Unit 1: What is Biology?
Unit 2: Ecology
Unit 3: The Life of a Cell
Unit 4: Genetics
Unit 5: Change Through Time
Unit 6: Viruses, Bacteria, Protists, and Fungi
Unit 7: Plants
Unit 8: Invertebrates
Unit 9: Vertebrates
Unit 10: The Human Body

Unit 1: What is Biology?
Chapter 1: Biology: The Study of Life
Unit 2: Ecology
Chapter 2: Principles of Ecology
Chapter 3: Communities and Biomes
Chapter 4: Population Biology
Chapter 5: Biological Diversity and Conservation
Unit 3: The Life of a Cell
Chapter 6: The Chemistry of Life
Chapter 7: A View of the Cell
Chapter 8: Cellular Transport and the Cell Cycle
Chapter 9: Energy in a Cell

Unit 4: Genetics
Chapter 10: Mendel and Meiosis
Chapter 11: DNA and Genes
Chapter 12: Patterns of Heredity and Human Genetics
Chapter 13: Genetic Technology
Unit 5: Change Through Time
Chapter 14: The History of Life
Chapter 15: The Theory of Evolution
Chapter 16: Primate Evolution
Chapter 17: Organizing Life’s Diversity

Unit 6: Viruses, Bacteria, Protists, and Fungi
Chapter 18: Viruses and Bacteria
Chapter 19: Protists
Chapter 20: Fungi
Unit 7: Plants
Chapter 21: What Is a Plant?
Chapter 22: The Diversity of Plants
Chapter 23: Plant Structure and Function
Chapter 24: Reproduction in Plants

Unit 8: Invertebrates
Chapter 25: What Is an Animal?
Chapter 26: Sponges, Cnidarians, Flatworms, and

Roundworms
Chapter 27: Mollusks and Segmented Worms
Chapter 28: Arthropods
Chapter 29: Echinoderms and Invertebrate
Chordates

Unit 9: Vertebrates
Chapter 30: Fishes and Amphibians
Chapter 31: Reptiles and Birds
Chapter 32: Mammals
Chapter 33: Animal Behavior
Unit 10: The Human Body
Chapter 34: Protection, Support, and Locomotion
Chapter 35: The Digestive and Endocrine Systems
Chapter 36: The Nervous System
Chapter 37: Respiration, Circulation, and Excretion
Chapter 38: Reproduction and Development
Chapter 39: Immunity from Disease

The Life of a Cell
The Chemistry of Life

A View of the Cell

Cellular Transport and the Cell Cycle

Energy in a Cell

Chapter 6 The Chemistry of Life
6.1: Atoms and Their Interactions
6.1: Section Check
6.2: Water and Diffusion
6.2: Section Check
6.3: Life Substances
6.3: Section Check
Chapter 6 Summary
Chapter 6 Assessment

What You’ll Learn
You will relate an atom’s interactions
with other atoms to its structure.
You will explain why water is
important in life.
You will compare the role of
biomolecules in organisms.

Section Objectives:
• Relate the structure of an atom to the identity
of elements.

• Relate the formation of covalent and ionic
chemical bonds to the stability of atoms.

Section Objectives:
• Distinguish
mixtures and
solutions.
• Define acids
and bases and
relate their
importance to
biological
systems.

Elements
• Everything – whether it is a rock, frog, or
flower – is made of substances called
elements.

• An element is a substance that can’t be
broken down into simpler chemical
substances.

Natural elements in living things
• Of the naturally occurring elements on Earth,
only about 25 are essential to living
organisms.
• Carbon, hydrogen, oxygen, and nitrogen
make up more than 96 percent of the mass of
a human body.

Trace elements
• Trace elements such as iron and copper, play
a vital role in maintaining healthy cells in all
organisms.

• Plants obtain trace elements by absorbing
them through their roots; animals get them
from the foods they eat.

Table 6.1 Some Elements That Make Up the Human Body
Percent By Percent By
Element Symbol Mass in Element Symbol Mass in
Human Body Human Body
Oxygen O 65.0 Iron Fe trace
Carbon C 18.5 Zinc Zn trace
Hydrogen H 9.5 Copper Cu trace
Nitrogen N 3.3 Iodine I trace
Calcium Ca 1.5 Manganese Mn trace
Phosphorus P 1.0 Boron B trace
Potassium K 0.4 Chromium Cr trace
Sulfur S 0.3 Molybdenum Mo trace
Sodium Na 0.2 Cobalt Co trace
Chlorine Cl 0.2 Selenium Se trace
Magnesium Mg 0.1 Fluorine F trace

Atoms: The Building Blocks of Elements
• An atom is the smallest particle of an
element that has the characteristics of that
element.
• Atoms are the basic building blocks of all
matter.

The structure of an atom
• The center of an atom is called the nucleus
(NEW klee us).
• All nuclei contain positively charged
particles called protons (p+).

• Most contain particles that have no charge,
called neutrons (n0).

The Structure of an atom
• The region of space surrounding the nucleus
contains extremely small, negatively charged
particles called electrons (e-)

• This region of
Nucleus
space is referred
to as an electron
Electron cloud.
energy
levels

The Structure of an atom
• Because opposites attract, the negatively
charged electrons are held in the electron
cloud by the positively charged nucleus.

Nucleus
Electron energy levels 8 protons (p+)
8 neutrons (n0)

• Electrons exist
around the nucleus
in regions known
as energy levels.
Oxygen atom

• The first energy level can hold only two
electrons. The second level can hold a
maximum of eight electrons. The third
level can hold up to 18 electrons.

Electron energy levels
• Atoms contain equal numbers of electrons
and protons; therefore, they have no net
charge.

Isotopes of an Element
• Atoms of the same element always have the
same number of protons but may contain
different numbers of neutrons.
• Atoms of the same element that have
different numbers of neutrons are called
isotopes (I suh tophs) of that element.

Compounds and Bonding
• A compound is a substance that is composed
of atoms of two or more different elements
that are chemically combined.
• Table salt
(NaCl) is a
compound
composed of the
elements sodium
and chlorine.

How covalent bonds form
• Atoms combine with other atoms only when
the resulting compound is more stable than
the individual atoms.
• For many elements, an atom becomes stable
when its outermost energy level is full.
• Sharing electrons with other atoms is
one way for elements to
become stable.

• Two hydrogen
atoms can combine Hydrogen molecule
with each other
by sharing their
electrons.
• Each atom
becomes stable
by sharing its
electron with
the other atom.


Click image to view movie.

• The attraction of Hydrogen molecule
the positively
charged nuclei
for the shared,
negatively
charged
electrons holds
the atoms
together.

• A covalent bond
holds the two
hydrogen atoms
together.
• A molecule is a
group of atoms
held together by
covalent bonds. Water
It has no overall molecule
charge.

How ionic bonds form
• An atom (or group of atoms) that gains or
loses electrons has an electrical charge and is
called an ion. An ion is a charged particle
made of atoms.
• The attractive force between two ions of
opposite charge is known as an ionic bond.

Chemical Reactions
• Chemical reactions occur when bonds are
formed or broken, causing substances to
recombine into different substances.
• All of the chemical reactions that occur within
an organism are referred to as that organism’s
metabolism.

Writing chemical equations
• In a chemical
reaction, substances
that undergo
chemical reactions,
are called reactants.
• Substances formed
by chemical
reactions, are called
products.

Writing chemical equations
• A molecule of table sugar can be represented
by the formula: C12H22O11.

• The easiest way to understand chemical
equations is to know that atoms are neither
created nor destroyed in chemical
reactions. They are simply rearranged.

Mixtures and Solutions
• A mixture is a combination of substances in
which the individual components retain their
own properties.

• Neither component of the mixture changes.

Mixtures and Solutions
• A solution is a mixture in which one or more
substances (solutes) are distributed evenly in
another substance (solvent).
• Sugar molecules
in a powdered
drink mix
dissolve easily in
water to form a
solution.

Acids and bases
• Chemical reactions can occur only when
conditions are right.
• A reaction may depend on:
- energy availability
- temperature
- concentration of a substance
- pH of the surrounding environment

Acids and bases
• The pH is a measure of how acidic or basic a
solution is.
• A scale with values ranging from below 0 to
above 14 is used to measure pH.

More acidic Neutral More basic

Acids and bases
• Substances with a pH below 7 are acidic. An
acid is any substance that forms hydrogen
ions (H+) in water.
• A solution is neutral if its pH equals seven.

More acidic Neutral More basic

Acids and bases
• Substances with a
pH above 7 are
basic. A base is any
substance
that forms hydroxide
ions (OH-) in water.

pH 11

Question 1
Which of the following is an element?

A. chlorophyll
B. carbon
C. sodium chloride
D. water

The answer is B. An element can't be broken
down into simpler chemical substances.
Chemical elements combine in different ways
to form a variety of substances useful to living
things.

Question 2
The smallest particle of an element that has the
characteristics of that element is a(n)
__________.

A. proton C. nucleus

B. electron D. atom

The answer is D. Atoms are the basic building
blocks of all matter and have the same general
structure, including a nucleus and electrons.
Elements found in both living and nonliving
things are made of atoms.

Nucleus An atom has a
nucleus and
electrons in
Electron energy levels energy levels.

Question 3
Which of the following can contain two types of
particles?

A. nucleus
B. protons
C. neutrons
D. electrons

The answer is A. The nucleus is the center of
the atom and may contain both positively
charged particles and particles that have no
charge.
Nucleus
8 protons (p+)
8 neutrons (n0)

Oxygen atom

Question 4
Sodium and chlorine combine to form table
salt. What do you know to be true?
A. Sodium and chlorine are sharing electrons in
their outer energy levels.
B. Sodium and chlorine atoms have no overall
electrical charge.

Question 4
Sodium and chlorine combine to form table
salt. What do you know to be true?
C. Sodium and chlorine are less stable in the
compound sodium chloride.
D. Sodium and chlorine atoms in table salt have
full outer energy levels.

The answer is D. Sodium and chlorine
atoms combine because the resulting
compound, table salt, is more stable than
the individual atoms. Sodium loses an
electron in its outer energy level, chlorine
gains that electron in its outer energy
level, and an ionic bond is formed.

Section Objectives
• Relate water’s unique features to polarity.
• Identify how the process of diffusion occurs
and why it is important to cells.

Water and Its
Importance
• Water is perhaps the
most important
compound in living
organisms.
• Water makes up 70
to 95 percent of most
organisms.

Water is Polar
• Sometimes, when atoms form covalent bonds
they do not share the electrons equally. This
is called a polar bond.

Water is Polar
• A polar molecule is a molecule with an
unequal distribution of charge; that is,
each molecule has a positive end and a
negative end.
• Water is an example of a polar molecule.
• Water can dissolve many ionic
compounds, such as salt, and many other
polar molecules, such as sugar.

Water is Polar
• Water molecules also attract other water
molecules. Hydrogen atom

• Weak hydrogen
bonds are formed
Hydrogen atom
between positively
charged hydrogen
atoms and negatively
charged oxygen
atoms. Oxygen atom

Water resists temperature changes
• Water resists changes in temperature.
Therefore, water requires more heat to
increase its temperature than do most other
common liquids.

Water expands when it freezes
• Water is one of the
few substances that
expands when it
freezes.
• Ice is less dense
than liquid water so
it floats as it forms
in a body of water.

Early observations: Bownian motion
• In 1827, Scottish scientist Robert Brown used a
microscope to observe pollen grains suspended
in water. He noticed that the grains moved
constantly in little jerks, as if being struck by
invisible objects.
• This motion is now called Brownian motion.
• Today we know that Brown was observing
evidence of the random motion of atoms
and molecules.

The process of diffusion
• Diffusion is the net movement of particles from
an area of higher concentration to an area of
lower concentration.
• Diffusion results because of the random
movement of particles (Brownian motion).
• Three key factors—concentration,
temperature, and pressure—affect the rate
of diffusion.

The results of diffusion
• When a cell is in
dynamic equilibrium
with its environment, Material moving
out of cell equals
materials move into material moving
into cell
and out of the cell at
equal rates. As a
result, there is no net
change in concentration
inside or outside the cell.

Diffusion in living systems
• The difference in concentration of a substance
across space is called a concentration gradient.
• Ions and molecules diffuse from an area of
higher concentration to an area of
lower concentration, moving with the
gradient.
• Dynamic equilibrium occurs when there
is no longer a concentration gradient.

Question 1
Explain why water is important to living
organisms.
Answer
Living organisms must have water for life
processes, because critical molecules and ions
must be free to move and collide, which only
happens when they are dissolved in water.
Water also transports materials in living
organisms, such as in blood or sap.

Question 2 + Positively
charged end

How does
water's chemical
structure impact
its role in living
organisms? +

― Negatively
charged end

Because water is polar, it can dissolve many
ionic compounds and polar molecules. Water
has the property of capillary action that enables
plants to get water from the ground. Water also
resists temperature changes, which allows cells
to maintain homeostasis.

Question 3
Which of the following best describes diffusion?
A. slow process resulting from random
movement of particles
B. net movement of particles from area of low
concentration to area of high concentration

Question 3
Which of the following best describes diffusion?

C. rapid process that is unaffected by increases
in temperature
D. net movement of particles from high to low
concentrations that accelerates when
pressure decreases

The answer is A. Diffusion is a slow process
resulting from the random movement of
particles, and is the net movement of particles
from areas of high concentration to areas of
lower concentration.

Section Objectives:
• Classify the variety of organic compounds.
• Describe how polymers are formed and broken
down in organisms.
• Compare the chemical structures of
carbohydrates, lipids, proteins, and nucleic
acids, and relate their importance to living
things.
• Identify the effects of enzymes.

The Role of Carbon in Organisms
• A carbon atom has four electrons available for
bonding in its outer energy level. In order to
become stable, a carbon atom forms four
covalent bonds that fill its outer energy level.

The Role of Carbon in Organisms
• Two carbon atoms can form various types of
covalent bonds—single, double or triple.

Single Bond Double Bond Triple Bond

Molecular chains
• Carbon compounds vary greatly in size.
• When carbon atoms bond to each other, they
can form straight chains, branched chains, or
rings.

Molecular chains
• Small molecules
bond together to
form chains
called polymers.
A polymer is a
large molecule
formed when
many smaller
molecules bond
together.

The structure of carbohydrates
• A carbohydrate is a biomolecule
composed of carbon, hydrogen, and
oxygen with a ratio of about two
hydrogen atoms and one oxygen atom for
every carbon atom.

The structure of carbohydrates
• The simplest type of carbohydrate is a
simple sugar called a monosaccharide (mah
noh SA kuh ride). (ie. glucose, fructose)
• The largest carbohydrate molecules are
polysaccharides, polymers composed of
many monosaccharide subunits. (ie.
potatoes, liver)

The structure of lipids
• Lipids are large biomolecules that are made
mostly of carbon and hydrogen with a small
amount of oxygen. (ie. fats, oils, waxes)
• They are insoluble in water because their
molecules are nonpolar and are not attracted
by water molecules.

The structure of proteins
• A protein is a large, complex polymer
composed of carbon, hydrogen, oxygen,
nitrogen, and sometimes sulfur.

• The basic building blocks of proteins are
called amino acids.
• There are about 20 common amino acids that
can make literally thousands of proteins.

• Peptide bonds are covalent bonds formed
between amino acids.

• Proteins are the building blocks of many
structural components of organisms.

• Enzymes are important proteins found in
living things. An enzyme is a protein that
changes the rate of a chemical reaction.
• They speed the
reactions in
digestion of food.

The structure of nucleic acids
• A nucleic (noo KLAY ihk) acid is a complex
biomolecule that stores cellular information in
the form of a code.

• Nucleic acids are polymers made of smaller
subunits called nucleotides.

• Nucleotides are arranged in three groups—a
nitrogenous base, a simple sugar, and a
phosphate group.
Phosphate
Sugar
Nitrogenous
base

• DNA, which stands for deoxyribonucleic acid
is a nucleic acid.

Phosphate
Sugar
Nitrogenous
base

• The information coded in DNA contains the
instructions used to form all of an organism’s
enzymes and structural proteins.
• Another important nucleic acid is RNA, which
stands for ribonucleic acid. RNA is a nucleic
acid that forms a copy of DNA for use in
making proteins.

Question 1
How many covalent bonds does a carbon atom
need to form in order to become stable?
A. 1
B. 2
C. 3
D. 4

The answer is D. A carbon atom has four
electrons available for bonding in its outer
energy level and needs to form four covalent
bonds in order to become stable.

Question 2
A __________ is a biomolecule composed of
carbon, hydrogen, and oxygen with a ratio of
about two hydrogen atoms and one oxygen
atom for every carbon atom.
A. carbohydrate C. protein
B. lipid D. fatty acid

The answer is A. Lipids are made mostly of
carbon and hydrogen, and proteins contain
nitrogen in addition to carbon, hydrogen and
oxygen.

Question 3
In which type of molecule will you find peptide
bonds?

B. lipid D. fatty acid

The answer is C. Amino acids are the basic
building blocks of proteins and are linked
together by peptide bonds.

Question 4
What biomolecule is represented in this
diagram?
Phosphate
Sugar
Nitrogenous
base

B. nucleotide D. lipid

The answer is B. Nucleotides are the smaller
subunits that make up nucleic acids. Nucleotides
are composed of three groups: a nitrogenous
base, a simple sugar, and a phosphate group.

Phosphate

Sugar Nitrogenous
base

Question 5
Describe an enzyme and its function.

An enzyme is a protein that enables other
molecules to undergo chemical changes to form
new products. Enzymes increase the speed of
reactions that would otherwise proceed too
slowly.
Substrate
Active
site

Atoms and Their Interactions
• Atoms are the basic building block of all
matter.
• Atoms consist of a nucleus containing protons
and usually neutrons. The positively charged
nucleus is surrounded by rapidly moving,
negatively charged electrons.
• Atoms become stable by bonding to other
atoms through covalent or ionic bonds.

Atoms and Their Interactions
• Components of mixtures retain their properties.

• Solutions are mixtures in which the
components are evenly distributed.
• Acids are substances that from hydrogen ions
in water. Bases are substances that form
hydroxide ions in water.

Water and Diffusion
• Water is the most abundant compound in
living things.
• Water is an excellent solvent due to the
polar property of its molecules.
• Particles of matter are in constant motion.
• Diffusion occurs from areas of higher
concentration to areas of lower
concentration.

Life Substances
• All organic compounds contain carbon atoms.
• There are four principal types of organic
compounds, or biomolecules, that make up
living things: carbohydrates, lipids, proteins,
and nucleic acid.
• The structure of a biomolecule will help
determine its properties and functions.

Question 1
What is the difference between a compound
and an element?

Answer
A compound is a substance that is composed
of atoms of two or more different elements
that are chemically combined. An element is
a substance that can't be broken down into
simpler chemical substances.

Question 2
What is it called when atoms share electrons?
Water
molecule

A. covalent bonding C. hydrogen bonding
B. ionic bonding D. diffusion

The answer is A.
Covalent bonds differ
from ionic bonds in
that the shared
electrons move about
the nuclei of both
atoms of the covalent Water
compound. molecule

Question 3
Which of the following combinations will
produce a solution?
A. chocolate chips and cookie dough
B. sand and sugar crystals
C. powdered drink mix and water
D. oil and vinegar

The answer is C. All
of the combinations
are mixtures because Water
molecules
the individual
components retain Sugar
molecules
their own properties. A
solution is a mixture in Sugar
crystal

which one or more
substances is dissolved
in another and will not
settle out of solution.

Question 4
What type of substance forms hydrogen ions in
water?
A. enzyme
B. acid
C. base
D. polar

The answer is B. Any substance that forms
hydrogen ions (H+) in water is an acid. The
pH of a substance is a measure of how acidic
or basic a solution is.

Question 5
Which of the following best describes a
molecule with an unequal distribution of
charge?
A. polar
B. acidic
C. basic
D. diffuse

The answer is A. Each polar molecule has a
positive end and a negative end. Polar water
molecules attract ions and other polar
molecules, and can dissolve many ionic
compounds.

Question 6
Name the chemical reaction illustrated in the
diagram.

CH2OH
O
CH2OH
O
A. hydrolysis
Glucose OH OH
HO OH HO
OH OH
+
B. condensation
+ O H2O

O O
HOCH2
Fructose HO
OH HOCH2

HO
C. Protein
OH
CH2OH
OH
CH2OH
synthesis
Sucrose D. glycolysis

The answer is B. In condensation
reactions, small molecules bond together
to produce a polymer and water.
CH2OH CH2OH
O O
Glucose OH OH
HO OH HO
OH OH
+ O + H2O

HOCH2 O OH HOCH2 O
Fructose HO HO
CH2OH CH2OH
OH OH

Sucrose

Question 7
An oxygen atom has 8 protons and 8 neutrons.
How many electrons does it have?
A. 8
B. 18
C. 32
D. 0

The answer is A. Atoms contain equal
numbers of electrons and protons and have no
net charge.

Nucleus
8 protons (p+)
8 neutrons (n0)

Oxygen atom

Question 8
Based on your CH2OH
A. nuclease
O
knowledge of
biomolecules, HO
OH B. lipase
which of the OH
following O
C. pepsin
substances
HOCH2 O D. water
would be most
effective at HO
CH2OH
breaking down OH
this polymer?

The answer is D. This is a sucrose molecule,
formed from glucose and fructose in a
condensation reaction. The products of this
reaction are the sucrose molecule and water. If
water is added to sucrose, hydrolysis occurs and
breaks the covalent bonds between the subunits.

Photo Credits

• Aaron Haupt
• Corbis
• Digital Stock
• Elaine Shay
• Mark Thayer
• PhotoDisc
• Alton Biggs

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