Chapter 4
Carbon: The
Basis of
Molecular
Diversity
Lecture Presentations by
Nicole Tunbridge and
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Kathleen Fitzpatrick
Carbon: The Backbone of Life
Living organisms consist mostly of carbon-based
compounds
Carbon is unparalleled in its ability to form large,
complex, and varied molecules
Proteins, DNA, carbohydrates, and other molecules
that distinguish living matter are all composed of
carbon compounds
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Figure 4.1
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Concept 4.1: Organic chemistry is the study of
carbon compounds
Organic chemistry is the study of compounds that
contain carbon, regardless of origin
Organic compounds range from simple molecules to
colossal ones
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Organic Molecules and the Origin of Life on
Earth
Stanley Miller’s classic experiment demonstrated the
abiotic synthesis of organic compounds
Experiments support the idea that abiotic synthesis
of organic compounds, perhaps near volcanoes,
could have been a stage in the origin of life
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Figure 4.2
“Atmosphere”
CH4
Water vapor
Electrode
Condenser
Cooled “rain”
containing Cold
organic water
molecules
H2O
“sea”
Sample for
chemical analysis
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The overall percentages of the major elements of
life—C, H, O, N, S, and P—are quite uniform from
one organism to another
Because of carbon’s ability to form four bonds, these
building blocks can be used to make an
inexhaustible variety of organic molecules
The great diversity of organisms on the planet is due
to the versatility of carbon
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Concept 4.2: Carbon atoms can form diverse
molecules by bonding to four other atoms
Electron configuration is the key to an atom’s
characteristics
Electron configuration determines the kinds and
number of bonds an atom will form with other atoms
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The Formation of Bonds with Carbon
With four valence electrons, carbon can form four
covalent bonds with a variety of atoms
This makes large, complex molecules possible
In molecules with multiple carbons, each carbon
bonded to four other atoms has a tetrahedral shape
However, when two carbon atoms are joined by a
double bond, the atoms joined to the carbons are
in the same plane as the carbons
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Figure 4.3
Molecule Molecular Structural Ball-and-Stick Model Space-Filling
Formula Formula Model
(a) Methane
CH4
(b) Ethane
C2H6
(c) Ethene (ethylene)
C2H4
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The number of unpaired electrons in the valence
shell of an atom is generally equal to its valence, the
number of covalent bonds it can form
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Figure 4.4
Hydrogen Oxygen Nitrogen Carbon
(valence = 1) (valence = 2) (valence = 3) (valence = 4)
H O N C
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The electron configuration of carbon gives it covalent
compatibility with many different elements
The valences of carbon and its most frequent
partners (hydrogen, oxygen, and nitrogen) are the
building code for the architecture of living molecules
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Carbon atoms can partner with atoms other than
hydrogen, such as the following:
Carbon dioxide: CO2
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Urea: CO(NH2)2
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Molecular Diversity Arising from Variation in
Carbon Skeletons
Carbon chains form the skeletons of most organic
molecules
Carbon chains vary in length and shape
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Figure 4.5
(a) Length (c) Double bond position
Ethane Propane 1-Butene 2-Butene
(b) Branching (d) Presence of rings
Butane 2-Methylpropane Cyclohexane Benzene
(isobutane)
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Animation: Carbon Skeletons
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Hydrocarbons
Hydrocarbons are organic molecules consisting of
only carbon and hydrogen
Many organic molecules, such as fats, have
hydrocarbon components
Hydrocarbons can undergo reactions that release a
large amount of energy
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Figure 4.6
Nucleus
Fat droplets
10 µm
(a) Part of a human adipose cell (b) A fat molecule
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Isomers
Isomers are compounds with the same molecular
formula but different structures and properties
Structural isomers have different covalent
arrangements of their atoms
Cis-trans isomers have the same covalent bonds but
differ in their spatial arrangements
Enantiomers are isomers that are mirror images
of each other
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Figure 4.7
(a) Structural isomers (c) Enantiomers
Pentane 2-Methylbutane L isomer D isomer
(b) Cis-trans isomers
cis isomer: trans isomer:
The two Xs are on The two Xs are
the same side. on opposite sides.
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Figure 4.7a
(a) Structural isomers
Pentane 2-Methylbutane
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Figure 4.7b
(b) Cis-trans isomers
cis isomer: The two Xs trans isomer: The two Xs
are on the same side. are on opposite sides.
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Figure 4.7c
(c) Enantiomers
CO2H CO2H
C C
H NH2 NH2 H
CH3 CH3
L isomer D isomer
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Animation: Isomers
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Enantiomers are important in the pharmaceutical
industry
Two enantiomers of a drug may have different
effects
Usually, only one isomer is biologically active
Differing effects of enantiomers demonstrate that
organisms are sensitive to even subtle variations
in molecules
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Figure 4.8
Effective Ineffective
Drug Effects Enantiomer Enantiomer
Reduces
Ibuprofen inflammation
and pain
S-Ibuprofen R-Ibuprofen
Relaxes bronchial
(airway) muscles,
Albuterol improving airflow
in asthma
patients R-Albuterol S-Albuterol
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Animation: L-Dopa
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Concept 4.3: A few chemical groups are key to
molecular function
Distinctive properties of organic molecules depend
on the carbon skeleton and on the chemical groups
attached to it
A number of characteristic groups can replace
the hydrogens attached to skeletons of organic
molecules
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The Chemical Groups Most Important in the
Processes of Life
Estradiol and testosterone are both steroids with
a common carbon skeleton, in the form of four fused
rings
These sex hormones differ only in the chemical
groups attached to the rings of the carbon skeleton
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Figure 4.UN04
Estradiol Testosterone
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Functional groups are the components of organic
molecules that are most commonly involved in
chemical reactions
The number and arrangement of functional groups
give each molecule its unique properties
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The seven functional groups that are most important
in the chemistry of life are the following:
Hydroxyl group
Carbonyl group
Carboxyl group
Amino group
Sulfhydryl group
Phosphate group
Methyl group
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Figure 4.9
Chemical Group Group Properties Examples
Hydroxyl group (—OH) Alcohol
Ethanol
Carbonyl group ( C ═ O) Ketone
Aldehyde
Acetone Propanal
Carboxyl group (—COOH) Carboxylic acid
or organic acid
Acetic acid
Amino group (—NH2) Amine
Glycine
Sulfhydryl group (—SH) Thiol
Cysteine
Phosphate group (—OPO32−) Organic
phosphate
Glycerol
phosphate
Methyl group (—CH3) Methylated
compound
5-Methylcytosine
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Figure 4.9a
Chemical Group Compound Name Examples
Hydroxyl group (—OH) Alcohol
Ethanol
Carbonyl group ( C ═ O) Ketone
Aldehyde
Acetone Propanal
Carboxyl group (—COOH) Carboxylic acid
or organic acid
Acetic acid
Amino group (—NH2) Amine
Glycine
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Figure 4.9aa
Hydroxyl group (—OH)
Ethanol, the alcohol
present in
alcoholic beverages
(may be written HO—)
Polar due to electronegative oxygen. Forms hydrogen bonds with water.
Compound name: Alcohol
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Figure 4.9ab
Carbonyl group ( C ═ O)
Acetone, Propanal,
the simplest ketone an aldehyde
Sugars with ketone groups are called ketoses; those with aldehydes
are called aldoses.
Compound name: Ketone or aldehyde
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Figure 4.9ac
Carboxyl group (—COOH)
Acetic acid, which Ionized form of —COOH
gives vinegar its (carboxylate ion),
sour taste found in cells
Acts as an acid.
Compound name: Carboxylic acid, or organic acid
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Figure 4.9ad
Amino group (—NH2)
Glycine, an amino acid Ionized form
(note its carboxyl group) of —NH2,
found in cells
Acts as a base.
Compound name: Amine
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Figure 4.9b
Chemical Group Compound Name Examples
Sulfhydryl group (—SH) Thiol
Cysteine
Phosphate group Organic
(—OPO32−) phosphate
Glycerol
phosphate
Methyl group (—CH3) Methylated
compound 5-Methylcytosine
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Figure 4.9ba
Sulfhydryl group (—SH)
Cysteine, a sulfur-
containing amino acid
(may be written HS—)
Two —SH groups can react, forming a “cross-link” that helps stabilize
protein structure.
Compound name: Thiol
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Figure 4.9bb
Phosphate group (—OPO32−)
Glycerol phosphate,
which takes part in
many important
chemical reactions
in cells
Contributes negative charge. When attached, confers on a molecule the
ability to react with water, releasing energy.
Compound name: Organic phosphate
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Figure 4.9bc
Methyl group (—CH3)
5-Methylcytosine, a
component of DNA
that has been modified
by addition of a methyl
group
Affects the expression of genes. Affects the shape and function of
sex hormones.
Compound name: Methylated compound
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ATP: An Important Source of Energy for
Cellular Processes
An important organic phosphate is adenosine
triphosphate (ATP)
ATP consists of an organic molecule called
adenosine attached to a string of three phosphate
groups
ATP stores the potential to react with water
This reaction releases energy that can be used by
the cell
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Figure 4.UN05
Adenosine
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Figure 4.UN06
Reacts
with H2O
P P P Adenosine P P Adenosine Pi Energy
ATP ADP Inorganic
phosphate
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