Water in plant life

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• Lecture 4
• Water in plant life
• Water transport

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The Importance of Water in Plant Life

• Water is the most abundant and best solvent known
• Helps in movement of molecules within the cell
• Influences structure of proteins, nucleic acids,
  polysaccharides etc.
• Forms environment in which most biochemical
  reactions of the cell occur
• Directly participates in chemical reactions
• Builds up hydrostatic pressures - turgor pressure
  - within cell due to cell wall
• Turgor pressure important for cell enlargement,
  gas exchange, transport processes, rigidity and
  stability of non-lignified plant tissue
• Transpiration, i.e. water loss through leaf
  surfaces, is important for dissipation of heat
  from sunlight
• Water uptake by roots brings in soil minerals for
  absorption

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Structure and Properties of Water

• Polarity of water gives rise to hydrogen bonds
• Its polarity makes it an excellent solvent
• Its thermal properties result form hydrogen
  bonding
• Specific heat the heat energy required to raise
  the temperature of a substance by a specific
  amount
• Latent heat of vaporization the energy needed to
  separate molecules from the liquid phase and move
  them into the gas phase at constant temperature
  (occurs during transpiration).
• Cohesive and adhesive properties are due to
  hydrogen bonds (surface tension and capillarity
  -gt important for transport through xylem)
• Has a high tensile strength due to cohesion

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The Water Molecule
Because oxygen is more electronegative than
hydrogen, it tends to attract the electrons of
the covalent bond ? polar molecule
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The Polarity of Water Molecules Gives Rise to
Hydrogen Bonds
Hydrogen bonding between water molecules results
in local aggregations of water molecules.
Because of continuous thermal agitation of water
molecules, these aggregations are short-lived.
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Water Transport Processes

• Diffusion following a concentration gradient
• Bulk Flow following a pressure gradient
• Osmosis following a concentration pressure
  gradient

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Diffusion of Water through the Plasma Membrane
(aquaporin)
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Diffusion
is the movement of molecules by random thermal
agitation In the 1880s, German Scientist Adolf
Fick ? Ficks First Law Js -Ds (?Cs/?x) Js
the amount of substance crossing a unit area per
unit time (mol m-2 s-1) Ds the
diffusion coefficient measures how easily a
substance S moves through a particular
medium (large molecules have smaller
Ds) depends on the medium (diffusion in air is
faster than in liquid) ?Cs difference
in concentration of a substance ?x distance
difference ?Cs/?x concentration
gradient negative sign indicates that flux moves
down a concentration gradient Rate of diffusion
is high when Ds is increased or ?Cs increased.
Diffusion is faster over short and slow over
long distances.
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Ficks Law Thermal motion of molecules leads to
diffusion
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Pressure-Driven Bulk (or mass) Flow Drives
Long-Distance Water Transport

• Bulk flow is the concerted movement of groups of
  molecules en masse, most often in response to a
  pressure gradient
• E.g. water through gardening hose, a river
  flowing
• Consider bulk flow through a tube ? the rate of
  volume flow depends on the radius (r) of the
  tube, the viscosity (?) of the liquid, and the
  pressure gradient (??p/?x) ? Poiseuilles
  equation Volume flow rate (pr4/8?)(??p/?x)
  m3 s-1
• Pressure-driven bulk flow is sensitive to the
  radius (if radius is
• doubled, volume flow rate increases by 16 (24)
• Pressure-driven bulk flow
• ? predominant mechanism for long-distance
  transport of water in the xylem!
• ? water flow through soil and cell walls
• ? independent of solute concentration gradient
  (as long as
• viscosity changes are negligible)

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Osmosis Is Driven by a Water Potential Gradient
What is Osmosis? The passage of water (solvent)
from a region of high water concentration through
a semi-permeable membrane to a region of low
water concentration.
Visit this link for an animation of
osmosis http//www.tvdsb.on.ca/westmin/science/sb
i3a1/Cells/Osmosis.htm
Hypertonic Solutions Contain a high
concentration of solute (e.g. salts, sugars)
relative to another solution (e.g. the cell's
cytoplasm). When a cell is in a hypertonic
solution, water diffuses out of the cell, causing
the cell to shrivel (plasmolysis). Hypotonic
Solutions Contain a low concentration of solute
relative to another solution. When a cell is in a
hypotonic solution, water diffuses into the cell,
causing the cell to swell and possibly explode
(cytolysis). Isotonic Solutions Contain the
same concentration of solute as an another
solution. When a cell is in an isotonic solution,
water diffuses into and out of the cell at the
same rate. The fluid that surrounds the cells is
isotonic.
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Plasmolysis
the contraction of cells within plants due to
the loss of water through osmosis. Cell membrane
peels off of the cell wall and vacuole collapses
when placed in a hypertonic environment. The
opposite of plasmolysis in plant cells is
cytolysis.
Epidermal cells of Rhoeo discolor (oyster
plant) Before plasmolysis After plasmolysis
Vacuoles (pink) fill out the whole cells.
Vacuoles (pink) have shrunken.
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Water Potential
a measure of the free energy of water per unit
volume expressed in pressure units, Pascal,
mainly MPa (megaPascals) hydrostatic pressure
for pure water in an open container is 0
MPa (the reference state of water)
?w ?s ?p ?g ?m
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Cell Water Potential
?w ?s ?p ?g ?m

• ?s solute (osmotic) potential the effect of
  solutes on water potential (solutes reduce the
  free energy of water by diluting the water)
• ?p pressure potential or hydrostatic potential
  (positive pressures raise water potential)
  turgor pressure is positive hydrostatic pressure
  within cells negative pressure between cells and
  in walls between cells ? important for long
  distance transport
• Measured as the deviation from ambient pressure ?
  ?p 0 MPa for water in the standard state
• ?g gravity potential depends on height and
  density of the water above the reference state of
  water, and acceleration due to gravity
  negligible on cell level, but important in tall
  plants or trees
• ?m matrix potential interaction between water
  and solid surfaces (soils, cell walls) often not
  used
• ?w ?s ?p

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Cell Water Potential
Osmotic potential may be estimated by the vant
Hoff equation ?s - RTcs R gas constant
(8.32 J mol-1K-1) T absolute temperature (K) cs
solute concentration, in osmolality (mol
L-1) The minus sign indicates that dissolved
solutes reduce the water potential of a solution
relative to the reference state of pure water.
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Water Enters The Cell Along a Water Potential
Gradient
?w ?s ?p
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Water enters the Cell along a Water Potential
Gradient
?w ?s ?p
Desiccated cell in air
0.1 M Sucrose solution
Water potential is a measure of how hydrated a
plant is!
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Water Can Also Leave the Cell in Response to a
Water Potential Gradient
Water flow is a passive process, i.e. water moves
in response to physical forces, toward regions of
low water potential or low free energy - there
are no metabolic pumps (driven by ATP
hydrolysis) that push water from one place to
another (if water is the only substance
being transported) - when solutes are
transported, water transported may be coupled,
i.e. water is moved against a water potential
gradient Example transport of sugars, amino
acids by membrane proteins may drag up to 260
water molecules across the membrane per molecule
of solute transported
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The Water Potential Concept Helps Us Evaluate
The Water Status of a Plant
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The Water Potential Concept Helps Us Evaluate
The Water Status of a Plant
By measuring the water potential we can also
determine - whether water in a leaf will
vaporize into the atmosphere - whether water
in the soil will move into the roots or
vacuole of a plant