opaque ????
the penetration of light in waterthe penetration of light in water
determines the distribution of aquaticdetermines the distribution of aquatic
plantsplants
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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VI. Chemically inertVI. Chemically inert ::
water does not react unless enzymaticcallywater does not react unless enzymaticcally
designed to reactdesigned to react
VII. Affect the shape, stability andVII. Affect the shape, stability and
properties of biological molecules.properties of biological molecules.
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Role of Water in plantsRole of Water in plants
Solvent for the uptake and transport of moleculesSolvent for the uptake and transport of molecules
Water is major component of cellsWater is major component of cells
Good medium for biochemical reactionsGood medium for biochemical reactions
React in many biochemical reactionReact in many biochemical reaction
Provide structural support via turger pressure in leavesProvide structural support via turger pressure in leaves
The medium for the transfer of plant gameteThe medium for the transfer of plant gamete
Plant movements are the result of water moving in toPlant movements are the result of water moving in to
and out of those plantsand out of those plants
Temperature stabilizationTemperature stabilization
Plays a role in cell elongation and growthPlays a role in cell elongation and growthBeira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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2.22.2 Concepts of water potentialConcepts of water potential
diverse multivarious role of water indiverse multivarious role of water in
plant functionplant function
Consider bothConsider both
the state of water andthe state of water and
rate of movement of water in plantsrate of movement of water in plants
determined by values of water potentialdetermined by values of water potential
or gradient in water potentialor gradient in water potential
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Defn.Defn. is the chemical potential of water in ais the chemical potential of water in a
solutionsolution compared tocompared to pure waterpure water at the sameat the same
temperature and pressuretemperature and pressure
a measure of the free energy of watera measure of the free energy of water
the energy available to movethe energy available to move waterwater from onefrom one
place to anotherplace to another
with out temperature changewith out temperature change
Determines direction of water movementDetermines direction of water movement
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Values of water potential:Values of water potential:
Pure water has a defined waterPure water has a defined water
potential of zero.potential of zero.
However it is possible for the waterHowever it is possible for the water
potential to bepotential to be positivepositive oror negativenegative
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Units of measurementUnits of measurement ::
measured in units of atmosphericmeasured in units of atmospheric
pressure:pressure:
Pascal (MPa)Pascal (MPa)
Pounds force/square inchPounds force/square inch
BarsBars
dynes/square cmdynes/square cm
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Equation of water potentialEquation of water potential
In a simple systemIn a simple system
• Pressure potential (Ψp)Pressure potential (Ψp)
• Osmotic potential(Ψπ) or solute potentialOsmotic potential(Ψπ) or solute potential
(Ψ(Ψss).).
Ψ = Ψp + Ψπ (Ψs)Ψ = Ψp + Ψπ (Ψs)
Complex SystemsComplex Systems
Gravity potential (Ψg)Gravity potential (Ψg)
Matric potential (Ψm)Matric potential (Ψm)
Ψ = Ψπ + Ψp + Ψg + ΨmΨ = Ψπ + Ψp + Ψg + ΨmBeira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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A. Solute potential / osmotic potential/A. Solute potential / osmotic potential/
A measure of tendency for HA measure of tendency for H22 O to cross aO to cross a
selectively permeable membraneselectively permeable membrane
from low concentration to highfrom low concentration to high
concentration of soluteconcentration of solute
Pure water has a solute potential ofPure water has a solute potential of zerozero
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Addition of solute makes the value ofAddition of solute makes the value of
solute potential negativesolute potential negative
decreasesdecreases the free energy of waterthe free energy of water
Negative contribution to water potentialNegative contribution to water potential
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Van’t Hoff equationVan’t Hoff equation
Shows relationship of solute concentrationShows relationship of solute concentration
(in molality) to solute potential(in molality) to solute potential
Ψπ = − miRTΨπ = − miRT
m,m, concentration of the soluteconcentration of the solute
ii , ionization constant of the solute (1 for, ionization constant of the solute (1 for
glucose, 2 for NaCl)glucose, 2 for NaCl)
RR , ideal gas constant, ideal gas constant
TT , temperature (K), temperature (K)
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For example,For example,
when a solute is dissolved in waterwhen a solute is dissolved in water
the water molecules are less likely tothe water molecules are less likely to
diffuse away via osmosis than whendiffuse away via osmosis than when
there is no solutethere is no solute
SSSSSS
WWWW
SSSSSS
WWWW
SSSSSS
WWWW
High free energy of water ?????
A B
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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regions
of higher
regions
of higher
water potential
water potential
•less
negative;
less
negative;
•high
free
energy;
high
free
energy;
•m
ore
positive;
m
ore
positive;
•less
solute)
less
solute)
Regions of lower water
Regions of lower water
potential
potential
•m
ore
negative,
m
ore
negative,
•less free
energy,
less free
energy,
•less positive,
less positive,
•high
solute
high
solute
Water movement
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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B. Pressure potential (turgor pressure) (Ψp)B. Pressure potential (turgor pressure) (Ψp)
• the hydrostatic pressure produced by athe hydrostatic pressure produced by a
solution in a space divided by asolution in a space divided by a
differentially permeable membranedifferentially permeable membrane
• due to a differential in the concentrationsdue to a differential in the concentrations
of soluteof solute
• is increased as water enters a plant cellis increased as water enters a plant cell
• It is usually positiveIt is usually positive
• may be negative (tension) as in the xylemmay be negative (tension) as in the xylem
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• When a biological cell is in a hypotonicWhen a biological cell is in a hypotonic
environment (the cell interior contains aenvironment (the cell interior contains a
lower concentration of water)lower concentration of water)
• water flows across the cell membrane intowater flows across the cell membrane into
the cellthe cell
• expand due to an increases in the totalexpand due to an increases in the total
amount of water inside the cellamount of water inside the cell
• exerts a pressure on the cell wallexerts a pressure on the cell wall
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C. Matric potentialC. Matric potential (Ψm)(Ψm)
• due to the force of attraction of water fordue to the force of attraction of water for
colloidal, charged surfacescolloidal, charged surfaces
• It is negative because it reduces the ability ofIt is negative because it reduces the ability of
water to movewater to move
• In large volumes of water it is very small andIn large volumes of water it is very small and
usually ignoredusually ignored
• Water adheres electrostatically to solidWater adheres electrostatically to solid
hydrophilic surfacehydrophilic surface
• Capillary rise is associated with matrix potential.Capillary rise is associated with matrix potential.
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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D.D. Gravitational potential ( ΨgΨg)
• The potential energy associated with movingThe potential energy associated with moving
water to heightwater to height
• when referring to the top of tall treeswhen referring to the top of tall trees
ΨgΨg== ρghρgh
= 1000kg/M= 1000kg/M33
*9.8 m/S*9.8 m/S22
*h(m)*h(m)
= 0.01Mpa/m= 0.01Mpa/m
water in leaves at the top of a 100m tall treewater in leaves at the top of a 100m tall tree
suck water.suck water.
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Basic principles of water potentialBasic principles of water potential
• Water potential of pure water is zero,Water potential of pure water is zero,
open to the atmosphereopen to the atmosphere
• Water potential in intact plant tissue isWater potential in intact plant tissue is
usually negative because of the largeusually negative because of the large
quantities of dissolved solutes in cellsquantities of dissolved solutes in cells
• The addition of solute decreases waterThe addition of solute decreases water
potentialpotential
• The addition of pressure increases waterThe addition of pressure increases water
potentialpotential
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20082008
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• Plant cells will gain or lose water toPlant cells will gain or lose water to
intercellular fluids depending upon theirintercellular fluids depending upon their
water potentialwater potential
– A flaccid cell placed in a hyperosmoticA flaccid cell placed in a hyperosmotic
solution (low Ψw)solution (low Ψw)
– lose water by osmosislose water by osmosis
– the cell will plasmolyzethe cell will plasmolyze
– protoplast moves away from cell wallprotoplast moves away from cell wall
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20082008
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• Gives a measure of water statusGives a measure of water status
– Leaves of well-watered plants have waterLeaves of well-watered plants have water
potential ranging from -0.2 and -0.6Mpapotential ranging from -0.2 and -0.6Mpa
– Leaves of plants in arid climates posesLeaves of plants in arid climates poses
water potential between -2 and -5water potential between -2 and -5
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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Effect on physiological changesEffect on physiological changes
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2.3 Water movement in cells and tissues2.3 Water movement in cells and tissues
Mechanisms of movementMechanisms of movement
1.1. Bulk (or Mass) FlowBulk (or Mass) Flow
• mass movement of molecules in responsemass movement of molecules in response
to ato a pressure gradientpressure gradient
• The molecules move from high to lowThe molecules move from high to low
pressurepressure
• functions in long-distance transportfunctions in long-distance transport
• is usually along the vertical axis of theis usually along the vertical axis of the
plantplant Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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2. Diffusion2. Diffusion
• the net random movement of individualthe net random movement of individual
molecules driven by random thermal motionmolecules driven by random thermal motion
• is rapid over short distancesis rapid over short distances
a region of higha region of high
concentrationconcentration //
high chemicalhigh chemical
potentialpotential
area of high free energy
low free energy
region of lowregion of low
concentrationconcentration//
low chemicallow chemical
potentialpotential
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20082008
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•Molecules move until they reach dynamic equilibrium
•At equilibrium the net movement stops
•the molecules continue to move randomly
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Factors influencing the rate of diffusionFactors influencing the rate of diffusion
i. Concentration Gradienti. Concentration Gradient
• Solutes move from an area of highSolutes move from an area of high
concentration to one of lowerconcentration to one of lower
concentrationconcentration
• Fick’s Law : relates the rate of diffusion toFick’s Law : relates the rate of diffusion to
the concentration gradient (C1–C2) andthe concentration gradient (C1–C2) and
resistance (r)resistance (r)
Js = (C1-C2)/r Js= flux densityJs = (C1-C2)/r Js= flux density
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20082008
45. 4545
• The rate of diffusion is directly proportionalThe rate of diffusion is directly proportional
to the concentration gradientto the concentration gradient
• The greater the difference in concentrationThe greater the difference in concentration
between two areas, the greater the rate ofbetween two areas, the greater the rate of
diffusiondiffusion
• if the gradient is zero, there will be no netif the gradient is zero, there will be no net
diffusiondiffusion
• The greater the resistance to diffusion, the lowerThe greater the resistance to diffusion, the lower
the rate of diffusionthe rate of diffusion
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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ii. Molecular Speedii. Molecular Speed
• atoms and molecules are always in motion atatoms and molecules are always in motion at
temperatures above absolute zerotemperatures above absolute zero
• Molecular speed is directly proportional toMolecular speed is directly proportional to
temperaturetemperature
• At room temperature, the average velocity of aAt room temperature, the average velocity of a
molecule is =2 km/secmolecule is =2 km/sec
• indirectly related to molecular weightindirectly related to molecular weight
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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iii. Temperatureiii. Temperature
• increases the rate of molecularincreases the rate of molecular
movementmovement
• therefore, increases the rate of diffusiontherefore, increases the rate of diffusion
iv. Pressureiv. Pressure
increases speed of moleculesincreases speed of molecules
increase the rate of diffusionincrease the rate of diffusion
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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V. Solute effect on the chemical potential ofV. Solute effect on the chemical potential of
the solventthe solvent
• Solute particles decrease the free energySolute particles decrease the free energy
of a solventof a solvent
• factor is the number of particlesfactor is the number of particles
• Mole fraction of solventMole fraction of solvent = Number of solvent= Number of solvent
molecules/total number of solvent molecules + solutemolecules/total number of solvent molecules + solute
moleculesmolecules
Water moves from an area of higher moleWater moves from an area of higher mole
fractionfraction
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20082008
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3.Osmosis3.Osmosis
• This is a specialized case of diffusionThis is a specialized case of diffusion
• the diffusion of a solvent across a biologicalthe diffusion of a solvent across a biological
selectively permeable membraneselectively permeable membrane
• Movement is driven by the sum of aMovement is driven by the sum of a
concentration gradient and pressure gradientconcentration gradient and pressure gradient
• Osmosis to occur :Osmosis to occur :
– two solutions of the same solventtwo solutions of the same solvent
– separated with selectively permeable membraneseparated with selectively permeable membrane
– pressure and concentration gradientpressure and concentration gradient
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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4. Dialysis:4. Dialysis:
• This is another specialized caseThis is another specialized case
of diffusionof diffusion
• it is the diffusion of solute acrossit is the diffusion of solute across
a semi-permeable membrane.a semi-permeable membrane.
High
salt
Low salt buffer
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Transport of water inTransport of water in
PlantsPlants
Levels of transport in plants:Levels of transport in plants:
Movement of water and solutes into and outMovement of water and solutes into and out
of individual cellsof individual cells
Localized transport of material from cell toLocalized transport of material from cell to
cell at the level of tissues and organscell at the level of tissues and organs
Long-distance transport of sap throughoutLong-distance transport of sap throughout
the vascular tissues at the whole-plantthe vascular tissues at the whole-plant
levellevel Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Water diffuse from theWater diffuse from the soilsoil to theto the plantplant
rootroot and then to theand then to the airair ::
Water potential gradient is establishedWater potential gradient is established
between the root cell sap and the soil solutionbetween the root cell sap and the soil solution
Water potential may be established by:Water potential may be established by:
• Increasing the concentration of solutesIncreasing the concentration of solutes ..
– Water potential of soil solution isWater potential of soil solution is
highest than airhighest than air
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20082008
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• Physical state of waterPhysical state of water
– highest whenhighest when water is a liquid andwater is a liquid and
– lowest when water is a gas in airlowest when water is a gas in air ..
intimate contact between root hairintimate contact between root hair
and soil particlesand soil particles
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Water Transport in the RootWater Transport in the Root
Water is taken in to the root hairWater is taken in to the root hair
move into and within the plantmove into and within the plant
root in a variety of routsroot in a variety of routs
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20082008
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A.A. Apoplastic path wayApoplastic path way
movement of water through intercellularmovement of water through intercellular
spaces and cell wallspaces and cell wall
continuum formed between the continuouscontinuum formed between the continuous
matrix of cell wallsmatrix of cell walls
Water and solute move without entering aWater and solute move without entering a
cellcell
involves the non-living vascular tissueinvolves the non-living vascular tissue
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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blocked by the casparian strip atblocked by the casparian strip at
endodermisendodermis
Impermeable band of suberinImpermeable band of suberin
inside walls of endodermal cellsinside walls of endodermal cells
regulates the quantity and type ofregulates the quantity and type of
minerals and ions reach the xylemminerals and ions reach the xylem
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20082008
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B. Syplastic path wayB. Syplastic path way
the continuum of cytoplasm within athe continuum of cytoplasm within a
plant tissueplant tissue
formed by the plasmodesmataformed by the plasmodesmata
which pass through pores in the cellwhich pass through pores in the cell
walls.walls.
responsible in order water andresponsible in order water and
minerals to reach the xylem.minerals to reach the xylem.
This path way involves the livingThis path way involves the living
part of the cellpart of the cellBeira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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C. Transmembrane path wayC. Transmembrane path way
• Water sequentially moves from one cellWater sequentially moves from one cell
to the next cellto the next cell
• by repeatedly crossing plasmaby repeatedly crossing plasma
membranes and cell walls.membranes and cell walls.
• NB. Water and solute molecules canNB. Water and solute molecules can
movemove
– by any one of these routes orby any one of these routes or
– a combination through switching from one toa combination through switching from one to
anotheranother.. Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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The xylem pathwayThe xylem pathway
1. Root Pressure1. Root Pressure
• This a push forceThis a push force
• Generated as solute accumulates in the xylemGenerated as solute accumulates in the xylem
Due to the root's active absorption of dissolvedDue to the root's active absorption of dissolved
nutrientsnutrients
the water in the soil tends to be lower inthe water in the soil tends to be lower in
solutes than the water inside the root's cellssolutes than the water inside the root's cells
solute potential gradient developssolute potential gradient develops
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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water flows into the roots through osmosis andwater flows into the roots through osmosis and
osmotic pressureosmotic pressure increasesincreases
called root pressurecalled root pressure
a mechanism used by vascular plants to transporta mechanism used by vascular plants to transport
water through the xylem to the plant's higherwater through the xylem to the plant's higher
reachesreaches
only provide modest push water up the stemonly provide modest push water up the stem
is not enough to account for the movement ofis not enough to account for the movement of
water to leaves at the top of treeswater to leaves at the top of trees
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2. TACT Mechanism2. TACT Mechanism
The transpirational pull on the xylem sapThe transpirational pull on the xylem sap
is transmitted to the soil solutionis transmitted to the soil solution
Four forces combine to transport waterFour forces combine to transport water
solutions from the roots through the xylemsolutions from the roots through the xylem
elements, and into the leaveselements, and into the leaves
These TACT forces are:These TACT forces are:
• TranspirationTranspiration
• AdhesionAdhesion
• CohesionCohesion
• TensionTension
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i. Transpirationi. Transpiration
involves the pulling of water upinvolves the pulling of water up
through the xylem of a plantthrough the xylem of a plant
Utilize the energy of evaporation andUtilize the energy of evaporation and
the tensile strength of water.the tensile strength of water.
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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ii. Adhesionii. Adhesion
is the attractive force between wateris the attractive force between water
molecules and other substances.molecules and other substances.
both water and cellulose are polarboth water and cellulose are polar
moleculesmolecules
there is a strong attraction for water to thethere is a strong attraction for water to the
hydrophilic walls of xylem cellshydrophilic walls of xylem cells
The small diameter of vessels andThe small diameter of vessels and
tracheids is important to the adhesiontracheids is important to the adhesion
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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iii. Cohesioniii. Cohesion
is the attractive force between molecules of theis the attractive force between molecules of the
same substance.same substance.
high cohesive force due to the 4 hydrogen bondshigh cohesive force due to the 4 hydrogen bonds
water's cohesive force within xylem give it awater's cohesive force within xylem give it a
tensile strength equivalent to that of a steel wiretensile strength equivalent to that of a steel wire
of similar diameter.of similar diameter.
Cohesion of water allows for the pulling of waterCohesion of water allows for the pulling of water
from the top of the plant without breaking thefrom the top of the plant without breaking the
"chain"."chain".
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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71. 7171
iv. Tensioniv. Tension
a stress placed on an object by a pulling force.a stress placed on an object by a pulling force.
is created by the surface tension whichis created by the surface tension which
develops in the leaf's air spaces.develops in the leaf's air spaces.
The upward pull of sap causes tension (negativeThe upward pull of sap causes tension (negative
pressure) in xylempressure) in xylem
decreases water potential anddecreases water potential and
allows passive flow of water from soil into steleallows passive flow of water from soil into stele
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008
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Soil factors affecting water absorptionSoil factors affecting water absorption
Reading AssignmentReading Assignment
Until Next classUntil Next class
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
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1.1. What is the importance of the water potentialWhat is the importance of the water potential
concept in plant physiology? What are theconcept in plant physiology? What are the
components of water potential?components of water potential?
2.2. List three unique properties of water thatList three unique properties of water that
make it such a good for cellular functioningmake it such a good for cellular functioning
3.3. Can plant cells have negative turgorCan plant cells have negative turgor
pressure values? Explainpressure values? Explain
4.4. Describe the casparian strip and its function.Describe the casparian strip and its function.
Beira Hailu Meressa, JUCAVMBeira Hailu Meressa, JUCAVM
20082008