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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 3
Molecules of Life
(Sections 3.4 - 3.6)
Albia Dugger • Miami Dade College
3.4 Lipids
• Cells use lipids as major sources of energy and as structural
materials
• lipid
• Fatty, oily, or waxy organic compound
• All are hydrophobic (nonpolar), or have regions that are
hydrophobic
Types of Lipids
• Phospholipids are the main structural component of cell
membranes
• Waxes are lipids that are part of water-repellent and
lubricating secretions
Key Terms
• fat
• Lipid that consists of a glycerol molecule with one, two, or
three fatty acid tails
Saturated and Unsaturated Fats
• Animal fats are saturated
• Tend to remain solid at room temperature because
their saturated tails pack tightly together
• Most vegetable oils are unsaturated
• Kinked tails do not pack tightly, so unsaturated fats are
typically liquid at room temperature
• Partially hydrogenated vegetable oils have a trans double
bond that allows them to pack tightly, like saturated fats
• Solid at room temperature
Fatty Acids
Trans and Cis
Phospholipids
• phospholipid
• Lipid with a highly polar phosphate group in its hydrophilic
head, and two nonpolar, hydrophobic fatty-acid tails
• Main part of cell membranes
• Opposing properties of a phospholipid molecule give rise to
cell membrane structure
• Two layers of lipids (lipid bilayer)
• Hydrophobic tails sandwiched between hydrophilic
heads
Phospholipids and Cell Membranes
• Head is hydrophilic –
tails are hydrophobic
• Lipid bilayer— the
structural foundation of
all cell membranes
hydrophilic
head
one layer
of lipids
one layer
of lipids
two hydrophobic
tails
Fig. 3.10, p. 42
Waxes
• wax
• Water-repellent mixture
with long fatty-acid tails
bonded to long-chain
alcohols or carbon rings
• Functions:
• Covers exposed surfaces
of plants
• Protects and lubricates
skin and hair
• Honeycomb
Key Concepts
• Lipids
• Lipids function as energy reservoirs and as waterproofing
or lubricating substances
• Lipids are the main structural component of all cell
membranes
3.5 Proteins—
Diversity in Structure and Function
• Structurally and functionally, proteins are the most diverse
molecules of life
• The shape of a protein is the source of its function
• protein
• Organic compound that consists of one or more chains of
amino acids (polypeptides)
Amino Acids
• Cells make thousands of different kinds of proteins from only
twenty kinds of monomers (amino acids)
• amino acid
• Small organic compound that is a subunit of proteins
Amino Acids
• Generalized structure of amino acids: Twenty amino acids
are used in eukaryotic proteins
Building Proteins
• Protein synthesis (making proteins) involves covalently
bonding amino acids into a chain polypeptide linked by
peptide bonds
• peptide bond
• Bond that joins the amine group of one amino acid and the
carboxyl group of another amino acid in a protein
• peptide
• Short chain of amino acids linked by peptide bonds
• Polypeptide
• Longer chain of amino acids linked by peptide bonds
• Primary structure of a protein
Polypeptide Formation
• Condensation: A peptide bond forms between the carboxyl
group of the methionine and the amine group of the serine
(condensation and functional groups again!)
• Additional amino acids are added to the carboxyl end to make
the polypeptide longer
Protein Structure
• Polypeptides (primary structure) twist into loops, sheets, and
coils (secondary structure) that can pack further into
functional domains (tertiary structure)
• Many proteins, including most enzymes, consist of two or
more polypeptides (quaternary structure)
• Fibrous proteins aggregate into much larger structures
Primary and Secondary Structure
• Primary structure (polypeptide) twists into secondary structure
Tertiary and Quaternary Structure
• Tertiary structure forms functional domains
• Hemoglobin has quaternary structure (4 globin chains)
Aggregate Proteins
• Many proteins aggregate by thousands into much larger
structures, such as keratin filaments that make up hair
Polypeptide Formation
lysine
glycine
A protein’s primary
structure consists of a
linear sequence of
amino acids (a
polypeptide chain).
Each type of protein
has a unique primary
structure.
1
arginine
glycine
2 Secondary structure
arises as a polypeptide
chain twists into a coil
(helix) or sheet held in
place by hydrogen
bonds between different
parts of the molecule.
The same patterns of
secondary structure
occur in many different
proteins.
3 Tertiary structure
occurs when a chain’s
coils and sheets fold
up into a functional
domain such as a
barrel or pocket. In
this example, the coils
of a globin chain form
a pocket.
4 Some proteins have
quaternary structure, in
which two or more
polypeptide chains
associate as one
molecule. Hemoglobin,
shown here, consists of
four globin chains (green
and blue). Each globin
pocket now holds a heme
group (red).
5
Many proteins aggregate
by the thousands into much
larger structures, such as the
keratin filaments that make
up hair.
Stepped Art
Fig. 3.14.1-4, p. 45
Combined Proteins
• Enzymes often attach sugars or lipids to proteins
• Glycoproteins allow a tissue or body to recognize its own cells
• Lipoproteins carry fats and cholesterol through the
bloodstream
• Low-density lipoprotein (LDL)
• High-density lipoprotein (HDL)
3.6 Importance of Protein Structure
• A protein’s structure dictates its function so, if a protein
unravels (denatures), it loses its function
• denature
• To unravel the shape of a protein or other large biological
molecule
• Caused by shifts in pH or temperature, exposure to
detergent or some salts that disrupt hydrogen bonds, and
other molecular interactions responsible for protein shape
Key Concepts
• Proteins
• Structurally and functionally, proteins are the most diverse
molecules of life
• They include enzymes and structural materials
• A protein’s function arises from and depends on its
structure
3.7 Nucleic Acids
• Nucleotides are small organic molecules consisting of a
sugar, a phosphate group, and a nitrogen-containing base
• nucleotide
• Monomer of nucleic acids; has five-carbon sugar,
nitrogen-containing base, and phosphate groups
A Nucleotide Monomer
• ATP, a nucleotide monomer of RNA, and also an essential
participant in many metabolic processes
Nucleic Acids
• Nucleotides are monomers of DNA and RNA, which are
nucleic acids
• nucleic acid
• Single- or double-stranded chain of nucleotides joined by
sugar–phosphate bonds; for example, DNA, RNA
A Nucleic Acid
• A chain of nucleotides is
a nucleic acid
• The sugar of one
nucleotide is covalently
bonded to the
phosphate group of the
next, forming a sugar–
phosphate backbone
DNA and RNA
• DNA (Deoxyribonucleic acid)
• DNA encodes heritable information that guides the
synthesis of RNA and proteins
• Consists of two nucleotide chains twisted in a double helix
• RNA (Ribonucleic acid)
• RNAs interact with DNA and with one another to carry out
protein synthesis
DNA
• DNA consists of two
chains of nucleotides,
twisted into a double
helix
• Hydrogen bonding
maintains the threedimensional structure
Other Nucleotides
• Some nucleotides have additional functions
• Example: ATP energizes many kinds of molecules by
phosphate-group transfers
• ATP Adenosine triphosphate
• Nucleotide that consists of an adenine base, a five-carbon
ribose sugar, and three phosphate groups
Key Concepts
• Nucleic Acids
• Nucleotides are the building blocks of nucleic acids
• Some have additional roles in metabolism
• DNA and RNA are part of a cell’s system of storing and
retrieving heritable information