Molecules of Life
Chapter 2
Part 1
2.1 Impacts/Issues
Fear of Frying
 All living things consist of the same kinds of
molecules, but small differences in the ways
they are put together have big effects on health
 Artificial trans fats found in manufactured and
fast foods raise cholesterol and increase risk of
atherosclerosis, heart attack, and diabetes
Video: Fear of frying
Fear of Frying
 Trans fats are made by adding hydrogen atoms
to liquid vegetable oils
trans fatty acid
Fig. 2-1, p. 20
2.2 Start With Atoms
 All substances consist of atoms
 Atom
• Fundamental building-block particle of matter
 Life’s unique characteristics start with the
properties of different atoms
Subatomic Particles and Their Charge
 Atoms consist of electrons moving around a
nucleus of protons and neutrons
 Electron (e-)
• Negatively charged subatomic particle that
occupies orbitals around the atomic nucleus
 Charge
• Electrical property of some subatomic particles
• Opposite charges attract; like charges repel
Subatomic Particles in the Nucleus
 Nucleus
• Core of an atom, occupied by protons and
neutrons
 Proton (p+)
• Positively charged subatomic particle found in the
nucleus of all atoms
 Neutron
• Uncharged subatomic particle found in the atomic
nucleus
An Atom
an atom
Fig. 2-2a, p. 21
Elements: Different Types of Atoms
 Atoms differ in numbers of subatomic particles
 Element
• A pure substance that consists only of atoms with
the same number of protons
 Atomic number
• Number of protons in the atomic nucleus
• Determines the element
Elements in Living Things
 The proportions of different elements differ
between living and nonliving things
 Some atoms, such as carbon, are found in
greater proportions in molecules made only by
living things – the molecules of life
Same Elements, Different Forms
 Isotopes
• Forms of an element that differ in the number of
neutrons their atoms carry
• Changes the mass number, but not the charge
 Mass number
• Total number of protons and neutrons in the
nucleus of an element’s atoms
Radioactive Isotopes
 Radioisotope
• Isotope with an unstable nucleus, such as
carbon 14 (14C)
 Radioactive decay
• Process by which atoms of a radioisotope
spontaneously emit energy and subatomic
particles when their nucleus disintegrates
Carbon 14: A Radioisotope
 Most carbon atoms have 6 protons and 6
neutrons (12C)
 Carbon 14 (14C) is a radioisotope with six
protons and eight neutrons
 When 14C decays, one neutron splits into a
proton and an electron, and the atom becomes a
different element – nitrogen 14 (14N)
Radioactive Tracers
 Researchers introduce radioisotope tracers into
living organisms to study the way they move
through a system
 Tracers
• Molecules with a detectable substance attached,
often a radioisotope
• Used in research and clinical testing
Why Electrons Matter
 Electrons travel around the nucleus in different
orbitals (shells) – atoms with vacancies in their
outer shells tend to interact with other atoms
• Atoms get rid of vacancies by gaining or losing
electrons, or sharing electrons with other atoms
 Shell model
• Model of electron distribution in an atom
Shell Models
Fig. 2-3 (top), p. 22
Fig. 2-3 (a-c), p. 22
first shell
1
1 proton
1 electron
hydrogen (H)
6
second shell
carbon (C)
11
third shell
sodium (Na)
2
helium (He)
8
oxygen (O)
17
chlorine (Cl)
10
neon (Ne)
18
argon (Ar)
Fig. 2-3 (a-c), p. 22
A) The first shell corresponds to
the first energy level, and it can
hold up to 2 electrons. Hydrogen
has one proton, so it has one
vacancy. A helium atom has 2
protons, and no vacancies. The
number of protons in each shell
model is shown.
B) The second shell corresponds
to the second energy level, and it
can hold up to 8 electrons. Carbon
has 6 protons, so its first shell is
full. Its second shell has 4
electrons, and four vacancies.
Oxygen has 8 protons and two
vacancies. Neon has 10 protons
and no vacancies.
first shell
6
second shell
carbon (C)
11
C) The third shell, which
corresponds to the third energy
level, can hold up to 8 electrons,
for a total of 18. A sodium atom
has 11 protons, so its first two
shells are full; the third shell has
one electron. Thus, sodium has
seven vacancies. Chlorine has 17
protons and one vacancy. Argon
has 18 protons and no vacancies.
third shell
2
1
1 proton
1 electron
hydrogen (H)
sodium (Na)
helium (He)
8
oxygen (O)
17
chlorine (Cl)
10
neon (Ne)
18
argon (Ar)
Stepped Art
Fig. 2-3 (a-c), p. 22
Animation: Shell models of common
elements
Ions
 The negative charge of an electron balances the
positive charge of a proton in the nucleus
 Changing the number of electrons may fill its
outer shell, but changes the charge of the atom
 Ion
• Atom that carries a charge because it has an
unequal number of protons and electrons
Ion Formation
electron
gain
17
Chlorine
atom
17p+
17e–
charge: 0
Chloride
ion
17
electron
loss
11
11
17p+
18e–
charge: –1
Sodium
atom
11p+
11e–
charge: 0
Sodium
ion
11p+
10e–
charge: +1
Fig. 2-4, p. 23
Fig. 2-4a, p. 23
electron
gain
Chlorine
atom
17
17p+
17e–
charge: 0
Chloride
ion
17
17p+
18e–
charge: –1
Fig. 2-4a, p. 23
Fig. 2-4b, p. 23
electron
loss
Sodium
atom
11
11p+
11e–
charge: 0
Sodium
ion
11
11p+
10e–
charge: +1
Fig. 2-4b, p. 23
electron
gain
17
Chlorine
atom
17p+
17e–
charge: 0
Chloride
ion
17
electron
loss
11
11
17p+
18e–
charge: –1
Sodium
atom
11p+
11e–
charge: 0
Sodium
ion
11p+
10e–
charge: +1
Stepped Art
Fig. 2-4, p. 23
Animation: How atoms bond
Animation: PET scan
Animation: The shell model of electron
distribution
Animation: Subatomic particles
Animation: Atomic number, mass number
Animation: Electron arrangements in
atoms
Animation: Isotopes of hydrogen
Video: ABC News: Nuclear Energy
Animation: Electron distribution
2.3 From Atoms to Molecules
 Atoms can also fill their vacancies by sharing
electrons with other atoms
 A chemical bond forms when the electrons of
two atoms interact
 Chemical bond
• An attractive force that arises between two atoms
when their electrons interact
From Atoms to Molecules
 Molecule
• Group of two or more atoms joined by chemical
bonds
 Compound
• Type of molecule that has atoms of more than
one element
Referring to a Molecule
Same Materials, Different Results
Animation: Building blocks of life
Ionic Bonds and Covalent Bonds
 Depending on the atoms, a chemical bond may
be ionic or covalent
 Ionic bond
• A strong mutual attraction formed between ions of
opposite charge
 Covalent bond
• Two atoms sharing a pair of electrons
An Ionic Bond: Sodium Chloride
ionic bond
11
sodium ion (Na+)
17
chloride ion (Cl–)
p. 24
Covalent Bonds
 Molecular hydrogen (H—H) and molecular
oxygen (O=O)
1
1
molecular hydrogen (H2)
8
8
molecular oxygen (O2)
p. 24
Polarity
 A covalent bond is nonpolar if electrons are
shared equally, and polar if the sharing is unequal
 Polarity
• Any separation of charge into distinct positive and
negative regions
Polar and Nonpolar Covalent Bonds
 Nonpolar
• Having an even distribution of charge
• When atoms in a covalent bond share electrons
equally, the bond is nonpolar
 Polar
• Having an uneven distribution of charge
• When the atoms share electrons unequally, the
bond is polar
Importance of Polar Molecules
 A water molecule (H-O-H) has two polar
covalent bonds – the oxygen is slightly negative
and the hydrogens are slightly positive – which
allows water to form hydrogen bonds
p. 25
1
8
1
water (H2O)
p. 25
Hydrogen Bonds
 Hydrogen bond
• Attraction that forms between a covalently
bonded hydrogen atom and another atom taking
part in a separate covalent bond
hydrogen bond
p. 25
Importance of Hydrogen Bonds
 Hydrogen bonds form and break more easily
than covalent or ionic bonds – they do not form
molecules
 Hydrogen bonds impart unique properties to
substances such as water, and hold molecules
such as DNA in their characteristic shapes
Animation: Ionic bonding
Animation: Examples of hydrogen bonds
Video: ABC News: Fuel Cell Vehicles
Animation: Sucrose synthesis
Animation: Covalent bonds
2.4 Water
 All living organisms are mostly water, and all
chemical reactions of life are carried out in water
 Hydrogen bonds between water molecules give
water unique properties that make life possible
• Capacity to dissolve many substances
• Cohesion (surface tension)
• Temperature stability
Polarity and the
Unique Properties of Water
Fig. 2-7a, p. 26
slight negative charge
slight positive charge
slight positive charge
Fig. 2-7a, p. 26
Fig. 2-7b, p. 26
Fig. 2-7c, p. 26
Animation: Structure of water
Water and Solutions
 Polar water molecules hydrogen-bond to other
polar (hydrophilic) substances, and repel
nonpolar (hydrophobic) substances
 Hydrophilic (water-loving)
• A substance that dissolves easily in water
 Hydrophobic (water-dreading)
• A substance that resists dissolving in water
Water and Solutions
 Water is an excellent solvent
 Solvent
• Liquid that can dissolve other substances
 Solute
• A dissolved substance
Water and Solutions
 Salts, sugars, and many polar molecules
dissolve easily in water
 Salt
• Compound that dissolves easily in water and
releases ions other than H+ and OH• Example: sodium chloride (NaCl)
Water and Solutions
 Water molecules surround the atoms of an ionic
solid and pull them apart, dissolving it
Animation: Spheres of hydration
Temperature Stability
 Temperature stability is an important part of
homeostasis
• Water absorbs more heat than other liquids
before temperature rises
• Hydrogen bonds hold ice together in a rigid
pattern that makes ice float
 Temperature
• Measure of molecular motion
Cohesion
 Cohesion helps sustain multicelled bodies and
resists evaporation
 Cohesion
• Tendency of water molecules to stick together
 Evaporation
• Transition of liquid to gas
• Absorbs heat energy (cooling effect)
2.5 Acids and Bases
 Water molecules separate into hydrogen ions
(H+) and hydroxide ions (OH-)
 pH
• A measure of the number of hydrogen ions (H+) in
a solution
• The more hydrogen ions, the lower the pH
 Pure water has neutral pH (pH=7)
• Number of H+ ions = OH- ions
Acids and Bases
 Acid
• Substance that releases hydrogen ions in water
• pH less than 7
 Base
• Substance that releases hydroxide ions (accepts
hydrogen ions) in water
• pH greater than 7
A pH Scale
battery acid
—1
gastric fluid
—2
acid rain
lemon juice
cola
vinegar
—3
—4
—5
—6
—7
more acidic
—0
orange juice
tomatoes,
wine
bananas
beer
bread
black coffee
urine, tea, typical rain
corn
butter
milk
pure water
more basic
blood, tears
egg white
—8
seawater
baking soda
detergents
—9
Tums
toothpaste
— 10 hand soap
milk of
— 11 magnesia
household ammonia
— 12 hair remover
bleach
— 13
oven cleaner
— 14 drain cleaner
Fig. 2-9, p. 27
Animation: The pH scale
Acid Rain
 Sulfur dioxide and other airborne pollutants
dissolve in water vapor to form acid rain
Buffer Systems
 Most molecules of life work only within a narrow
range of pH – essential for homeostasis
 Buffers keep solutions in cells and tissues within
a consistent range of pH
 Buffer
• Set of chemicals that can keep the pH of a
solution stable by alternately donating and
accepting ions that contribute to pH
CO2 and the Bicarbonate Buffer System
 CO2 forms carbonic acid in water
• CO2 + H2O
→ H CO
2
3
(carbonic acid)
 Bicarbonate buffer system
• Excess H+ combines with bicarbonate
• H+ + HCO3- (bicarbonate)
↔ H CO
2
3
Video: ABC News: Bottle Backlash
Video: ABC News: Water Use
Video: ABC News: Water Wars
3D Animation: Dissolution