essential chemistry for biology
TRANSCRIPT
© 2010 Pearson Education, Inc.
Lectures by Chris C. Romero, updated by Edward J. Zalisko
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Chapter 2
Essential Chemistry for Biology
Biology and Society:
Fluoride in the Water
• Fluoride
– Is a common ingredient in Earth’s crust
– Helps maintain healthy teeth
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• Fluoride prevents cavities by
– Affecting the metabolism of oral bacteria
– Promoting the replacement of lost minerals on the tooth surface
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SOME BASIC CHEMISTRY
• Take any biological system apart, and you eventually end up at
the chemical level.
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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.
– There are 92 naturally occurring elements on Earth.
• All of the elements are listed in the periodic table.
Copper Cu Mercury Hg Lead Pb
H
Rb
K
Na
Li
Fr
Cs
Sr
Ca
Mg
Be
Ra
Ba
Y
Sc
Ac
La
Zr
Ti
Rf
Hf
Nb
V
Db
Ta
Mo
Cr
Sg
W
Tc
Mn
Bh
Re
Ru
Fe
Hs
Os
Rh
Co
Mt
Ir
Pd
Ni
Uun
Pt
Xe
Kr
Uuo
Rn
Ag
Cu
Uuu
Au
Cd
Zn
Uub
Hg
Ar
Ne
In
Ga
Tl
Al
B
Sn
Ge
Uuq
Pb
Si
C
Sb
As
Bi
P
N
Te
Se
Uuh
Po
S
O
I
Br
At
Cl
F
He
Th
Ce
Pa
Pr
U
Nd
Np
Pm
Pu
Sm
Am
Eu
Lr
Lu
Cm
Gd
Bk
Tb
Cf
Dy
Es
Ho
Fm
Er
Md
Tm
No
Yb
6
C 12
Figure 2.1
Atomic number
(number of protons)
Atomic mass
(number of
protons +
neutrons)
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• Twenty-five elements are essential to life.
• Four elements make up about 96% of the weight of the human
body:
– Oxygen
– Carbon
– Hydrogen
– Nitrogen
Carbon C: 18.5%
Hydrogen H:
9.5%
Nitrogen N:
3.3%
Calcium Ca: 1.5%
Trace elements: less than 0.01%
Boron B Manganese Mn
Oxygen O:
65.0%
Magnesium Mg: 0.1%
Phosphorus P: 1.0%
Potassium K: 0.4%
Sulfur S: 0.3%
Sodium Na: 0.2%
Chlorine Cl: 0.2%
Cobalt Co Chromium Cr
Iron Fe Iodine I Fluorine F Copper Cu Silicon Si
Zinc Zn Vanadium V Tin Sn
Molybdenum Mo Selenium Se
Figure 2.2
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• Trace elements
– Required in smaller amounts
– Are essential for life
• An iodine deficiency causes goiter.
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• Elements can combine to form compounds.
– Compounds are substances that contain two or more elements in a
fixed ratio.
– Common compounds include
– NaCl (table salt)
– H2O (water)
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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.
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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.
• Atoms do not have any charge
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• Most atoms have protons and neutrons packed tightly into the
nucleus.
– The nucleus is the atom’s central core.
– Electrons orbit the nucleus.
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• 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.
• Isotopes have the same number of protons and electrons, but they
have a different number of neutrons.
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• The nucleus of a radioactive isotope decays, giving off particles
and energy.
• Radioactive isotopes have many uses in research and medicine.
– They can be used to determine the fate of atoms in living
organisms.
– They are used in PET scans to diagnose heart disorders and some
cancers.
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• Uncontrolled exposure to radioactive isotopes can harm living
organisms by damaging DNA.
– The 1986 Chernobyl nuclear accident released large amounts of
radioactive isotopes.
– Naturally occurring radon gas may cause lung cancer.
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 farther an electron is from the nucleus, the greater its energy.
• The number of electrons in the outermost shell determines the
chemical properties of an atom.
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First electron shell
can hold 2 electrons
Outer electron shell
can hold 8 electrons
Hydrogen H
Atomic number = 1
Carbon C
Atomic number = 6
Nitrogen N
Atomic number = 7
Oxygen O
Atomic number = 8
Electron
Figure 2.5
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Chemical Bonding and Molecules
• Chemical reactions enable atoms to give up or acquire electrons
to complete their outer shells.
• Chemical reactions usually result in atoms
– Staying close together
– Being 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.
Animation: Ionic Bonds
Outer shell has 1 electron
Outer shell has 7 electrons
The outer electron is stripped from sodium and completes the chlorine atom’s outer shell
Na Sodium atom
Cl Chlorine atom
Figure 2.6-1
Electron transfer and ionic bonding
Outer shell has 1 electron
Outer shell has 7 electrons
The outer electron is stripped from sodium and completes the chlorine atom’s outer shell
Na Sodium atom
Cl Chlorine atom
Complete outer shells
The attraction between the ions—an ionic bond—holds them together
Na Sodium ion
Cl Chlorine ion
Sodium chloride (NaCl)
Figure 2.6-2
Electron transfer and ionic bonding
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Covalent Bonds
• A covalent bond forms when two atoms share one or more pairs
of outer-shell electrons.
• Atoms held together by covalent bonds form a molecule.
• The number of covalent bonds an atom can form is equal to the
number of additional electrons needed to fill its outer shell.
Animation: Covalent Bonds
Blast Animation: Covalent Bonds
Name molecular formula
Hydrogen gas H2
Oxygen gas O2
Methane CH4
Electron configuration Structural formula Space-filling model Ball-and-stick model
Single bond a pair of shared electrons
Double bond two pairs of shared electrons
Figure 2.7
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The reactive properties (chemical behavior) of an
atom mostly depend on the number of
a) electrons in each electron shell of the atom.
b) neutrons found in the nucleus.
c) filled electron shells.
d) electrons in the outer electron shell of the atom.
Concept Check
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Hydrogen Bonds
• Water is a compound in which the electrons in its covalent bonds
are shared unequally.
– This causes water to be a polar molecule, one with opposite
charges on opposite ends.
Animation: Water Structure
Blast Animation: Hydrogen Bonds in Water
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• The polarity of water results in weak electrical attractions
between neighboring water molecules.
– These interactions are called hydrogen bonds.
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Concept Check
Water molecules form hydrogen
bonds because
a) the water molecule is polar.
b) the oxygen molecule is
positively charged.
c) the water molecule
forms a tetrahedron.
d) the hydrogen atoms
are negatively charged.
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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.
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• Chemical reactions include
– Reactants, the starting materials
– Products, the end materials
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• Chemical reactions can rearrange matter but cannot create or
destroy matter.
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Interpreting Data
This is the general equation for photosynthesis—the process of capturing
sunlight energy and converting it to chemical energy. Which of the
following are the reactants of this reaction?
a) C6H12O6 and O2
b) CO2 and H2O
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WATER AND LIFE
• Life on Earth began in water and evolved there for 3 billion years.
– Modern life remains tied to water.
– Your cells are composed of 70%–95% water.
• The abundance of water is a major reason Earth is habitable.
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Water’s Life-Supporting Properties
• The polarity of water molecules and the hydrogen bonding that
results explain most of water’s life-supporting properties.
– Water molecules stick together.
– Water has a strong resistance to change in temperature.
– Frozen water floats.
– Water is a common solvent for life.
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The Cohesion of Water
• Water molecules stick together as a result of hydrogen bonding.
– This is called cohesion.
– Cohesion is vital for water transport in plants.
Animation: Water Transport
Microscopic tubes
Cohesion due to
hydrogen bonds
between water
molecules
Evaporation from the leaves
SE
M
Figure 2.10
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• 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.
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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.
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• 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.
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The Biological Significance of Ice Floating
• When water molecules get cold enough, they move apart, forming
ice.
• A chunk of ice has fewer molecules than an equal volume of
liquid water.
• Ice floats because it is less dense than the liquid water around it.
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• If ice did not float, ponds, lakes, and even the oceans would
freeze solid.
• Life in water could not survive if bodies of water froze solid.
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Water as the Solvent of Life
• A solution is a liquid consisting of a homogeneous mixture of
two or more substances.
– The dissolving agent is the solvent.
– The dissolved substance is the solute.
• When water is the solvent, the result is an aqueous solution.
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Acids, Bases, and pH
• A chemical compound that releases H+ to solution is an acid.
• A compound that accepts H+ and removes it from solution is a
base.
• To describe the acidity of a solution, chemists use the pH scale.
Basic
solution
Neutral
solution
Acidic
solution
Oven cleaner
Household
bleach
Human blood
Pure water
Grapefruit juice,
soft drink
Lemon juice,
gastric juice
Household ammonia
Milk of magnesia
Seawater
Tomato juice
Urine
pH scale 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Inc
rea
sin
gly
ac
idic
g
rea
ter
H c
on
ce
ntr
ati
on
Inc
rea
sin
gly
ba
sic
l
ow
er
H c
on
ce
ntr
ati
on
Neutral [H+] [OH–]
Figure 2.16
Each pH unit represents a
tenfold change in the
concentration of H+.
Lemon juice, pH 2 has
100 times more of H+
than tomato juice at pH 4
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• Buffers are substances that resist pH change.
• Buffers
– Accept H+ ions when they are in excess
– Donate H+ ions when they are depleted
• Increases in global CO2 concentrations may lead to the
acidification of the oceans.