2. Water Relations - Part 1 chapter 3

Question 1

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

Answer 1

absorbed by the roots, transported through the plant body and lost to the atmosphere.

Even slight imbalances in this flow of water can cause water deficits and severe malfunctioning of many cellular processes.
Thus, every plant must delicately balance its uptake and loss of water.

Question 2

This water balancing in plants is a serious challenge for land plants. To carry on photosynthesis, they need to

Answer 2

draw carbon dioxide from the atmosphere, but doing so exposes them to water loss and the threat of dehydration.

Question 3

A major difference between plant and animal cells that affects virtually all aspects of their relation with water is the existence in plants of

Answer 3

the cell wall.

Question 4

Cell walls allow plant cells to build up

Answer 4

large internal hydrostatic pressures, called turgor pressure, which are a result of their normal water balance.

Question 5

Turgor pressure is essential for

Answer 5

many physiological processes, including
cell enlargement
gas exchange in the leaves
transport in the phloem, and various transport processes across membranes.

Turgor pressure also contributes to the rigidity and mechanical stability of nonligni-fied plant tissues.

Question 6

Water makes up most of the mass of plant cells, as we can readily appreciate if we look at microscopic sections of mature plant cells: Each cell contains a large water-filled

Answer 6

vacuole.

In such cells the cytoplasm makes up only 5 to 10% of the cell volume; the remainder is vacuole.

Question 7

Water typically constitutes 80 to 95% of the mass of

Answer 7

growing plant tissues.

Question 8

Common vegetables such as carrots and lettuce may contain how much % water?

Answer 8

Common vegetables such as carrots and lettuce may contain 85 to 95% water.

Question 9

Wood, which is composed mostly of dead cells, has a lower

Answer 9

water content
heartwood has a slightly lower water content

Question 10

sapwood, which functions in transport in the xylem, contains how much water?

Answer 10

contains 35 to 75% water;

Question 11

Seeds, with a water content of 5 to 15%, are among the

Answer 11

driest of plant tissues, yet before germinating they must
absorb a considerable amount of water.

Question 12

Water is the most abundant and arguably the best solvent known. As a solvent, it

Answer 12

makes up the medium for the movement of molecules within and between cells and greatly influences the structure of proteins, nucleic acids, polysaccharides, and other cell constituents.

Water forms the environment in which most of the biochemical reactions of the cell occur

it directly participates in many essential chemical reactions.

Question 13

Plants continuously absorb and lose water. Most of the
water lost by the plant evaporates from the leaf as

Answer 13

the CO2 needed for photosynthesis is absorbed from the atmosphere.

Question 14

On a warm, dry, sunny day a leaf will exchange up
to

Answer 14

100% of its water in a single hour.

During the plant’s life-time, water equivalent to 100 times the fresh weight of the plant may be lost through the leaf surfaces. Such water loss is called transpiration

Question 15

what is transpiration

Answer 15

transpiration is a passive process by which water evaporate from the leaves of the plant

Question 16

Transpiration is an important means of dissipating the

Answer 16

heat input from sunlight.

Question 17

Heat dissipates because

Answer 17

the water molecules that escape into the atmosphere have higher-than-average energy, which breaks the bonds holding them in the liquid.

When these molecules escape, they leave behind a mass of molecules with lower-than-average energy and thus a cooler body of water.

For a typical leaf, nearly half of the net heat input from sunlight is dissipated by transpiration.

In addition, the stream of water taken up by the roots is an important means of bringing dissolved soil minerals to the root surface for absorption.

Question 18

For a typical leaf, nearly half of the net heat input from sunlight is dissipated by

Answer 18

transpiration.

Question 19

In addition, the stream of water taken up by the roots is an important means of

Answer 19

bringing dissolved soil minerals to the root surface for absorption.

Question 20

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

The fact that water is limiting is the reason for the practice of

Answer 20

crop irrigation.
Water availability likewise limits the productivity of natural ecosystems

Thus an understanding of the uptake and loss of water by plants is
very important.

Question 21

Water has special properties that enable it to act as a solvent and to be readily transported through the body of the plant. These properties derive primarily from the

Answer 21

polar structure of the water molecule.

Question 22

The water molecule consists of an

Answer 22

oxygen atom covalently bonded to two hydrogen atoms.

The two O—H bonds form an angle of 105°

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

Question 23

in the water molecule the two O—H bonds form

Answer 23

an angle of 105°

Question 24

in a water molecule the oxygen atom is more electronegative than the hydrogen which means

Answer 24

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

Oxygen has 6 electrons in the outer orbitals; each hydrogen has 1

Question 25

in the water molecule These partial charges are equal, so the water molecule carries

Answer 25

no net charge.

Question 26

This separation of partial charges, together with the shape of the water molecule, makes water a polar molecule, and the opposite partial charges between neighboring water molecules tend to

Answer 26

attract each other.

Question 27

what is a hydrogen bond

Answer 27

The weak electrostatic attraction between water molecules, known as a hydrogen bond, is responsible for many of the unusual
physical properties of water.

Question 28

Hydrogen bonds can also form between water and other
molecules that contain

Answer 28

electronegative atoms (O or N).

Question 29

In aqueous solutions, hydrogen bonding between water molecules leads to

Answer 29

local, ordered clusters of water that, because
of the continuous thermal agitation of the water molecules,
continually form, break up, and re-form

Question 30

Hydrogen bonding between water molecules results in local aggregations of water molecules.
Because of the continuous thermal agitation of the water molecules, these aggregations are very

Answer 30

short-lived; they break up rapidly to form much more random configurations

Question 31

Water is an excellent solvent: It dissolves greater amounts of a wider variety of substances than do other related solvents. This versatility as a solvent is due in part to

Answer 31

the small size of the water molecule and in part to its polar nature.

Question 32

the polar nature of water makes water a particularly good solvent for

Answer 32

ionic substances and for molecules such as sugars and proteins
that contain polar —OH or —NH2 groups.

Question 33

Hydrogen bonding between water molecules and ions, and between water and polar solutes, in solution effectively decreases the

Answer 33

electrostatic interaction between the charged substances and thereby increases their solubility.

Question 34

The polar ends of water molecules can orient themselves next to charged or partially charged groups in macromolecules, forming

Answer 34

shells of hydration. Hydrogen bonding between macromolecules and water reduces the interaction between the macromolecules and helps draw them into solution

Question 35

The extensive hydrogen bonding between water molecules
results in

Answer 35

unusual thermal properties, such as high specific heat and
high latent heat of vaporization.

Question 36

Specific heat is

Answer 36

The heat energy required to raise the temperature of a substance by a specific amount.

Question 37

HOW DOES WATER HAVE a high specific heat

Answer 37

When the temperature of water is raised, the molecules
vibrate faster and with greater amplitude.
To allow for this motion, energy must be added to the system to break the hydrogen bonds between water molecules.

Thus, compared with other liquids, water requires a relatively large
energy input to raise its temperature.

This large energy
input requirement is important for plants because it helps
buffer temperature fluctuations

Question 38

Latent heat of vaporization is

Answer 38

the energy needed to separate molecules from the liquid phase
and move them into the gas phase at constant temperature—a process that occurs during transpiration.

Question 39

For water at 25°C, the heat of vaporization is

Answer 39

44 kJ mol –1 —the highest value known for any liquid.

Most of this energy is used to break hydrogen bonds between
water molecules.

Question 40

The high latent heat of vaporization of water enables plants to

Answer 40

cool themselves by evaporating water from leaf surfaces, which
are prone to heat up because of the radiant input from the sun.

Transpiration is an important component of temperature regulation in plants

Question 41

The Cohesive and Adhesive Properties of Water
Are Due to

Answer 41

Hydrogen Bonding

Question 42

Water molecules at an air–water interface are more strongly
attracted to neighboring water molecules than to the

Answer 42

gas phase in contact with the water surface.
As a consequence of this unequal attraction, an air–water interface minimizes its surface area.

To increase the area of an air–water interface,hydrogen bonds must be broken, which requires an input of energy.

The energy required to increase the surface area is known as surface tension.

Question 43

Surface tension not only influences the shape of the surface but also may create a

Answer 43

pressure in the rest of the liquid.

Question 44

The extensive hydrogen bonding in water also gives rise
to the property known as cohesion which is,

Answer 44

the mutual attraction between molecules.

Question 45

adhesion is

Answer 45

The attraction of water to a solid phase such as a cell wall
or glass surface.

Question 46

Cohesion, adhesion, and surface tension give rise to a phenomenon known as

Answer 46

capillarity, the movement of water along a capillary tube.

Question 47

In a vertically oriented glass capillary tube, the upward
movement of water is due to

Answer 47

(1) the attraction of water to the polar surface of the glass tube (adhesion) and

(2) the surface tension of water, which tends to minimize the area
of the air–water interface.

Together, adhesion and surface tension pull on the water molecules, causing them to move up the tube until the upward force is balanced by the weight of the water column.

The smaller the tube, the higher the capillary rise

Question 48

The smaller the tube, the higher the

Answer 48

capillary rise.

Question 49

Cohesion gives water a high tensile strength, defined as
the

Answer 49

maximum force per unit area that a continuous column
of water can withstand before breaking.

We do not usually
think of liquids as having tensile strength; however, such a
property must exist for a water column to be pulled up a
capillary tube.

Question 50

We can demonstrate the tensile strength of water by

Answer 50

placing it in a capped syringe

When we push on the plunger, the water is compressed and a positive hydrostatic pressure builds up.

Question 51

Pressure is measured in units called

Answer 51

pascals (Pa) or, more conveniently, megapascals (MPa).

Question 52

One MPa equals approximately

Answer 52

9.9 atmospheres.

Question 53

Pressure is equivalent to a force per unit

Answer 53

area (1 Pa = 1 N m –2 ) and
to an energy per unit volume (1 Pa = 1 J m–3).

A newton (N) = 1 kg m s–1.

Question 54

If instead of pushing on the plunger we pull on it, a tension, or negative hydrostatic pressure, develops in

Answer 54

the water to resist the pull.

How hard must we pull on the plunger before the water molecules are torn away from each other and the water column breaks? Breaking the water column requires sufficient energy to break the hydrogen bonds that attract water molecules to one another.

Question 55

Careful studies have demonstrated that water in small
capillaries can resist tensions more negative than

Answer 55

–30 MPa (the negative sign indicates tension, as opposed to com-
pression).

This value is only a fraction of the theoretical tensile strength of water computed on the basis of the strength of hydrogen bonds.

Nevertheless, it is quite substantial.

Question 56

The presence of gas bubbles reduces the

Answer 56

tensile strength of a water column.

Question 57

expansion of microscopic bubbles often interferes with the ability of the water to resist the pull exerted by the plunger. If a tiny gas bubble forms in a column of water under tension, the gas bubble may expand indefinitely, with the result that

Answer 57

the tension in the liquid phase collapses, a phenomenon known as cavitation.

Question 58

A sealed syringe can be used to create

Answer 58

positive and negative pressures in a fluid like water.

Pushing on the plunger compresses the fluid, and a positive pressure
builds up.

If a small air bubble is trapped within the syringe, it shrinks as the pressure increases.

Pulling on the plunger causes the fluid to develop a tension, or negative pressure.
Any air bubbles in the syringe will expand as the pressure is reduced

Question 59

When water moves from the soil through the plant to the
atmosphere, it travels through a widely variable medium
(cell wall, cytoplasm, membrane, air spaces), and the mechanisms of water transport also vary with the type of

Answer 59

medium.

Question 60

Some studies indicated that diffusion directly across the lipid bilayer was not sufficient to account for observed rates of water movement across membranes, but the evidence in support of microscopic pores was not compelling.
This uncertainty was put to rest with the recent discovery
of

Answer 60

aquaporins (page 59)

Aquaporins are integral membrane proteins that form water-selective channels across the membrane.

Question 61

Because water diffuses faster through

Answer 61

water selective channels than through a lipid bilayer, aquaporins facilitate water movement into plant cells

Question 62

although the presence of aquaporins may alter the rate of
water movement across the membrane, they do not change

Answer 62

the direction of transport or the driving force for water
movement.

Question 63

Water molecules in a solution are not static; they are

Answer 63

in continuous motion, colliding with one another and exchanging kinetic energy.

The molecules intermingle as a result of heir random thermal agitation.
This random motion is called diffusion.

As long as other forces are not acting on the molecules, diffusion causes the net movement of molecules from regions of high concentration to regions of low concentration—that is, down a concentration gradient

Question 64

As long as other forces are not acting on the molecules, diffusion causes

Answer 64

the net movement of molecules from regions of high concentration to regions of low concentration—that is, down a concentration gradient

Question 65

In the 1880s the German scientist Adolf Fick discovered
that the rate of diffusion is

Answer 65

directly proportional to the concentration gradient (∆cs /∆x)—that is, to the difference in concentration of substances (∆cs) between two points separated by the distance ∆x.

Question 66

The rate of transport, or the flux density (Js ), is the

Answer 66

amount of substances crossing a unit area per unit time

(e.g., Js may have units of moles per square meter per sec-
ond [mol m–2 s–1]).

Question 67

The diffusion coefficient (Ds) is a proportionality constant that measures

Answer 67

how easily substances moves through a particular medium.

Question 68

The diffusion coefficient is a characteristic of

Answer 68

The diffusion coefficient is a characteristic of the substance (larger molecules have smaller diffusion coefficients) and depends on the
medium (diffusion in air is much faster than diffusion in a
liquid, for example).

Question 69

The negative sign in Ficks equation indicates that

Answer 69

the flux moves down a concentration gradient.

Question 70

Fick’s first law says that a

Answer 70

substance will diffuse faster when the concentration gradient becomes steeper (∆cs is large) or when the diffusion coefficient is increased.

This equation accounts only for movement in response to a concentration gradient, and not for movement in response to
other forces (e.g., pressure, electric fields, and so on)

Question 71

speed of diffusion in solids, liquids and gases

Answer 71

Diffusion is fastest in gases, slower in liquids, and slowest in
solids

Question 72

the average time required for a substance to diffuse a given distance
increases in proportion to

Answer 72

the square of that distance.

Question 73

The diffusion coefficient for glucose in water is about

Answer 73

10–9 m 2s–1.

Thus the average time required for a glucose molecule to diffuse across a cell with a diameter of 50 μm is 2.5 s.

However, the average time needed for the same glucose
molecule to diffuse a distance of 1 m in water is approximately 32 years.
These values show that diffusion in solutions can be effective within cellular dimensions but is far too slow for mass transport over long distances.

Question 74

diffusion in solutions can be effective within

Answer 74

cellular dimensions but is far too slow for mass transport over long distances.

Question 75

A second process by which water moves is known as bulk
flow or mass flow. Bulk flow is the

Answer 75

concerted movement of groups of molecules en masse, most often in response to a pressure gradient.

Question 76

Among many common examples of bulk flow are

Answer 76

water moving through a garden hose, a river flowing, and rain falling.