Transcript Water Potential

Plant Physiology 2014
RETNO MASTUTI
TRANSPORT AND TRANSLOCATION
OF WATER AND SOLUTES
1 Water and Plant Cells
1 - 1Properties of Water
Water is absolutely essential for all
living organisms
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Most organisms are comprised of at least 70% or
more water.
80-90% of a growing plant cell is water. This varies
between types of plant cells
Carrot has 85-95% water
Wood has 35-75% water
Seeds have 5-15% water
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Water is the limiting resource for crop productivity
in most agricultural systems
Plant continuously absorb and lose water
Water
Water is polar and readily forms
hydrogen bonds
Cohesion, adhesion → capillary action
The Properties of Water
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a liquid at physiological temperatures (0 – 100C)
high heat of vaporization → cooling system
high specific heat (heat capacity). → thermal buffer
high surface tension → capillary action, a meniscus forms. (negative
presssure  tension)
high tensile strength and incompressibility → water is good for
hydraulic systems and produces positive pressures (hydrostatic
pressures). This pressure provides the driving force for cell growth
and other plant movements
dissociates into protons and hydroxide ions.
Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321
 High heat of vaporization. Large quantities of energy
(about 44 kJ mol-1) is required to change water from liquid
phase to gas phase at constant temperature.
 Specific heat is the specific amount of heat in calories
(4.184 J g-1 C-1) needed to raise the temperature of 1 gram
of water by 1 degree Celsius  to beak the hydrogen bond
Surface tension and capillary action of water
Acids and Bases
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Water ionizes to form a hydrogen ion (or proton/H+) and
hydroxide ion (OH-)
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An acid is a substance that increases the [H+], or as a proton
donor.
eg. HCl → H+ + Cl-
3.
A base is a substance that increases the [OH-]; or from the
perspective of a proton, a base is a substance that decreases
the proton concentration; it is a proton acceptor.
e.g. NaOH → Na+ + OH- (accepts protons to make water)
e.g. NH3 (ammonia) + H+ → NH4+ (ammonium ion)
4.
pH increases, the [H+] decreases and the [OH-] increases
pH decreases, the [H+] increases and the [OH-] decrease
Living systems are very sensitive to pH
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Organisms must maintain pH within tolerable
ranges. This is a good example of homeostasis.
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A buffer is a solution that resists fluctuations in
pH when additional OH- or H+ are added.
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A buffer maintain a constant pH and usually
consist of a proton donor and a proton acceptor.
Functions of Water
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a major component of cells
a solvent for the uptake and transport of materials - a universal
solvent
a good medium for biochemical reactions - chemically inert
a reactant in many biochemical reactions (i.e., photosynthesis) ;
transparent to light → photosynthesis
cell elongation and growth
thermal buffer
provides structural support via turgor pressure (i.e., leaves)
the medium for the transfer of plant gametes (sperms swim to
eggs in water, some aquatic plants shed pollen underwater)
offspring (propagule) dispersal (think "coconut")
plant movements are the result of water moving into and out of
those parts (i.e., diurnal movements, stomatal opening, flower
opening)
Water and Plant Cells
1 – 2 Water Potential
The factors affecting water moves
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Its concentration (chemical potential): water will diffuse from a
dilute solution to concentrated solution (osmosis).
Its pressure (mechanical potential): water will move from a high
pressure system (hose-pipe) to a low pressure system (vacuum).
Its height (gravitational potential): water will flow downhill.
These factors sum together to give the overall free energy of a
mass of water
Water Potential
Measure of the energy state of water.
This concept is important in plant
physiology because it determines the
direction and movement of water.
A. Some definitions :
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Free energy of water : energy available to do work (J
= N m)
Chemical potential (µ) : free energy/unit quantity (per
mole) (J mol-1)
Water potential (Ψw) - chemical potential of water,
compared to pure water at the same temperature and
pressure (atm pressure).
The water potential of pure water is zero
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Water potential is defined as the chemical
potential of water divided by the partial molar
volume.
J mol-1/ L mol-1 = J / liter = energy / volume =
(weight x distance)/area x distance =
force/area = pressure
J mol-1/ L mol-1 = (N x m mol-1)/ (m3 mol-1)
= N m-2 = MPa
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Pressure is measured in MPa (megapascals).
1 MPa = 10 bars = 10 atm (1 atm = 760 mm Hg
= 14. 7 lbs sq in-1)
B. Equation for water potential
Ψw = Ψp + Ψs + Ψm + Ψg
Water potential is the sum of the contributions of the
various factors that influence water potential
 Ψw = water potential
 Ψp = pressure potential
 Ψs = solute or osmotic potential
 Ψm = matrix potential, and
 Ψg = gravity potential
Pressure (Pressure Potential; Ψp)
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Due to the pressure build up in cells against to the wall.
It is usually positive, although may be negative (tension) as in
the xylem.
Pressure can be measured with an osmometer.
a large positive pressure potential
flaccid or plasmolysed cell
(right) have a pressure
potential of zero
Solute (or osmotic) potential (Ψs)
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This is the contribution due to dissolved solutes.
Solutes always decrease the free energy of water, thus there
contribution is always negative.
The solute potential of a solution can be calculated with the van’t
Hoff equation:
Ψs = - miRT where,
m = molality (moles/1000 g);
i = ionization constant (often 1.0);
R = gas constant (0.0083 liter x MPa/mol)
T = temperature (K).
Matric potential (Ψm)
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This is the contribution to water potential due to the
force of attraction of water for colloidal, charged
surfaces.
It is negative because it reduces the ability of water
to move.
In large volumes of water it is very small and
usually ignored.
However, it can be very important in the soil,
especially when referring to the root/soil interface.
Gravity (Ψg)
 Contributions due to gravity which is usually ignored unless
referring to the tops of tall trees.
Ψw = Ψp + Ψs
Water and Plant Cells
1 – 3 Water Movement
Water Movement
There are two major ways to move molecules:
A. Diffusion : spontaneous, random movement of
molecules from an area of high free energy (higher
concentration) to one of low free energy (lower
concentration) → concentration gradient.
Does not require energy (exergonic). Net diffusion stops
when concentration on both sides equal (if crossing a
membrane) or when there is a uniform distribution of
particles. When equilibrium is reached : Molecules continue to
move, but no net change in concentration.
B. Osmosis: diffusion through selective permeable membrane
C. Bulk (or Mass) Flow. : mass movement of molecules in
response to a pressure gradient, from high to low
pressure, following a pressure gradient.
Factors influencing the rate of
diffusion
Fick’s Law
Jv = (C1 - C2)
r
Diffusion rate (Jv), concentration gradient (C1 – C2) and resistance (r)
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Concentration Gradient. directly proportional to the
concentration gradient, indirectly proportional to resistance,
inversely proportional to distance traveled
Molecular Speed : directly proportional to temperature and
indirectly related to molecular weight
Temperature
Pressure
Solute effect on the chemical potential of the solvent. Solute
particles decrease the free energy of a solvent. The critical factor
is the number of particles, not charge or particle size.
Diffusion - Osmosis
- Osmosis - diffusion of water.
- Osmosis affects the turgidity of
cells, different solution can affect
the cells internal water amounts
Turgor pressure occurs in plants cells as their central vacuoles fill with water.