CHAPTER 2
Essential Chemistry
for Biology
TRACING LIFE DOWN TO THE CHEMICAL
LEVEL
• Biology includes the study of life at many levels
• In order to understand life, we began at the
macroscopic level, with the ecosystem and the
organisms in it
• We now need to work our way down to the
microscopic and biochemical level of cells
• Cells consist of enormous numbers of chemicals
that give the cell the properties we recognize as
life
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Ecosystem:
African savanna
Community:
All organisms in the savanna
Organism: Zebra
Population:
Herd of zebras
Organ system:
Circulatory system
Organ: Heart
Tissue: Heart
muscle tissue
Cell: Heart
muscle cell
Molecule:
DNA
Atom:
Oxygen atom
Figure 2.1
Matter: Elements and Compounds
• Matter is anything that occupies space and has mass
• Matter is found on the Earth in three physical states
– Solid
– Liquid
– Gas
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• Matter is composed of chemical elements
– Elements are substances that cannot be broken down
into other substances by ordinary chemical means
(on Earth, only physicists can split atoms)
– There are 92 naturally occurring elements on Earth
• All the elements are listed in the Periodic Table
Atomic number
Element symbol
Mass number
Figure 2.2
3
• Twenty-five elements are essential to life
– Four of these
make up about
96% of the
weight of the
human body
– Trace elements
occur in smaller
amounts
Figure 2.3
• Trace elements are essential for life
– An iodine deficiency causes goiter
Figure 2.4
4
• Elements can combine to form compounds,
substances that contain two or more elements in a
fixed ratio
– Example: NaCl (table salt)
Na
Cl
NaCl
Atoms
• Each element consists of one kind of atom
– An atom is the smallest unit of matter that still retains
the properties of an element
Nucleus
(a)
(b)
Cloud of negative
charge (2 electrons)
2
Protons
2
Neutrons
2
Electrons
Figure 2.5
5
The Structure of Atoms
• Atoms are composed of subatomic particles
– A proton is positively charged
– An electron is negatively charged
– A neutron is electrically neutral
• Most atoms have protons and neutrons packed
tightly into the nucleus
– The nucleus is the atom’s central core
– The electrons orbit the nucleus
• Elements differ in the number of subatomic particles
in their atoms
– The number of protons, the atomic number,
determines which element it is
– An atom’s mass number is the sum of the number of
protons and neutrons
– Mass is a measure of the amount of matter in an
object
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Isotopes
• Isotopes are alternate mass forms of an element
– they have the same number of protons and electrons
– but they have a different number of neutrons
Table 2.1
Electron Arrangement and the Chemical Properties
of Atoms
• Electrons determine how an atom behaves when it
encounters other atoms
• Electrons orbit the nucleus of an atom in specific
electron shells
– The number of electrons in the outermost shell
determines the chemical properties of an atom
– Imagine a ping-pong ball placed inside a baseball
placed inside a basketball placed inside a beach ball.
The surface of each ball is like the shell of an atom on
which electrons travel.
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• Atoms of the four elements most abundant in life
First
electron shell
(can hold
2 electrons)
Outermost
electron shell
(can hold
8 electrons)
Electron
Hydrogen (H)
Atomic number = 1
Carbon (C)
Atomic number = 6
Nitrogen (N)
Atomic number = 7
Oxygen (O)
Atomic number = 8
Figure 2.7
Chemical Bonding and Molecules
• Chemical reactions enable atoms to give up or
acquire electrons in order to complete their outer
shells
– These interactions usually result in atoms staying
close together
– The atoms are held together by chemical bonds
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Ionic Bonds
• When an atom loses or
gains electrons, it
becomes electrically
charged
– Charged atoms are
called ions
– Ionic bonds are
formed between
oppositely charged
ions
Sodium atom (Na)
Chlorine atom (Cl)
Complete
outer shells
Sodium ion (Na +) Chloride ion (Cl−)
Sodium chloride (NaCl)
Figure 2.8
Covalent Bonds
• A covalent bond forms when two atoms share one
or more pairs of outer-shell electrons
Figure 2.9
9
Chemical Reactions
• Cells constantly rearrange molecules by breaking
existing chemical bonds and forming new ones
– Such changes in the chemical composition of matter
are called chemical reactions
Hydrogen gas
Oxygen gas
Reactants
Water
Products
Unnumbered Figure 2.1
• Chemical reactions can be symbolized with
equations
– On the left side of the equation are the reactants, the
starting materials
– On the right side of the equation are the products, the
end materials
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WATER AND LIFE
• Life on Earth began in water and evolved there for
3 billion years
– Modern life still remains tied to water
– Your cells are composed of 70%–95% water
• The abundance of water is a major reason Earth is
habitable
Figure 2.10
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The Structure of Water
• Studied in isolation, the water molecule is
deceptively simple
– Its two hydrogen atoms are joined to one oxygen
atom by single covalent bonds
H
H
O
Unnumbered Figure 2.2
• But the electrons of the covalent bonds are not
shared equally between oxygen and hydrogen
– Oxygen has a larger surface area, and it takes
electrons longer to travel around it
– This unequal sharing makes water a polar molecule
(+)
(+)
(−)
(−)
Figure 2.11a
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• The polarity of
water results in
weak electrical
attractions between
neighboring water
molecules
– These
interactions
are called
hydrogen
bonds
(−)
(+)
(+)
Hydrogen bond
(−)
(−)
(+)
(−)
(+)
(b)
Figure 2.11b
Water’s Life-Supporting Properties
• The polarity of water molecules and the hydrogen
bonding that results explain most of water’s lifesupporting properties
– water molecules are cohesive, meaning they tend to
stay together
– through hydrogen bonding, water is able to absorb a
lot of energy before turning into a gas and thus
moderates temperature
– ice floats because it has enclosed air in between
water molecules
– water is an excellent solvent
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The Cohesion of Water
• Water molecules
stick together as a
result of hydrogen
bonding
– This is called
cohesion
Microscopic tubes
– Cohesion is
vital for water
transport in
plants
Figure 2.12
• Surface tension is the measure of how difficult it is
to stretch or break the surface of a liquid
– Hydrogen bonds
give water an
unusually high
surface tension
Figure 2.13
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How Water Moderates Temperature
• Because of hydrogen bonding, water has a strong
resistance to temperature change
• Heat and temperature are related, but different
– Heat is the amount of energy associated with the
movement of the atoms and molecules in a body of
matter
– Temperature measures the intensity of heat
• Water can absorb and store large amounts of heat
while only changing a few degrees in temperature
• Water can moderate
temperatures
– Earth’s giant water
supply causes
temperatures to stay
within limits that
permit life
– Evaporative cooling
removes heat from the
Earth and from
organisms
Figure 2.14
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The Biological Significance of Ice Floating
• When water molecules get cold, they move apart,
forming ice
– A chunk of ice has fewer molecules than an equal
volume of liquid water
• The density of ice is lower than liquid water
– This is why ice floats
Hydrogen bond
Ice
Liquid water
Stable hydrogen bonds
Hydrogen bonds
constantly break and re-form
Figure 2.15
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• Since ice floats, ponds, lakes, and even the oceans
do not freeze solid
– Marine life could not survive if bodies of water froze
solid
Water as the Solvent of Life
• A solution is a liquid consisting of two or more
substances evenly mixed
– The dissolving agent is called the solvent
– The dissolved substance is called the solute
Ion in solution
Salt crystal
Figure 2.16
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Acids, Bases, and pH
• Acid
– A chemical compound that donates H+ ions to
solutions
– pH 0–7
• Base
– A compound that accepts H+ ions and removes them
from solution
– pH 8–14
• A pH near 7 is considered neutral
• To describe the
acidity of a
solution, we use
the pH scale
Oven cleaner
Household bleach
Household ammonia
Basic
solution
Milk of magnesia
Seawater
Human blood
Pure water
Neutral
solution
Urine
Tomato juice
Grapefruit juice
Acidic
solution
Lemon juice;
gastric juice
pH scale
Figure 2.17
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• Buffers are substances that resist pH change
– They accept H+ ions when they are in excess
– They donate H+ ions when they are depleted
– Buffers are not foolproof. As your body chemistry
changes (through medication, drug abuse, hormone
problems, metabolic diseases, stress, smog) buffers
may lose their effectiveness. Thus, when you clean
your face with a cleanser, you should tone to restore
the skin’s pH.
Figure 2.18
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