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Chapter 2
The Chemical Context of Life
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Elements and Compounds
• An element is a substance that cannot be
broken down to other substances by chemical
reactions
• A compound is a substance consisting of
two or more elements in a fixed ratio
Sodium
Chlorine
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sodium chloride
Essential Elements of Life
• Carbon, hydrogen, oxygen, and nitrogen make
up 96% of living matter
• Trace elements are those required by an
organism in minute quantities
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 2-3
Nitrogen deficiency
Iodine deficiency
LE 2-4
Cloud of negative
charge (2 electrons)
Electrons
Nucleus
Atomic Number and Atomic Mass
• Atoms of the various elements differ in number of
subatomic particles
– Atomic number
– Mass number
– Atomic mass
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Isotopes
• Atoms of an element have the same number of
protons but may differ in number of neutrons
• some are radioactive
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• Some applications of radioactive isotopes
in biological research:
– Dating fossils
– Tracing atoms through metabolic
processes
– Diagnosing medical disorders
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LE 2-5a
TECHNIQUE
Ingredients including
radioactive tracer
(bright blue)
Human cells
Incubators
1
2
10°C
15°C
4
5
25°C
30°C
35°C
7
40°C
8
45°C
9
50°C
DNA (old and new)
3
20°C
6
LE 2-5b
LE 2-5c
Counts per minute
(x 1,000)
RESULTS
30
Optimum
temperature
for DNA
synthesis
20
10
0
10
20
30
40
Temperature (°C)
50
LE 2-6
Cancerous
throat
tissue
The Energy Levels of Electrons
• Energy is the capacity to cause change
• Potential energy is the energy that matter
has because of its location or structure
• electron shell
A ball bouncing down a flight
of stairs provides an analogy for
energy levels of electrons.
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LE 2-7b
Third energy level (shell)
Second energy level (shell)
Energy
absorbed
First energy level (shell)
Energy
lost
Atomic
nucleus
LE 2-8
Hydrogen
1H
2
Atomic number
He
Atomic mass
First
shell
4.00
Helium
2He
Element symbol
Electron-shell
diagram
Lithium
3Li
Beryllium
4Be
Boron
5B
Carbon
6C
Nitrogen
7N
Oxygen
8O
Fluorine
9F
Neon
10Ne
Sodium
11Na
Magnesium
12Mg
Aluminum
12Al
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Second
shell
Third
shell
Electron Orbitals
• An orbital is the three-dimensional space
where an electron is found 90% of the time
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 2-9
Electron
orbitals
y
x
z
1s orbital
2s orbital
Three 2p orbitals
1s, 2s, and 2p orbitals
Electron-shell
diagrams
First shell
(maximum
2 electrons)
Second shell
(maximum
8 electrons)
Neon, with two
filled shells
(10 electrons)
Covalent Bonds
Hydrogen atoms (2 H)
• A covalent bond is
the sharing of a pair
of valence electrons
by two atoms
Hydrogen
molecule (H2)
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• A molecule consists of two or more atoms held
together by covalent bonds
– single covalent bond
– double covalent bond
Oxygen (O2)
Animation: Covalent Bonds
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 2-11c
Name
(molecular
formula)
Water (H2O)
Electronshell
diagram
Structural
formula
Spacefilling
model
LE 2-11d
Name
(molecular
formula)
Methane (CH4)
Electronshell
diagram
Structural
formula
Spacefilling
model
LE 2-12
–
O
H
+
H
H2O
+
Ionic Bonds
• Atoms sometimes strip electrons from
their bonding partners
– Example: sodium to chlorine
Na
Cl
Na+
Cl–
Sodium atom
(an uncharged
atom)
Chlorine atom
(an uncharged
atom)
Sodium ion
(a cation)
Chlorine ion
(an anion)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sodium chloride (NaCl)
LE 2-8
Hydrogen
1H
2
Atomic number
He
Atomic mass
First
shell
4.00
Helium
2He
Element symbol
Electron-shell
diagram
Lithium
3Li
Beryllium
4Be
Boron
5B
Carbon
6C
Nitrogen
7N
Oxygen
8O
Fluorine
9F
Neon
10Ne
Sodium
11Na
Magnesium
12Mg
Aluminum
12Al
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Second
shell
Third
shell
• An anion is a negatively charged ion
• A cation is a positively charged ion
• An ionic bond is an attraction between an anion
and a cation
Na+
Cl–
Animation: Ionic Bonds
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Weak Chemical Bonds
• Weak chemical bonds reinforce shapes of
large molecules and help molecules
adhere to each other
– Ionic bonds
– Hydrogen bonds
– Van der Waals Interactions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hydrogen Bonds
–
+
Water
(H2O)
+
Hydrogen bond
–
Ammonia
(NH3)
+
+
+
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Van der Waals Interactions
• Molecules or atoms that are very close
together can be attracted by fleeting
charge differences
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Molecular Shape and Function
• A molecule’s shape is usually very
important to its function
– determined by the positions of its atoms’
valence orbitals
s orbital
z
Three p orbitals
Four hybrid orbitals
x
y
Tetrahedron
Hybridization of orbitals
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LE 2-16b
Space-filling
model
Ball-and-stick
model
Hybrid-orbital model
(with ball-and-stick
model superimposed)
Unbonded
electron
pair
104.5°
Water (H2O)
Methane (CH4)
Molecular shape models
LE 2-17a
Carbon
Nitrogen
Hydrogen
Sulfur
Natural
endorphin
Oxygen
Morphine
Structures of endorphin and morphine
LE 2-17b
Natural
endorphin
Brain cell
Morphine
Endorphin
receptors
Binding to endorphin receptors
Concept 2.4: Chemical reactions make and break
chemical bonds
• The starting molecules of a chemical reaction
are called reactants
• The final molecules of a chemical reaction
are called products
2 H2
O2
Reactants
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2 H2O
Reaction
Products
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings