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Campbell Biology: Concepts &

Connections Tenth Edition

Chapter 2

The Chemical Basis of Life

Figure 2.0_0

Introduction

• How might a chemical compound in the air harm coral

reefs? The answer is chemistry.

– When carbon dioxide (C O2) dissolves in water, it reacts

with water to form an acid, which can disrupt the

formation of coral skeletons.

Figure 2.0_1 (1 of 2)

Elements, Atoms, and

Compounds

2.1 Organisms Are Composed of

Elements, Usually Combined into

Compounds (1 of 2)

• Living organisms are composed of matter.

• Matter is composed of chemical elements.

– About 25 elements are essential for human life.

– Four elements make up about 96% of the weight of

most living organisms.

• A compound is a substance consisting of two or more

different elements in a fixed ratio.

2.1 Organisms Are Composed of Elements,

Usually Combined into Compounds (2 of 2)

Checkpoint question Explain how table salt illustrates the

themes of emergent properties.

Figure 2.1b

2.2 Connection: Trace Elements Are

Common Additives to Food and

Water (1 of 2)

• Some trace elements are required to prevent disease.

• Fluoride is usually added to municipal water and dental

products to help reduce tooth decay.

• Several chemicals are added to food to

– help preserve it,

– make it more nutritious, and/or

– make it look better.

2.2 Connection: Trace Elements Are

Common Additives to Food and

Water (2 of 2)

Checkpoint question A serving of cereal fortified to provide

100% of the recommended daily intake contains 18 m g of

iron. The recommended tolerable upper intake level is 45 m g

(before causing gastrointestinal upset). How many bowls of

cereal can you eat before you exceed that level?

Figure 2.2a

Figure 2.2b

Figure 2.2c

2.3 Atoms Consist of Protons,

Neutrons, and Electrons (1 of 2)

• Each element consists of one kind of atom.

• An atom is the smallest unit of matter that still retains the

properties of an element.

• Three subatomic particles are relevant to our discussion of

elements.

– Neutrons and protons are packed into an atom’s

nucleus.

– Electrons orbit the nucleus.

• The unique number of protons is an element’s atomic

number.

2.3 Atoms Consist of Protons,

Neutrons, and Electrons (2 of 2)

• An atom’s mass number is the sum of its protons and

neutrons in the nucleus.

• The atomic mass is approximately equal to its mass

number.

• Isotopes of an element have the same number of protons

but different numbers of neutrons.

Checkpoint question A nitrogen atom has 7 protons, and

its most common isotope has 7 neutrons. A radioactive

isotope of nitrogen has 9 neutrons. What is the atomic

number and mass number of this radioactive nitrogen?

Figure 2.3

Table 2.3 Isotopes of Carbon

2.4 Connection: Radioactive Isotopes

Can Help or Harm Us

• Radioactive isotopes are useful as tracers for monitoring

the fate of atoms in living organisms.

• Sophisticated imaging instruments can detect them.

• There are also dangers associated with using radioactive

substances.

Checkpoint question Why are radioactive isotopes useful

as tracers in research on the chemistry of life?

Figure 2.4a

Figure 2.4b

Chemical Bonds

2.5 The Distribution of Electrons

Determines an Atom’s Chemical

Properties (1 of 2)

• Electrons can be located in different electron shells, each

with a characteristic distance from the nucleus.

• An atom whose outer electron shell is not full tends to

interact with other atoms and share, gain, or lose

electrons, resulting in attractions called chemical bonds.

2.5 The Distribution of Electrons

Determines an Atom’s Chemical

Properties (2 of 2)

• The actual transfer of an electron between atoms results in

an attraction called an ionic bond.

• In a covalent bond, atoms do not transfer electrons but

actually share electrons between them.

Checkpoint question Looking at the electron distribution

diagrams for hydrogen, oxygen, nitrogen, and carbon, how

many covalent bonds can each form?

Figure 2.5a

Figure 2.5b

Figure 2.5b_1

Figure 2.5b_2

2.6 Visualizing the Concept: Covalent

Bonds Join Atoms into Molecules

Through Electron Sharing

• In a nonpolar covalent bond, electrons are shared

equally.

• In polar covalent bonds, such as those found in water,

electrons are pulled closer to the more electronegative

atom.

Checkpoint question Polarity refers to a separation of

charges (think of the positive and negative poles of a

battery). Explain why the bonds in a water molecule are

polar covalent bonds.

Figure 2.6_1

Figure 2.6_2

Figure 2.6_2a

Figure 2.6_2b

Figure 2.6_2c

Animation: Covalent Bonds

2.7 Ionic Bonds are Attractions

Between Ions of Opposite Charge

• An ion is an atom or molecule with an electrical charge

resulting from gain or loss of one or more electrons.

• Two ions with opposite charges attract each other.

– When the attraction holds the ions together, it is called

an ionic bond.

– Salt is a synonym for an ionic compound.

Checkpoint question Explain what holds together the ions

in a crystal of table salt.

Figure 2.7a_1

Figure 2.7a_2

Figure 2.7b_1

Figure 2.7b_2

Animation: Ionic Bonds

2.8 Hydrogen Bonds are Weak Bonds

Important in the Chemistry of Life

• One of the most important types of weak bonds is the

hydrogen bond.

• The hydrogen atoms of a water molecule are attached to

oxygen by polar covalent bonds.

• Because of these polar bonds and the wide V shape of the

molecule, water is a polar molecule. Water has an

unequal distribution of charges.

Checkpoint question What enables neighboring water

molecules to hydrogen-bond to one another?

Figure 2.8

Animation: Water Structure

2.9 Chemical Reactions Make and

Break Chemical Bonds

• The formation of water from hydrogen and oxygen is an

example of a chemical reaction, breaking existing

chemical bonds and forming new ones.

• The composition of matter is changed as bonds are broken

and formed to convert reactants to products.

• Chemical reactions do not create or destroy matter; they

only rearrange it in various ways.

Checkpoint question Fill in the blanks with the correct

numbers in the following chemical process:

C6H12O6 + ⸺⸺ O2 →in th + ⸺⸺ CO2 + fill in + ⸺⸺ l inH2O

Figure 2.9

Water’s Life-Supporting

Properties

2.10 Hydrogen Bonds Make Liquid

Water Cohesive

• The tendency of molecules of the same kind to stick

together is cohesion.

• The clinging of one substance to another is adhesion.

• Cohesion is related to surface tension—a measure of

how difficult it is to break the surface of a liquid.

Checkpoint question After a hard workout, you may notice

“beads” of sweat on your face. Can you explain what holds

the sweat in droplet form on your face?

Figure 2.10

Animation: Water Transport

2.11 Water’s Hydrogen Bonds

Moderate Temperature

• Thermal energy is the energy associated with the random

movement of atoms and molecules.

– Thermal energy in transfer from a warmer to a cooler

body of matter is defined as heat.

– Temperature measures the intensity of heat.

• When a substance evaporates, the surface of the liquid

that remains behind cools down; this is the process of

evaporative cooling.

Checkpoint question Explain the popular adage “It’s not the

heat, it’s the humidity.”

Figure 2.11

2.12 Ice Floats Because It Is Less

Dense than Liquid Water

• Water can exist as a gas, liquid, or solid.

• Water is less dense as a solid than a liquid because of

hydrogen bonding.

• When water freezes, each molecule forms a stable

hydrogen bond with its neighbors.

– As ice crystals form, the molecules are less densely

packed than in liquid water.

– Because ice is less dense than water, it floats.

Checkpoint question Explain how freezing water can crack

boulders.

Figure 2.12

Figure 2.12_1

2.13 Water is the Solvent of Life

• A solution is a liquid consisting of a uniform mixture of two

or more substances.

• Water’s versatility as a solvent, a dissolving agent, results

from the polarity of its molecules.

• Polar or charged solutes dissolve when water molecules

surround them, forming aqueous solutions.

Checkpoint question Why are blood and most other

biological fluids classified as aqueous solutions?

Figure 2.13

2.14 The Chemistry of Life Is Sensitive to

Acidic and Basic Conditions

• In liquid water, a very small percentage of water molecules

break apart into ions.

• We use the p H scale to describe how acidic or basic a

solution is.

• A buffer minimizes changes in p H.

Checkpoint question Compared to a basic solution at p H9,

the same volume of an acidic solution at p H 4 has fill in the

times more H+.

Figure 2.14

Figure 2.14_1

Figure 2.14_2

Figure 2.14_3

2.15 Scientific Thinking: Scientists Study

the Effects of Rising Atmospheric C O2 on

Coral Reef Ecosystems (1 of 4)

• Carbon dioxide is

– the main product of fossil fuel combustion,

– increasing in the atmosphere, and

– linked to global climate change.

• About 25% of this human-generated C O2 is absorbed by

the vast oceans.

• C O2 dissolved in seawater lowers the p H of the ocean in a

process known as ocean acidification.

2.15 Scientific Thinking: Scientists Study

the Effects of Rising Atmospheric C O2 on

Coral Reef Ecosystems (2 of 4)

• As seawater acidifies, the extra hydrogen ions (H +)

combine with carbonate ions (C O 3 2−) to form bicarbonate

ions (H C O 3 −).

• This reaction reduces the carbonate ion concentration

available to corals and other shell-building animals.

2.15 Scientific Thinking: Scientists Study

the Effects of Rising Atmospheric C O2 on

Coral Reef Ecosystems (3 of 4)

• In a controlled experiment, scientists looked at the effect of

decreasing carbonate ion concentration on the rate of

calcium deposition by reef organisms.

• The lower the concentration of carbonate ions, the lower

the rate of calcification, and thus the slower the growth of

coral animals.

2.15 Scientific Thinking: Scientists Study

the Effects of Rising Atmospheric C O2 on

Coral Reef Ecosystems (4 of 4)

Source: Adaptation of figure 5 from “Effect of Calcium Carbonate Saturation State on the

Calcification Rate of an Experimental Coral Reef” by C. Langdon, et al., from Global

Biogeochemical Cycles, June 2000, Volume 14(2). American Geophysical Union.

Checkpoint question Identify the independent and dependent

variables in the experiment depicted in Figure 2.15A.

Figure 2.15b

2.16 Evolution Connection: The Search for

Extraterrestrial Life Centers on the Search

for Water

• The emergent properties of water support life on Earth and

may contribute to the potential for life to have evolved on

other planets.

Checkpoint question Why is the presence of water

important in the search for extraterrestrial life?

You Should Now Be Able to (1 of 2)

1. Describe the importance of chemical elements to living

organisms.

2. Explain the formation of compounds.

3. Describe the structure of an atom.

4. Distinguish between ionic, hydrogen, and covalent bonds.

5. Define a chemical reaction and explain how it changes

the composition of matter.

You Should Now Be Able to (2 of 2)

6. List and define the life-supporting properties of water.

7. Explain the p H scale and the formation of acid and base

solutions.

8. Explain how rising C O 2 levels affect coral reefs.

Figure 2.0

Figure 2.0_1 (2 of 2)

Figure 2.0_2

Figure 2.0_3

Figure 2.1a_1

Figure 2.1a_2

Figure 2.1a_3

Figure 2.U N01

Figure 2.U N02

Figure 2.U N03

Figure 2.U N03_1

Figure 2.U N03_2

Figure 2.U N04

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