Chapter 3

Water and the Fitness of the Environment

1

Why is water so important?

2

Importance of water

• Cell chemistry occurs in the watery cytoplasm

• Water moderates temperatures

• Universal solvent

How much of the earth is water?

4

Water Facts• Three-quarters of

the Earth’s surface is water

• The abundance of water is the main reason the Earth is habitable

What important fact about water have we already learned?

6

Water Facts

• The water molecule is polar • The polarity results in hydrogen bonding

7

How does hydrogen bonding relate to the properties of water?

8

Hydrogenbonds

+

+

H

H+

+

–

–

–

–

Figure 3.2

Properties of water

• Six emergent properties of water contribute to Earth’s fitness for life

1.Cohesion/Adhesion2.Surface tension3.Temperature Moderation4.High specific heat5.Evaporative cooling6.Universal solvent

9

Emergent Properties

• Emergent properties – result from the arrangement and interaction of parts

within a system – may characterize nonbiological entities as well

• a functioning bicycle emerges only when all of the necessary parts connect in the correct way

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Power and Limitations of Reductionism

• Reductionism – the reduction of complex systems to

simpler components that are more manageable to study

• Ex. the molecular structure of DNA

• Understanding biology– balances reductionism with the study of

emergent properties• Ex. new understanding comes from studying

the interactions of DNA with other molecules

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

What do we mean by cohesion of water molecules?

12

Cohesion

• Cohesion– the bonding of a high percentage of the

molecules to neighboring water molecules– Results from hydrogen bonding

• water attracting other water molecules

13

How does the cohesion of water affect living things?

14

Cohesion

– Helps pull water up through the microscopic vessels of plants (capillarity)

15

Water conducting cells

100 µm

What is surface tension?

16

Surface Tension

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

– related to cohesion

17

Why do desert temperatures range from very hot during the day to very cold at night but areas near large bodies of water have a smaller temperature range?

18

Moderation of Temperature

• Water absorbs heat from air that is warmer and releases the stored heat to air that is cooler–heats up slowly and cools down

slowly

19

Heat and Temperature

• Heat–a measure of the total amount of

kinetic energy due to molecular motion

• Temperature–Measures the intensity of heat

20

Water’s High Specific Heat

• specific heat –the amount of heat that must be

absorbed or lost for 1 gram of that substance to change its temperature by 1ºC

21

Relate water’s molecular structure to its high specific heat.

22

Specific Heat

• When heat is absorbed, hydrogen bonds break

• When heat is released, hydrogen bonds form–Water’s high specific heat allows it to

minimize temperature fluctuations within limits that permit life

23

Why is evaporation a cooling process?

24

Evaporative Cooling

• Evaporation– the transformation of a substance from a

liquid to a gas– Requires energy

• Heat of vaporization– the quantity of heat a liquid must absorb

for 1 gram of it to be converted from a liquid to a gas

25

Evaporative Cooling

• due to water’s high heat of vaporization• Sweating cools the body as heat energy

from the body changes sweat into a gas

26

Insulation of Bodies of Water by Floating Ice

• Solid water, or ice–less dense than liquid water–Floats in liquid water–Insulates water & organisms below ice

layer

27

Why is ice less dense than water?

28

Floating Ice

• The hydrogen bonds in ice– Are more “ordered” than in liquid water, making

ice less dense (crystal lattice)

29

Liquid water

Hydrogen bonds constantly break and re-form

IceHydrogen bonds are stable

Hydrogen bond

Figure 3.5

30

31

Floating Ice

• Since ice floats in water–Life can exist under frozen lakes and seas

32

Why do so many substances dissolve easily in water?

33

The Solvent of Life

–Water is a solvent due to its polarity• Forms aqueous solutions• the universal solvent

–because so many substances dissolve in it

34

• Different regions of the polar water molecule interact with ionic compounds called solutes and dissolve them

35

Negative oxygen regions

of polar water molecules are

attracted to sodium cations

(Na+).

+

+

+

+Cl –

–

–

–

–

Na+Positive hydrogen regions

of water molecules cling to chloride

anions (Cl–).

+

+

+

+

–

–

–

–

–

–

Na+

Cl–

36

• Water can also interact with polar molecules such as proteins

37

This oxygen is attracted to a slight positive charge on the lysozyme molecule.

This hydrogen is attracted to a slight

negative charge on the lysozyme molecule.

(a) Lysozyme molecule

in a nonaqueous

environment

(b) Lysozyme molecule (purple)

in an aqueous environment

such as tears or saliva

(c) Ionic and polar regions on the protein’s

surface attract water molecules.

+

–

Figure 3.7

What kind of substances do not mix well with water?

38

Hydrophilic and Hydrophobic Substances

• A hydrophobic substance– No affinity for water– Nonpolar– Lipids – oil, fat, wax

• A hydrophilic substance– Has an affinity for water– Polar or ionic– Carbohydrates, salts

39

Solute Concentration in Aqueous Solutions

• Since most biochemical reactions occur in water inside cells–It is important to learn to calculate

the concentration of solutes in an aqueous solution

40

Moles and Molarity

• A mole–Represents an exact number of

molecules of a substance in a given mass

• Molarity–Is the number of moles of solute per

liter of solution41

Acids and Bases

• Dissociation of water molecules leads to acidic and basic conditions that affect living organisms

• Organisms must maintain homeostasis in the pH of their internal and external environments

42

Effects of Changes in pH

• Water can dissociate Into hydronium ions (H+ or H3O+) and hydroxide (OH-) ions

• Changes in the concentration of these ions Can have a great affect on pH in living organisms

43

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure on p. 53 of water dissociating

Acids and Bases

• An acid–any substance that increases the

hydrogen ion concentration of a solution

• A base–any substance that reduces the

hydrogen ion concentration of a solution (more OH- ions)

44

The pH Scale• Scale goes from 0-14 with 7 neutral• The pH of a solution Is determined by the

relative concentration of hydrogen ions• Difference of 10X in hydrogen ion

concentration between any two pH values• Acids have a higher number of H+ ions than a

base• Acids produce H+ ion in solution• Bases produce OH- ions in solution

45

46

Incr

easi

ngly

Aci

dic

[H+]

> [

OH

–]

Incr

easi

ngly

Bas

ic[H

+]

< [

OH

–]

Neutral[H+] = [OH–]

Oven cleaner

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

pH Scale

Battery acid

Digestive (stomach) juice, lemon juice

Vinegar, beer, wine,cola

Tomato juice

Black coffee Rainwater

Urine

Pure waterHuman blood

Seawater

Milk of magnesia

Household ammonia

Household bleach

Figure 3.8

Buffers

• The internal pH of most living cells– Must remain close to pH 7

• Buffers– substances that minimize changes in the

concentrations of hydrogen and hydroxide ions in a solution

– Consist of an acid-base pair that reversibly combines with hydrogen ions

– Made by organisms

47

The Threat of Acid Precipitation

• Acid precipitation–Refers to rain, snow, or fog with a pH

lower than pH 5.6–Caused by the mixing of different

pollutants with water in the air

48

• Acid precipitation–Damages life in Earth’s ecosystems

49

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Moreacidic

Acidrain

Normalrain

Morebasic

Figure 3.9