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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