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3
Theories of the Origin of Life
• In the 1950s, Stanley Miller and Harold Urey set
up an experimental “primitive” atmosphere and
used a spark to simulate lightning.
• Within days, the system contained numerous
complex molecules.
Figure 3.1 Synthesis of Prebiotic Molecules in an Experimental Atmosphere
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Macromolecules: Giant Polymers
• There are four major types of biological
macromolecules:
 Proteins
 Carbohydrates
 Lipids
 Nucleic acids
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Macromolecules: Giant Polymers
• Macromolecules are giant polymers.
• Polymers are formed by covalent linkages of
smaller units called monomers.
• Molecules with molecular weights greater than
1,000 daltons (atomic mass units) are usually
classified as macromolecules.
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Macromolecules: Giant Polymers
• The functions of macromolecules are related to
their shape and the chemical properties of their
monomers.
• Some of the roles of macromolecules include:
 Energy storage
 Structural support
 Transport
 Protection and defense
 Regulation of metabolic activities
 Means for movement, growth, and development
 Heredity
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Condensation and Hydrolysis Reactions
• Macromolecules are made from smaller
monomers by means of a condensation or
dehydration reaction.
• Energy must be added to make or break a
polymer.
• The reverse reaction, in which polymers are
broken back into monomers, is a called a
hydrolysis reaction.
Figure 3.3 Condensation and Hydrolysis of Polymers (Part 1)
Figure 3.3 Condensation and Hydrolysis of Polymers (Part 2)
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Proteins: Polymers of Amino Acids
• Proteins are polymers of amino acids. They are
molecules with diverse structures and functions.
• Proteins range in size from a few amino acids to
thousands of them.
• Folding is crucial to the function of a protein and is
influenced largely by the sequence of amino
acids.
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Proteins: Polymers of Amino Acids
• An amino acid has four groups attached to a
central carbon atom:
 A hydrogen atom
 An amino group (NH3+)
 The acid is a carboxyl group (COO–).
 Differences in amino acids come from the side
chains, or the R groups.
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Proteins: Polymers of Amino Acids
• Amino acids can be classified based on the
characteristics of their R groups.
 Five have charged hydrophilic side chains.
 Five have polar but uncharged side chains.
 Seven have nonpolar hydrophobic side chains.
 Cysteine has a terminal disulfide (—S—S—).
 Glycine has a hydrogen atom as the R group.
 Proline has a modified amino group that forms
a covalent bond with the R group, forming a
ring.
Table 3.2 The Twenty Amino Acids Found in Proteins (Part 1)
Table 3.2 The Twenty Amino Acids Found in Proteins (Part 2)
Table 3.2 The Twenty Amino Acids Found in Proteins (Part 3)
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Proteins: Polymers of Amino Acids
• Proteins are synthesized by condensation
reactions between the amino group of one amino
acid and the carboxyl group of another. This
forms a peptide linkage.
• Proteins are also called polypeptides. A
dipeptide is two amino acids long; a tripeptide,
three. A polypeptide is multiple amino acids long.
Figure 3.5 Formation of Peptide Linkages
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Proteins: Polymers of Amino Acids
• There are four levels of protein structure: primary,
secondary, tertiary, and quaternary.
• The precise sequence of amino acids is called its
primary structure.
• Enormous numbers of different proteins are
possible.
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Proteins: Polymers of Amino Acids
• A protein’s secondary structure consists of
regular, repeated patterns in different regions in
the polypeptide chain.
• The two common secondary structures are the a
helix and the b pleated sheet.
Figure 3.6 The Four Levels of Protein Structure (Part 1)
Figure 3.6 The Four Levels of Protein Structure (Part 2)
Figure 3.6 The Four Levels of Protein Structure (Part 3)
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Proteins: Polymers of Amino Acids
• Tertiary structure is the three-dimensional shape
of the completed polypeptide.
• Other factors can include the location of disulfide
bridges, which form between cysteine residues.
Figure 3.4 A Disulfide Bridge
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Proteins: Polymers of Amino Acids
• Other factors determining tertiary structure:
 Hydrophobic side-chain aggregation and van
der Waals forces.
 The ionic interactions between the positive
and negative charges deep in the protein,
away from water
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Proteins: Polymers of Amino Acids
• It is now possible to determine the complete
description of a protein’s tertiary structure.
• The location of every atom in the molecule is
specified in three-dimensional space.
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Proteins: Polymers of Amino Acids
• Quaternary structure results from the ways in
which multiple polypeptide subunits bind together
and interact.
• Hemoglobin is an example of such a protein; it
has four subunits.
Figure 3.8 Quaternary Structure of a Protein
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Proteins: Polymers of Amino Acids
• Shape is crucial to the functioning of some
proteins:
 Enzymes need certain surface shapes in order
to bind substrates correctly.
 Carrier proteins in the cell surface membrane
allow substances to enter the cell.
 Chemical signals such as hormones bind to
proteins on the cell surface membrane.
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Proteins: Polymers of Amino Acids
• Changes in temperature, pH, salt concentrations,
and oxidation or reduction conditions can change
the shape of proteins.
• This loss of a protein’s normal three-dimensional
structure is called denaturation.
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Carbohydrates: Sugars and Sugar Polymers
• Carbohydrates are carbon molecules with
hydrogen and hydroxyl groups.
• They act as energy storage and transport
molecules.
• They also serve as structural components.
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Carbohydrates: Sugars and Sugar Polymers
• There are four major categories of carbohydrates:
 Monosaccharides
 Disaccharides, which consist of two
monosaccharides
 Oligosaccharides, which consist of between
3 and 20 monosaccharides
 Polysaccharides, which are composed of
hundreds to hundreds of thousands of
monosaccharides
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Carbohydrates: Sugars and Sugar Polymers
• All living cells contain the monosaccharide
glucose (C6H12O6).
• Glucose exists as a straight chain and a ring, with
the ring form predominant.
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Carbohydrates: Sugars and Sugar Polymers
• Different monosaccharides have different
numbers or different arrangements of carbons.
• Most monosaccharides are optical isomers.
• Hexoses (six-carbon sugars) include the
structural isomers glucose, fructose, mannose,
and galactose.
• Pentoses are five-carbon sugars.
Figure 3.14 Monosaccharides Are Simple Sugars (Part 1)
Figure 3.14 Monosaccharides Are Simple Sugars (Part 2)
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Carbohydrates: Sugars and Sugar Polymers
• Monosaccharides are bonded together covalently
by condensation reactions. The bonds are called
glycosidic linkages.
• Disaccharides have just one such linkage:
sucrose, lactose, maltose, cellobiose.
Figure 3.15 Disaccharides Are Formed by Glycosidic Linkages
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Carbohydrates: Sugars and Sugar Polymers
• Oligosaccharides contain more than two
monosaccharides.
• Many proteins found on the outer surface of cells
have oligosaccharides attached to the R group of
certain amino acids, or to lipids.
• The human ABO blood types owe their specificity
to oligosaccharide chains.
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Carbohydrates: Sugars and Sugar Polymers
• Polysaccharides are giant polymers of
monosaccharides connected by glycosidic
linkages.
• Cellulose is a giant polymer of glucose joined by
b-1,4 linkages.
• Starch is a polysaccharide of glucose with a-1,4
linkages.
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Carbohydrates: Sugars and Sugar Polymers
• Carbohydrates are modified by the addition of
functional groups:
 Glucose can acquire a carboxyl group (—COOH),
forming glucuronic acid.
 Phosphate added to one or more hydroxyl (—OH)
sites creates a sugar phosphate such as fructose
1,6-bisphosphate.
 Amino groups can be substituted for —OH groups,
making amino sugars such as glucosamine and
galactosamine.
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Lipids: Water-Insoluble Molecules
• Lipids are insoluble in water.
• This insolubility results from the many nonpolar
covalent bonds of hydrogen and carbon in lipids.
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Lipids: Water-Insoluble Molecules
• Roles for lipids in organisms include:
 Energy storage (fats and oils)
 Cell membranes (phospholipids)
 Capture of light energy (carotinoids)
 Hormones and vitamins (steroids and modified
fatty acids)
 Thermal insulation
 Electrical insulation of nerves
 Water repellency (waxes and oils)
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Lipids: Water-Insoluble Molecules
• Fats and oils store energy.
• Fats and oils are triglycerides, composed of
three fatty acid molecules and one glycerol
molecule.
Figure 3.18 Synthesis of a Triglyceride
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Lipids: Water-Insoluble Molecules
• Saturated fatty acids have only single carbon-tocarbon bonds and are said to be saturated with
hydrogens.
• Saturated fatty acids are rigid and straight, and
solid at room temperature. Animal fats are
saturated.
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Lipids: Water-Insoluble Molecules
• Unsaturated fatty acids have at least one
double-bonded carbon in one of the chains —the
chain is not completely saturated with hydrogen
atoms.
• The double bonds cause kinks that prevent easy
packing. Unsaturated fatty acids are liquid at room
temperature. Plants commonly have unsaturated
fatty acids.
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Lipids: Water-Insoluble Molecules
• Phospholipids have two hydrophobic fatty acid
tails and one hydrophilic phosphate group
attached to the glycerol.
• As a result, phospholipids orient themselves so
that the phosphate group faces water and the tail
faces away.
• In aqueous environments, these lipids form
bilayers, with heads facing outward, tails facing
inward. Cell membranes are structured this way.
Figure 3.21 Phospholipids Form a Bilayer
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Lipids: Water-Insoluble Molecules
• Steroids are signaling molecules.
• Steroids are organic compounds with a series of
fused rings.
• The steroid cholesterol is a common part of
animal cell membranes.
• Cholesterol is also is an initial substrate for
synthesis of the hormones testosterone and
estrogen.
Figure 3.23 All Steroids Have the Same Ring Structure
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• Nucleic acids are polymers that are specialized
for storage and transmission of information.
• Two types of nucleic acid are DNA
(deoxyribonucleic acid) and RNA (ribonucleic
acid).
• DNA encodes hereditary information and transfers
information to RNA molecules.
• The information in RNA is decoded to specify the
sequence of amino acids in proteins.
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• Nucleic acids are polymers of nucleotides.
• A nucleotide consists of a pentose sugar, a
phosphate group, and a nitrogen-containing base.
• In DNA, the pentose sugar is deoxyribose; in RNA
it is ribose.
Figure 3.24 Nucleotides Have Three Components
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• DNA typically is double-stranded.
• The two separate polymer chains are held
together by hydrogen bonding between their
nitrogenous bases.
• The base pairing is complementary: At each
position where a purine is found on one strand, a
pyrimidine is found on the other.
• Purines have a double-ring structure.
Pyrimidines have one ring.
Figure 3.25 Distinguishing Characteristics of DNA and RNA
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• The linkages that hold the nucleotides in RNA and
DNA are called phosphodiester linkages.
• In DNA, the two strands are antiparallel.
• The DNA strands form a double helix, a molecule
with a right-hand twist.
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• Most RNA molecules consist of only a single
polynucleotide chain.
• Instead of the base thymine, RNA uses the base
uracil.
• Hydrogen bonding between ribonucleotides in
RNA can result in complex three-dimensional
shapes.
Figure 3.26 Hydrogen Bonding in RNA
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• DNA is an information molecule. The information
is stored in the order of the four different bases.
• This order is transferred to RNA molecules, which
are used to direct the order of the amino acids in
proteins.
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• Closely related living species have DNA base
sequences that are more similar than distantly
related species.
• The comparative study of base sequences has
confirmed many of the traditional classifications of
organisms.
• DNA comparisons confirm that our closest living
relatives are chimpanzees: We share more than
98 percent of our DNA base sequences.
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Nucleic Acids: Informational Macromolecules
That Can Be Catalytic
• Nucleotides have other important roles:
 The ribonucleotide ATP acts as an energy
transducer in many biochemical reactions.
 The ribonucleotide GTP powers protein
synthesis.
 cAMP (cyclic AMP) is a special ribonucleotide
that is essential for hormone action and the
transfer of information by the nervous system.
Figure 3.28 Disproving the Spontaneous Generation of Life (Part 1)
Figure 3.28 Disproving the Spontaneous Generation of Life (Part 2)