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Water comprises about 80-95% of
the mass of the young herbaceous plants Root,
Seed, corn (dry) Water
Water Content of Plant Tissues
Water is a key limiting factor for productivity
of agricultural crops and natural ecosystems Why? Blaming photosynthesis!
500:1 ratio for H2O/CO2
Major functions of water in plants
1] Water constitutes the large Central Vacuole in most plant cells.
This accounts for approximately 90% of the Protoplast.
Cytoplasm accounts for the remaining 10% and it also contains water.
2] It is the most abundant and best Solvent in nature.
It is the medium for major transport processes in plants.
It affects the structure of proteins, carbohydrates, & nucleic acids.
3] Water is the medium in which biochemical reactions occur in plants.
4] Water participates directly in some important biochemical reactions.
The oxygen we breathe comes from the splitting of water inphotosynthesis.
Water is continuously absorbed and lost by plants.
A leaf may loose 100% of its water content in one hour under dry, hot conditions.
A plant may loose its entire fresh weight 100 times during its lifetime.
5] Water evaporation is important for regulating Leaf Temperatures.
Transpiration is the process by which leaves regulate water loss.
Evaporation cools leaves and prevents them from overheating.
This may dissipate 50% of the heat from solar irradiation. Water
Water is important because it is polar and readily forms hydrogen bonds 20 kJ/mol . 1. Water molecules are attracted to one
another due to the bipolar nature of the
2. This weak "electrostatic" attraction is
called a "Hydrogen Bond".
3. much weaker than covalent bonds,
however, many H-bonds can be "strong". Polarity
Polarity makes it an excellent solvent--shells of hydration
1. Water dissolves substances to which it can readily hydrogen bond
2. For a substance to dissolve in water, the substance must displace water-to-water hydrogen bonds
3. Consequently, for a substance to readily dissolve in water, it must be something that water will
hydrogen bond to at least as well as water hydrogen bonds to itself
4. Furthermore, the substance must also more-readily hydrogen bond to water than it interacts with
molecules of its own kind; that way the molecule tends to leave the solid state and enter into solution
5. Within solution, such a substance will be surrounded by water molecules which are hydrogen
bonded to it
6. This surrounding array of water molecules is called a hydration shell Hydrogen
Hydrogen bonding is also responsible for other properties of water
1. Cohesion (Mutual attraction)
Cohesion refers to the tendency of water molecules to hydrogen bond to each other
It explains the ability of water to be siphoned as well
the related property of transport of water from the roots to the leaves of plants
Adhesion is the tendency of water to stick to substances other than water
3. Surface tension (the energy required to break H-bonding…)
Surface tension is an emergent property of water that results from the tendency of water
molecules to stick to each other (by hydrogen bonding) better than they adhere to air molecules QuickTime™ and a
are needed to see this picture. Capillary action is the result of adhesion, cohesion, and surface tension.
Adhesion of water to the walls of a vessel will cause an upward force on the liquid
at the edges and result in a meniscus which turns upward.
The surface tension acts to hold the surface intact, so instead of just the edges moving
upward, the whole liquid surface is dragged upward. Adhesion
Adhesion can be quantified
by the contact angel
“from the solid surface through the liquid
to the gas-liquid interface” Temperature buffering and cooling properties of water
High specific heat
The heat energy required to raise the temperature of a substance by 1 OC
1 calorie is required to raise 1 g of pure water 1OC
this value is higher than that of any other substance except liquid ammonia
water (1.00 cal/g/deg) > alcohol (0.58) > air (0.25) > copper (0.09) @ standard pressure,
Thus water can absorb large quantities of energy without much temperature increase Such a thermal property is important to buffer temperature fluctuations
High latent heat of vaporization
the energy needed to separate molecules from the liquid phase and move them
into the gas phase at constant temperature
For water at 25OC, the heat of vaporization is 44 kJ mol-1 Help plants to cool themselves by evaporating water from leaf surfaces. Water
Water has a high tensile strength
Defined as the maximum force per unit area that a continuous column of
water can withstand before breaking
Cohesion gives water a high tensile strength. A simple demonstration of water tensile strength
Hydrostatic pressure (+) Tension (-)
[1 MP = 9.9 atomspheres Pressure = force/unit area
1 Pal = 1N/m2 = 1 J/m3
The presence of gas bubble residues reduces the tensile strength of a
water column (cavitation). Water transport mechanisms
driven by concentration gradient
2. Bulk flow
driven by pressure gradient
driven by a water potential gradient Water
Water moving from the soil through the plant
to the atmosphere passes through a widely
variable medium such as cell wall, cytoplasm,
membrane, and air spaces, and the
mechanisms of water transport also varies
with the types of medium Diffusion
Diffusion is the movement of molecules by
random thermal agitation
resulting in the net movement of substances from
areas of High Concentration to areas of Low Concentration. Frick's
Frick's First Law
The Rate of Movement (the amount of substances
crossing a unit area per unit time)
is directly proportional to the Concentration Gradient
flux density Js = -Ds [Cs]/[x]
The Diffusion coefficient (Ds) is a proportionality
constant that measure how easily a given substance
moves through a particular medium The Ds is related to molecular size, the medium in
which it moves, and the temperature. Diffusion
Diffusion is rapid over short distances but extremely slow over long distances
The average time needed for a particle to diffuse a distance L is equal to
t = L2/Ds
where Ds is the diffusion coefficient
It takes 2.5 sec. for a glucose molecule to diffuse from one side of a 50 micron cell to the other.
However, it would take 32 years for the molecule to travel 1 meter by diffusion!
The rate of Diffusion can not match the requirements for the sustenance of large plant Bulk
Bulk Flow or Mass Flow
Bulk flow is the concerted movement of molecules en masse.
This is usually due to a Pressure Gradient
Pressure 1. Bulk Flow occurs regardless of solute concentration gradients!
2. The Rate of Bulk Flow through a hollow cylinder or pipe is related to many parameters.
Volume flow rate = [r4/8][p/x] (Poiseuilles Equation).
r: radius of the tube
, the viscosity of the liquid
[p/x], the pressure gradient that drives the flow
Thus, the Radius or Diameter of the pipe is the most important factor
If the Radius of a pipe is doubled the rate of Bulk Flow is increased by 16X! Question
Why do Vessel Members transport more water than Tracheids? Osmosis is the movement of a solvent like water
through a Membrane.
The plasma membrane of plant cells are selectively permeable
It is permeable to small, uncharged molecules.
It is relatively impermeable to large molecules and charged molecules like ions.
The direction and rate of water flow across a membrane are determined by
both concentration gradients and pressure gradients.
The Total Driving Force for Osmosis is called the Water potential.
Movement will occur
from areas of High Water Potential towards areas of Low Water Potential.
If the stem of the funnel is removed, there will be no outlet for the
Positive Hydrostatic Pressure that is produced by the influx of Water.
If the semi-permeable membrane is flexible and stretchable, it will
become bow towards the outside.
If the membrane is delicate, like the Plasmalemma, it will break.
If the membrane remains intact, water influx will continue until the Hydrostatic Pressure
inside the funnel makes the Water Potential of both compartments equal.
Water Water can cross plant membranes by diffusion of individual water molecules through
the membrane bilayer and by microscopic bulk flow of water molecules through a
water selective pore formed by integral membrane proteins such as aquaporins Three
Three major factors contribute to cell Water Potential
(free energy of water/unit volume; J m-3) w = s + p + g s Solute potential or osmotic potential
p Hydrostatic pressure of the solution
g Gravity potential
s = -RTCs
Where R is the gas constant [8.32 J mol-1K-1]
T is the absolute temperature
Cs is the solute concentration of the solution It
It is very important to remember that
[For ionic solutes that dissociate into 2 or more particles, C s must be multiplied by
the # of dissociated particles] The sign
The - sign indicates that dissolved solutes reduces the water
potential of a solution is relative to that of water
The water potential of pure water is 0 Pressure p The
The term Hydrostatic Pressure is used to indicate
the pressure component of Water Potential.
Positive Pressures increase the Water Potential.
[Positive Pressure would bring Water Molecules closer together.
This would foster a quasi-crystalline organization which has less
Entropy and greater Potential Energy] Negative Pressures reduce Water Potential.
[They pull water molecules further apart.
This disrupts Hydrogen Bonding & increases Entropy.
This lowers the potential energy of water]
The terms Tension & Negative Hydrostatic Pressure are used interchangeably.
Turgor Pressure is the Positive Hydrostatic Pressure that occurs inside plant cells. Gravity g g = wgh
Gravity causes water to move downward unless the gravity force
Is opposed by an equal and opposite force The
g depends on
The height (h) of the water above the reference-state water
The density of water (w)
The acceleration due to gravity (g)
Gravity is not a major consideration for plants less than 5 meters in height.
It is significant for taller trees. Examples
Examples illustrating the concept of water potential and its components 0.3 M Sucrose Examples
Examples illustrating the concept of water potential and its components One important point to remember:
Water flow is a passive process, water moves in response to physical forces,
Toward regions of low water potential or low free energy. ...
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This note was uploaded on 11/30/2011 for the course BIOLOGY 321 taught by Professor Min during the Winter '11 term at University of Michigan.
- Winter '11