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Chapter 3
Water and plant cells
Importance of water on crop yield
Productivity of various ecosystems as a
function of annual precipitation
 Most (~97%) of the water absorbed by roots is
carried through the plant and evaporates from leaf
surfaces. => transpiration
 In contrast, only a small amount of the water
absorbed by roots actually remains in the plant to
supply growth (~2%) or to be used in
photosynthesis and other metabolic processes (~1%).
 The uptake of CO2 is coupled to water loss.
 The concentration gradient for water loss from
leaves is much larger than for CO2 uptake.
Physical and chemical properties of water
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Polar
H-bonds
High melting point
High boiling point
High specific heat
High thermal conductivity
High heat of vaporization : energy required to convert a
substance from the liquid to the vapor state
(The heat of vaporization accounts for the cooling effect
associated with heat loss as water evaporate from leaves.)
 An excellent solvent => hydration shell
 Cohesion (內聚力)
 Adhesion (附著力)
(continued)
Physical and chemical properties of water
 Cohesion (內聚力) : mutual attraction between water
molecules
 Surface tension : cohesion force between water
molecules is higher than that between water and air
 Tensile strength (抗張強度) : the ability to resist a
pulling force
The maximum force for a continuous column of water
to withstand before breaking
 Adhesion (附著力) : attraction of water to a solid phase
such as cell wall
The Solubilization of Sodium Chloride
 Pushing on the plunger compresses the fluid, and a positive
pressure builds up.
=> positive hydrostatic pressure (流體靜壓)
 Pulling on the plunger causes the fluid to develop a tension,
or a negative pressure.
=> negative hydrostatic pressure.
 The hydrostatic pressure is positive in plant cells and is
referred as turgor pressure (p ).
The combination of cohesion, adhesion, and tensile
strength result in capillarity (毛細管作用).
Translocation of water
 Diffusion
 Bulk flow
The size of the pore is about 0.3nm = the size of a water molecule.
Flow rate = 1 X 109 molecules/sec
Diffusion (擴散)
 Diffusion means molecules move down a concentration
gradient.
 Fick’s first law (1855) :
The rate of diffusion is directly proportional to the
concentration gradient and is inversely proportional to
the length of the path.
 Diffusion is rapid over short distances but extremely
slow over long distances.
Bulk flow
 also called “mass flow”.
 Movement of groups of molecules en mass in
response to a pressure gradient.
 Bulk flow is driven by pressure.
 Bulk flow of water is the predominant
mechanism responsible for long-distance
transport of water in plants via the xylem and
the water flow through the soil and the cell
walls of plant tissue.
Osmosis
 Movement of a solvent (water) through a selectively
permeable membrane is called osmosis (滲透).
 Occurrence of osmosis is dependent on the gradient in free
energy of water across the membrane.
 The free-energy gradient of water = driving force of water
movement.
 In osmosis, both concentration gradient and pressure
gradient will influence transport.
 The forces involved in osmosis is called osmotic pressure.
 For an isolated solution, it has only an osmotic potential.
Osmotic potential (滲透勢能) is the negative of the osmotic
pressure, since they are equal but opposite forces.
Osmotic pressure
Hydrostatic
pressure
developed in
the tube
Osmosis
 The magnitude of the osmotic pressure is a function
of solute concentration.
 If addition pressure were applied, we might expect
the net movement of water to reverse its direction
and instead flow out of the solution.
 Therefore, osmosis is driven by the solute
concentration as well as by the pressure differences.
 In osmosis, both concentration gradient and pressure
gradient will influence the overall chemical potential
of water (water potential), which is the ultimate
driving force for water movement in plants.
 Osmosis is driven by a water potential gradient.
Water potential
 Also called chemical potential of water.
 A quantitative expression of the free energy
associated with water
 Free energy means a potential for performing work
or the energy that is free and available for
performing work.
 Water potential is a measure of the free energy of
water per unit volume (J/m3)
 Water moves down a chemical potential gradient
from a region of high chemical potential to a region
of low chemical potential.
Water potential (水分勢能)
 Water potential in plants is influenced by
three major factors :
Concentration
Pressure
gravity
 w = s + p + g
w : water potential
s : osmotic potential (solute potential)
p : hydrostatic pressure of the solution
g : the effect of gravity on water potential
w : water potential
 w inside plant cells is negative.
 w is proportional to the work required
to move 1 mole of pure water.
s : osmotic potential (solute potential)
 The effect of dissolved solutes on water potential
 Dissolving of solutes increases the disorder of water and
reduce the free energy of water by diluting the water.
 Solutes reduce the vapor pressure of a solution, raise its
boiling point, and lower its freezing point.
 s = -RTCs
R : gas constant
T: absolute temp.
Cs: osmolality ([Solute]/L)
The minus sign indicates that dissolved solute reduces the
water potential of a solution.
p : hydrostatic pressure of the solution
 Positive hydrostatic pressures raise the water potential;
negative hydrostatic pressure reduce the water potential.
 The positive hydrostatic pressure within plant cells is
referred as turgor pressure.
 Water moves from high turgor pressure to the region with
low turgor pressure.
 The xylem and cell walls between cells have negative
hydrostatic pressure (or tension).
 During daytime, transpiration increases =>
p decrease (negative)
 During nighttime, transpiration decrease =>
p increase (plants rehydrated)
p => +
g : the effect of gravity on water potential
 g is influenced by height (h) of the water, the
density of water (rw), and the acceleration due to
gravity (g).
 g = rwgh
 Gravity causes water to move downward.
 When dealing with water transport at the cell level,
the gravitational component (g) is generally
omitted because it is negligible compared to the
osmotic potential and the hydrostatic pressure.
 Water flow is a passive process.
 Water moves in response to physical forces toward
region of low water potential or free energy; that is
the direction of water flow is determined by the
direction of the gradient, which is the driving force
for transport.
 The rate of water transport depends on
Driving forces (Δψw )
Water potential difference across the
membrane
Hydraulic conductivity (Lp) = membrane
permeability
 Flow rate = driving force X hydraulic
conductivity
Hydraulic conductivity
 Volume of water per unit area of membrane per
unit time per unit driving force (MS-1MPa-1)
 The larger the hydraulic conductivity, the larger
the flow rate.
Why do we study water potential?
 Water potential is the quantity that govern
water transport across cell membranes.
 Water potential is usually used as a
measure of the water status of a plant.
Water potential of plants under various
growing conditions
END
The concentration gradient of a solute that is
diffusing according to Fick’s law
The pressure chamber method for measuring plant water potential. The diagram at
left shows a shoot sealed into a chamber, which may be pressurized with compressed
gas. The diagrams at right show the state of the water columns within the xylem at
three points in time: (A) The xylem is uncut and under a negative pressure, or
tension. (B) The shoot is cut, causing the water to pull back into the tissue, away
from the cut surface, in response to the tension in the xylem. (C) The chamber is
pressurized, bringing the xylem sap back to the cut surface.
Measuring cell turgor pressure