Plant Cells and Water

Chapter 10

Evaporation

• Heat energy from the sun causes water in puddles, streams, rivers, seas or lakes to change from a liquid to a water vapor.

• This is called evaporation.

• The vapor rises into the air and collects in clouds.  

Condensation

• Water vapor collects in clouds. As the clouds cool the water vapor condenses into water drops.

• This is called condensation.

• These drops fall to the earth as rain, snow or hail.

Precipitation

• Water falls to the earth from clouds. Mainly as rain, but sometimes as snow and hail.

• This is called precipitation.

Transpiration

• Transpiration is the process by which plants lose water out of their leaves. 

• Transpiration gives evaporation a bit of a hand in getting the water vapor back up into the air.

Important Properties of Water
• Density
• Thermal expansivity
• Polarizability
(Dielectric constant, permittivity; index of refraction)
• Surface tension (capillary rise)
• Vapor pressure
• Viscosity
• Solvent power
• pH
• Electrical conductivity
• Total dissolved solids
• Thermal capacity
• Latent heat of fusion
• Latent heat of evaporation
• Compressibility

WATER HAS UNIQUE PHYSICAL AND CHEMICAL PROPERTIES

• The key to understanding many of the unique properties of water is found in the structure of the water molecule and the strong intermolecular attractions that result from that structure.

• Water consists of an oxygen atom covalently bonded to two hydrogen atoms (Fig. 10.1). The oxygen atom is strongly electronegative, which means that it has a tendency to attract electrons.

• One consequence of this strong electronegativity is that, in the water molecule, Oxygen tends to draw electrons away from the Hydrogen.

ELECTRONEGATIVITY

• THE MEASURE OF AN ATOM'S ATTRACTION FOR ELECTRONS OF A COVALENT BOND IS CALLED ITS ELECTRONEGATIVITY. The more electronegative an atom, the more strongly it pulls shared electrons toward itself.

Figure 2.12 Covalent bonding in four molecules

•The shared electrons that make up the O-H bond are closer to the oxygen nucleus than to hydrogen.

•As a consequence, the oxygen atom carries a partial negative charge and a corresponding partial positive charge is shared between the two hydrogen atoms.

•This asymmetric electron distribution makes water a polar molecule..

•This attraction is called hydrogen bonding (Fig 10.1).

Figure 2.13 Polar covalent bonds in a water molecule

• Multiple hydrogen bonds

• hold the two strands of the DNA double helix together

• hold polypeptides together in such secondary structures as the alpha helix and the beta conformation;

• help enzymes bind to their substrate;

• help antibodies bind to their antigen

• help transcription factors bind to each other;

• help transcription factors bind to DNA

Molecular Movement

• Diffusion

– Movement of molecules from a region of higher concentration to a region of lower concentration.

• Move along a concentration gradient.

• Move until equilibrium reached.

Diffusion

Osmosis

• Osmosis is diffusion of water through a differentially permeable membrane from a region where the water is more concentrated to a region where it is less concentrated.

– Water enters a cell by osmosis until the osmotic potential is balanced by the resistance to expansion of the cell wall.

• Turgor Pressure

– Pressure Potential

Osmosis

• Water Potential of a plant is essentially its osmotic potential and pressure potential combined.

• Water flows from the xylem to the leaves, evaporates within the leaf air spaces, and transpires through the stomata into the atmosphere.

Osmosis

• Overview: The Molecule That Supports All of Life

– Water is the biological medium here on Earth

– All living organisms require water more than any other substance

•Water is the most common and yet unusual substance in the universe

• Solutions vs. mixtures

– exclusion of salts in freezing water

– dissolving water in air and air in water

•• Molecular properties

– Electronic structure

– Electrical polarity

– cohesion and adhesion

• Surface Properties

– Surface tension

– Capillarity

•Thermal properties

– density vs. temperature

– heat capacity

– Heat of fusion and vaporization

• Air is warmer near the surface because air can hold more water vapor in solution

It gets cooler at higher elevations because air can hold less water vapor in solution

•Electronic structure

• When hydrogen combines with oxygen to form water, the electrons shared by hydrogen go to the two half empty p orbitals.

• This results in an asymmetrical charge distribution that creates an electrical dipole across the molecule.

•• The side of the molecule nearest the hydrogen atoms will exhibit a slight + charge, while the side opposite will exhibit a slight (–) charge.

• This gives the water molecule a range of very abnormal physical properties, that make water the most useful substance on our planet.

• The polarity of water molecules results in hydrogen bonding

• The water molecule is a polar molecule

• This property accounts for the cohesiveness, the temperature-stabilizing capacity and the solvent properties of water.

Moderation of Temperature

• Water moderates air temperature

– By absorbing heat from air that is warmer and releasing the stored heat to air that is cooler

THE THERMAL PROPERTIES OF WATER ARE BIOLOGICALLY IMPORTANT

•Perhaps the single most important property of water is that it is a liquid over the range of temperatures most compatible with life.

• Both the melting and boiling points of water are higher than expected when compared with other molecules of similar size, especially ammonia (NH3) and methane (CH4) (Table 10.1).

• The introduction of oxygen raises the boiling points of both methanol (CH3-OH) and ethanol (CH3CH2OH) to temperatures much closer to that of water. This is because the presence of oxygen introduces polarity and the opportunity to form hydrogen bonds.

Heat and Temperature

• Kinetic energy

– Is the energy of motion

• Heat

– Is a measure of the total amount of kinetic energy due to molecular motion

• Temperature

– Measures the intensity of heat

Water’s High Specific Heat

• The specific heat of a substance

– Is the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1ºC

• Water has a high specific heat, which allows it to minimize temperature fluctuations to within limits that permit life

– Heat is absorbed when hydrogen bonds break

– Heat is released when hydrogen bonds form

WATER EXHIBITS A UNIQUE THERMAL CAPACITY

•The term specific heat is used to describe the thermal capacity of a substance or the amount of energy that can be absorbed for a given temperature rise.

•Specific heat is defined as the amount of energy required to raise the temperature of one gram of substance by 1°C (usually at 20°C).

•The specific heat of water is the basis for the definition of a quantity of energy called the calorie.

•The specific heat of water was therefore assigned the value of 1.0 calorie.

•In accordance with the International System of Units (Systeme Internationale d'Unites, or SI), the preferred unit for energy is the joule U). 1 calorie = 4.184 joules.

•The specific heat of water is

•4.184 J g^-1°C^{-1 ,} higher than that of any other substance except liquid ammonia (Table 10.1).

•Because of its highly ordered structure, liquid water also has a high thermal conductivity.

•This means that it rapidly conducts heat away from the point of application.

•The combination of high specific heat and thermal conductivity enables water to absorb and redistribute large amounts of heat energy without correspondingly large increases in temperature.

Having so much water on earth, having so much water in organisms, rapid temperature changes tend not to occur in organisms and bodies of water...helping control the climate! It also takes a lot of energy to vaporize water. However the evaporation cools because all the hot molecules left!

Evaporative Cooling

• Evaporation

– Is the transformation of a substance from a liquid to a gas

• Heat of vaporization

– Is the quantity of heat a liquid must absorb for 1 gram of it to be converted from a liquid to a gas

• Evaporative cooling

– Is due to water’s high heat of vaporization

– Allows water to cool a surface

WATER EXHIBITS A HIGH HEAT OF FUSION AND HEAT OF VAPORIZATION

•Energy is required to cause changes in -the state of any substance, such as from solid to liquid or liquid to gas, without a change in temperature.

•The energy required to convert a substance from the solid to the liquid state is known as the heat of fusion.

•The heat of fusion for water is 335 Jg-1, which means that 335 J of energy are required to convert 1 gram of ice to 1 gram of liquid water at 0°C (Table 10.1).

.

•The heat of fusion of water is one of the highest known, second only to ammonia.

•The high heat of fusion of water is attributable to the large amount of energy necessary to overcome the strong intermolecular forces associated with hydrogen bonding.

•The density of ice is another important property.

•At 0°C, the density of ice is less than that of liquid water.

•Thus water, unlike other substances, reaches its maximum density in the liquid state (near 4°C), rather than as a solid.

•This occurs because molecules in the liquid state are able to pack more tightly than in the highly ordered crystalline state of ice.

• Consequently, ice floats on the surface of lakes and ponds rather than sinking to the bottom where it might remain year round.

•This is extremely important to the survival of aquatic organisms of all kinds.

•Just as hydrogen bonding increases the amount of energy required to melt ice, it also increases the energy required to evaporate water.

•The heat of vaporization of water, or the energy required to convert one mole of liquid water to one mole of water vapor, is about 44 kJ mol-1 at 25°C.

•Because this energy must be absorbed from its surroundings, the heat of vaporization accounts for the pronounced cooling effect associated with evaporation.

The Solvent of Life

• Water is a versatile solvent due to its polarity

• It can form aqueous solutions

WATER IS THE “UNIVERSAL”? SOLVENT

The excellent solvent properties of water are due to the highly polar character of the water molecule.

The polarity of molecules can be measured by a quantity known as the dielectric constant.

Water has one of the highest known dielectric constants (Table 2).

The dielectric constants of alcohols are somewhat lower and those of nonpolar organic liquids such as benzene and hexane are very low.

Water is thus an excellent solvent for charged ions or molecules, which dissolve very poorly in nonpolar organic liquids.

Many of the solutes of importance to plants are charged.

On the other hand, the low dielectric constants of nonpolar molecules helps to explain why charged solutes do not readily cross the predominantly nonpolar, hydrophobic lipid regions of cellular membranes (Chap. 1).

TABLE 2 Dielectric constants for some common solvents at 25°C.

Water 78.4

Methanol 33.6

Ethanol 24.3

Benzene 2.3

Hexane 1.910.4

As a solvent

Terms...a liquid that is a mixture of two or more substances sis called a solution. The dissolving agent of a solution is the SOLVENT, and the substance that is dissolved is the SOLUTE. If water is the solvent, then the solution is called an AQUEOUS SOLUTION.

• Water can also interact with polar molecules such as proteins

 While there is no universal solvent, water is one of the best and guess why? That's right, the Hydrogen bonds. Example: put salt in the water, the hydrogen regions (+) surround the chlorine atoms (-) and the oxygen (-) surround the sodium (+). and non-ionic compounds can form weak H bonds with the water as long as they are polar.

If the substance has an affinity for water, its said to be HYDROPHILIC (cotton absorbs H2O without dissolving) and some have no affinity to water (HYDROPHOBIC) because of the nonpolar bonds (especially C--H bonds). (See table 3.1 P46).

Hydrophilic and Hydrophobic Substances

• A hydrophilic substance

– Has an affinity for water

• A hydrophobic substance

– Does not have an affinity for water

POLARITY OF WATER MOLECULES RESULTS IN COHESION AND ADHESION

•The strong mutual attraction between water molecules resulting from hydrogen bonding is also known as cohesion.

•One consequence of cohesion is that water has an exceptionally high surface tension, which is most evident at interfaces between water and air.

•Surface tension arises because the cohesive force between water molecules is much stronger than interactions between water and air.

•Cohesion is directly responsible for the unusually high tensile strength of water. Tensile strength is the maximum tension that an uninterrupted column of any material can withstand without breaking.

These attractions tend to hold the molecules together in a hydrogen bond which makes the water COHESIVE. The bonds are short lasting because they are so weak but collectively hold it together providing such possibilities as water reaching through the force of gravity through small pores through plants to leaves. Adhesion to the walls of the vessels also aid this process. The surface tension is a measure of how difficult it is to break the surface of a liquid.

Cohesion

• Water molecules exhibit cohesion

• Cohesion

– Is the bonding of a high percentage of the molecules to neighboring molecules

– Is due to hydrogen bonding

WATER ON GLASS WATER ON PLASTIC

WATER WITH WATERp

Cohesive force changes physical properties

• In the liquid state water molecules form chains that are constantly breaking up and reforming.This increases the viscosity of water.

Cohesion - Tension Theory

• When the negatively charged end of one water molecule comes close to the positively charged end of another water molecule, weak hydrogen bonds hold the molecules together.

– Water molecules adhering to capillary walls, and each other, create a certain amount of tension.

Cohesion - Tension Theory

• When water transpires, the cells involved develop a lower water potential than the adjacent cells.

– Creates tension on water columns, drawing water from one molecule to another, throughout the entire span of xylem cells.

•The same forces that attract water molecules to each other will also attract water to solid surfaces, a process known as adhesion.

•Adhesion is an important factor in the capillary rise of water in small-diameter conduits.

•The combined properties of cohesion, adhesion, and tensile strength help to explain why water rises in capillary tubes and are exceptionally important in maintaining the continuity of water columns in plants.

•Cohesive and adhesive forces

• The polarity of water causes water molecules to be:

– electrically attracted to each other = cohesive force

– electrically attracted to molecules of another species (such as glass) = adhesive force

• Surface tension

– Is a measure of how hard it is to break the surface of a liquid

– Is related to cohesion

•Surface tension

• Molecules located at the air-water interface will be attracted to both air and water molecules.

• The sum of all these force vectors is creates a net downward force.

• The result is that the surface acts as an elastic “skin.”

• The tensile strength of this skin to intrusion and tearing is called the surface tension.

Water expands when it freezes. Water is one of the few substance that is less dense as a solid than a liquid (ice floats). The H bonds are the reason for this and above 4'C, it is like other substances, contracting when cool and expanding when warm but when water starts to freeze,,the molecules aren't moving fast enough to break the H bonds and the crystal lattice the molecules get locked in keep the molecules far enough apart to make ice about 10% less dense than water at 4'C

• The hydrogen bonds in ice

– Are more “ordered” than in liquid water, making ice less dense

The pH Scale

• The pH of a solution

– Is determined by the relative concentration of hydrogen ions

– Is low in an acid

– Is high in a base

Molecular Movement

• Plasmolysis

– Loss of water through osmosis is accompanied by shrinkage of protoplasm away from the cell wall.

• Imbition

– Colloidal material and large molecules usually develop electrical charges when they are wet, and thus attract water molecules.

Molecular Movement

• Active Transport

– Plants absorb and retain solutes against a diffusion, or electrical, gradient through the expenditure of energy.

• Involves proton pump.

Water and Its Movement Through The Plant

• More than 90% of the water entering a plant passes into leaf air spaces and then evaporates through

the stomata into

the atmosphere

(Transpiration).

– Usually less than 5% of water escapes through the cuticle.

Plant Water Relations

• Plants need water for

– Nutrient transport

– Metabolic solvent

– Turgor pressure

– Cooling

Plant Water Relations

• Plants obtain water passively

• Plants transport water without a systemic pump

– Plants transport water through two different, but interconnected, systems

FIGURE 10.5 Osmosis is the directed movement of the solvent molecule (usually water) across a selectively permeable membrane. Chamber A is separated from chamber B by a selectively permeable membrane. The selectively permeable membrane allows the free movement of the solvent (water) molecules between chambers A and B but restricts the movement of the solute molecules. At time zero (to), all the solute molecules are retained in chamber A and chambers A and B exhibit identical volumes as indicated by the broken line.

After a certain time t, all solute molecules are still retained in chamber A but the volume of chamber A has increased while the volume in chamber B has decreased due to the diffusion of water across the selectively permeable membrane from chamber B to chamber A. This change in volume is represented by Oh.

chamber B to chamber A because the energy of pure water in chamber B is greater than the energy of thewater in the solution in chamber A. Net movement of water stops when there is no longer an energy gradient across the selectively permeable membrane.