lecture 3 [water and plant cell).pdf
TRANSCRIPT
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Water and Plant Cell
What are the role of water to plant?What are the role of water to plant?
Water plays a crucial role in the life of the plant.
For every gram of organic matter made by the plant,
approximately 500 g of water is absorbed by the roots,
transported through the plant body and lost to the
atmosphere.
Every plant must delicately balance its uptake and loss of
water.
To carry on photosynthesis, they need to draw carbon
dioxide from the atmosphere, but doing so exposes them
to water loss and the threat of dehydration.
Plants cell wall distinguishes plant cell from animal cell;
Cell walls allow plant cells to build up large internal
hydrostatic pressures, called turgor pressure, which are
a result of their normal water balance.
Why is Turgor pressure important?
Play roles in cell enlargement, gas exchange in the leaves,
transport in the phloem, and various transport processes
across membranes; and the rigidity and mechanical
stability of nonlignified plant tissues.
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So, what are we going to learn in this
chapter?
How water moves into and out of plant cells, emphasizing the molecular properties of water and the physical forces that influence water movement at the cell level.
WATER IN PLANT LIFE
Water typically constitutes:
80 to 95% of the mass of growing plant tissues;
Common vegetables such as carrots and lettuce may
contain 85 to 95% water;
Sapwood, which functions in transport in the xylem,
contains 35 to 75% water;
Seeds, with a water content of 5 to 15% (before
germinating must absorb water).
Water is the most abundant and arguably the best solvent
known.
During the plants lifetime, water equivalent to 100 times
the fresh weight of the plant may be lost through the leaf
surfaces, called transpiration.
Transpiration is an important means of dissipating the
heat input from sunlight
For a typical leaf, nearly half of the net heat input from
sunlight is dissipated by transpiration.
The stream of water taken up by the roots is an
important means of bringing dissolved soil minerals to the
root surface for absorption.
Of all the resources that plants need to grow and
function, water is the most abundant and at the same
time the most limiting for agricultural productivity (Figure
3.1).
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Yes, water availability likewise limits the productivity of
natural ecosystems (Figure 3.2).
THE STRUCTURE AND PROPERTIES
OF WATER
Water has special properties that enable it to act as a
solvent and to be readily transported through the body of
the plant.
The Polarity of Water Molecules Gives Rise
to Hydrogen Bonds
The water molecule consists of
an oxygen atom covalently
bonded to two hydrogen atoms.
Because the oxygen atom is
more electronegative than
hydrogen, it tends to attract the
electrons of the covalent bond.
This attraction results in a
partial negative charge at the
oxygen end of the molecule and
a partial positive charge at each
hydrogen.
The Polarity of Water Molecules Gives Rise
to Hydrogen Bonds
This unequal distribution of
electrons makes water a polar
molecule, meaning that the two
ends of the molecule have
opposite charges
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A covalent bond is the
sharing of a pair of valence
electrons by two atoms.
The anomalous properties of
water arise from attractions
between its polar molecules:
The slightly positive
hydrogen molecule is
attracted to the slightly
negative oxygen of a nearby
molecule.
The two molecules are thus
held together by a
Hydrogen bond (Figure
3.2).
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\Vhen water is in its liquid form, its hydrogen bonds are
very fragile, each about 1/20 as strong as a covalent bond.
The hydrogen bonds form, break, and reform with great
frequency. Each lasts only a few trillionths of a second, but
the molecules are constantly forming new hydrogen bonds
with a succession of partners.
Therefore, at any instant, a substantial percentage of all
the water molecules are hydrogen-bonded to their
neighbors.
The extraordinary qualities of water are emergent
properties resulting from the hydrogen bonding that
orders molecules into a higher level of structural
organization.
The Polarity of Water Makes It an Excellent Solvent
Excellent solvent = dissolves a variety of substances more
than other related solvents, because:
1. Small molecule size of water;
2. Its polarity nature (good for ionic substances and sugar
and proteins with OH or NH22 groups).
The hydrogen bonding (water&ion; water&polar solutes)
reduce electrostatic interaction between charged
substances increase solubility.
The Thermal Properties of Water Result from Hydrogen Bonding
The extensive hydrogen
bonding between water
molecules results in high
specific heat and high latent
heat of vaporization.
Specific heat:
the heat energy required to
raise the temperature of a
substance by a specific
amount.
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Energy is required to break the hydrogen bond.
Water requires a large energy input to raise its
temperature (compare to other liquid);
This large energy input requirement is important for
plants because it helps buffer temperature fluctuations.
Latent heat of vaporization : The energy needed to
separate molecules from the liquid phase and move them
into the gas phase at constant temperaturea process
that occurs during transpiration.
Water: 25C, the heat of vaporization is 44 kJ mol-1
(highest value known for any liquid).
Plants have hight latent heat of
vaporization, why?
Allow plants to cool themselves by evaporating water from leaf surfaces,
which are prone to heat up because of the radiant input from the sun.
The Cohesive and Adhesive Properties of
Water Are Due to Hydrogen Bonding
Surface tension: The energy required to increase the
surface area.
Surface tension at the evaporative surfaces of leaves
generates the physical forces that pull water through the
plants vascular system;
The extensive hydrogen bonding in water also gives rise
to the property known as cohesion, the mutual
attraction between molecules. [the hydrogen bonds hold
the substance together, a phenomenon called cohesion]
Arelated property, called adhesion, is the attraction of
water to a solid phase such as a cell wall or glass surface.
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Surface tension, a measure of how
difficult it is to stretch or break the
surface of a liquid.
Water has a greater surface
tension than most other liquids.
Water behave as though coated
with an invisible film. (Figure 3.4).
Cohesion, adhesion, and surface tension give rise to a phenomenon known as the movement
Cohesion, adhesion, and surface tension give rise to a phenomenon known as capillarity, the movement of water along a capillary tube.
The Cohesive and Adhesive Properties of
Water Are Due to Hydrogen Bonding
Water Has a High Tensile Strength
Tensile strength: the maximum force per unit area that
a continuous column of water can withstand before
breaking (given by cohesion).
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Water Has a High Tensile Strength
PUSH Positive hydrostatic pressure
PULL Negative hydrostatic pressure
Pressure is measured in units called pascals (Pa) or, more
conveniently, megapascals (MPa).
1 MPa equals approximately 9.9 atmospheres.
Pressure is equivalent to a force per unit area (1 Pa = 1 N
m2) and to an energy per unit volume (1 Pa = 1 J m3).
A newton (N) = 1 kg m s1.
WATER TRANSPORT PROCESSES
Diffusion Is the Movement of Molecules by
Random Thermal Agitation
Water molecules in a solution collide to each other
exchange kinetc energy;
Thermal agitation causes the molecules to intermingle;
Diffusion: movement of molecules from regions of high concentration to regions of low concentrationdown a concentration gradient (Figure 3.7)
Diffusion: movement of molecules from regions of high concentration to regions of low concentrationthat is, down a concentration gradient (Figure 3.7)
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Pressure-Driven Bulk Flow Drives Long-Distance Water Transport
Pressure-driven bulk flow is the concerted
movement of groups of molecules en masse, most
often in response to a pressure gradient.
Example: Water moving through a garden hose, a river
flowing, and rain falling.
The predominant mechanism responsible for longthe xylem. The predominant mechanism responsible for long-distance transport of water in the xylem.
The water flow through the soil and through the cell walls of plant tissues;The water flow through the soil and through the cell walls of plant tissues;
Unlike driven bulk flow is independent of solute concentration gradients,Unlike diffusion, pressure-driven bulk flow is independent of solute concentration gradients,
Osmosis Is Driven by a Water Potential
Gradient
Membranes of plant cells are selectively permeable;
that is, they allow the movement of water and other
small uncharged substances across them more readily
than the movement of larger solutes and charged
substances.
Osmosis: The direction and rate of water flow across a membrane are determined not solely by the concentration gradient of water or by the pressure gradient, but by the sum of these two driving forces.
The Chemical Potential of Water Represents the Free-Energy Status of Water
All organisms need energy to survive;
In plants, processes (biochemical reactions, solute
accumulation, and long-distance transport) are all driven
by an input of free energy into the plant.
The Chemical potential of water = amount of the free energy associated with water; [Energy per mole of substance (J mol-1)]
Historically, plant physiologists use Water potential: the
chemical potential of water divided by the partial molal volume
of water (the volume of 1 mol of water): 18
These units = unit for pressure:
Historically, plant physiologists use Water potential: the
chemical potential of water divided by the partial molal volume
of water (the volume of 1 mol of water): 18 106 m3 mol1.
These units = unit for pressure: Pascal.
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What is Water
Potential (W)?
It is a quantitative description of the free energy states of water. The concepts of free energy and water potential are derived from the second law of hermodynamics.
In thermodynamics, free energy is defined as the potential for performing work. A water fall is a good example. The water at the top of the fall has a higher potential for performing work than the water at the base of the fall. The water is moving from an area of higher free energy to an area of lower free energy. The free energy from water is the power source for waterwheels and hydroelectric facilities.
Water potential is a useful measurement to determine water-deficit stress in plants. Scientists use water potential measurements to determine drought tolerance in plants, the irrigation needs of different crops and how the water status of a plant affects the quality and yield of plants.
Water available for
uptake by plant roots
Atmospheric
Water
Potential
Water potential affects plants in many ways. Atmospheric water
potential is one of the factors that influences the rate of
transpiration or water loss in plants. Soil water potential
influences the water available for uptake by plant roots.
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YW = P + S + E + G
Where,
P = pressure potential
S = osmotic or solute potential
E = electrical potential
- ignore because water is uncharged
G = gravitational potential
- ignore because gravity is not a
large force for small trees
Current Convention Defines w as:
Yw = P + S
Where,
P = pressure potential
- represents the pressure in addition to
atmospheric pressure
S = osmotic or solute potential
- represents the effect of dissolved solutes on
water potential; addition of solutes will always
lower the water potential
Simplified Definition of w:
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Pressure Potential Positive Turgor (in cells with
membranes)
Negative Tension (in xylem)
Osmotic or Solute Potential
- Negative
SUMMARY:
Water Potential of Plant Tissue
has two components
and is always negative
Q U I Z