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Chapter 3
The Molecules of Cells
Got Lactose?
• Lactose intolerance illustrates the importance
of biological molecules to the functioning of
living cells and to human health
• Molecular interactions, such as those between
the gene for lactase production, the enzyme
lactase, and the milk sugar lactose, drive all
biological processes
INTRODUCTION TO ORGANIC COMPOUNDS
3.1 Life's molecular diversity is based on the properties
of carbon
•
Organic compounds contain at least one carbon atom
•
Covalent bonding enables carbon to form complex
structures
– A carbon atom has four electrons in its outer shell
– To complete the shell, it can form four covalent
bonds
– The way bonding occurs among atoms determines
the overall shape of the molecule
LE 3-1a
Structural
formula
Ball-and-stick
model
Space-filling
model
Methane
The 4 single bonds of carbon point to the corners of a tetrahedron.
LE 3-1b
Ethane
Propane
Carbon skeletons vary in length.
LE 3-1c
Butane
Isobutane
Skeletons may be unbranched or branched.
LE 3-1d
1-Butene
2-Butene
Skeletons may have double bonds, which can vary in location.
LE 3-1e
Cyclohexane
Benzene
Skeletons may be arranged in rings.
Animation: Carbon Skeletons
Animation: Isomers
Animation: L-Dopa
• Hydrocarbons are composed of only hydrogen
and carbon
– A series of covalently bonded carbons
forms the carbon skeleton of the molecule
– Isomers are molecules with the same
molecular formula but different structures
and properties
3.2 Functional groups help determine the
properties of organic compounds
• Functional groups are groups of atoms
attached to the carbon skeleton of molecules
– Usually participate in chemical reactions
– Give organic molecules their particular
properties
LE 3-2
Estradiol
Female lion
Testosterone
Male lion
• Five main functional groups are important in the
chemistry of life:
– Hydroxyl group
– Carbonyl group
– Carboxyl group
– Amino group
– Phosphate group
• These groups are all polar and make
compounds containing them hydrophilic (waterloving)
3.3 Cells make a huge number of large
molecules from a small set of small molecules
• Four main classes of biological
macromolecules
– Carbohydrates
– Lipids
– Proteins
– Nucleic acids
• Cells make the most of their large molecules by
joining smaller organic monomers into chains
called polymers
– Monomers are usually linked by dehydration
reactions
• A water molecule is removed
Animation: Polymers
LE 3-3a
Short polymer
Unlinked monomer
Dehydration
reaction
Longer polymer
– Polymers are broken down to monomers by
the reverse process, hydrolysis
• A water molecule is added
LE 3-3b
Hydrolysis
CARBOHYDRATES
3.4 Monosaccharides are the simplest
carbohydrates
• Monosaccharides (single sugars) are
carbohydrate monomers
• A monosaccharide has a formula that is a
multiple of CH2O
– Contains hydroxyl groups and a carbonyl
group
– May be isomers, such as glucose and
fructose
– May take chain or ring forms
LE 3-4b
Glucose
Fructose
LE 3-4c
Structural
formula
Abbreviated
structure
Simplified
structure
3.5 Cells link two single sugars to form
disaccharides
• Two monosaccharides can join to form a
disaccharide
– Linked by a dehydration reaction
– Example: two glucose monomers form the
disaccharide maltose
Animation: Disaccharides
LE 3-5
Glucose
Glucose
Maltose
CONNECTION
3.6 How sweet is sweet?
• We perceive a sweet taste when a chemical
binds to the sweet receptor on the tongue
– The structure of a compound determines
how well it fits into a receptor
– The more strongly the chemical binds to the
receptor, the sweeter it is perceived to be
– The chemical can be sugar or another
compound, such as aspartame
3.7 Polysaccharides are long chains of sugar
units
•
Polysaccharides are polymers of
monosaccharides linked together by dehydration
reactions
•
Some polysaccharides are storage molecules
– Starch in plants
– Glycogen in animals
•
Some polysaccharides serve as structural
compounds
– Cellulose in plants
Animation: Polysaccharides
LE 3-7
Starch granules in
potato tuber cells
Glycogen
granules in
muscle
tissues
Cellulose fibrils in
a plant cell wall
Cellulose
molecules
STARCH
Glucose
monomer
GLYCOGEN
CELLULOSE
LIPIDS
3.8 Fats are lipids that are mostly energy-storage
molecules
• Lipids are diverse compounds consisting
mainly of carbon and hydrogen atoms
– Linked by nonpolar covalent bonds
– Hydrophobic (water-fearing)
Animation: Fats
• Fats, also called triglycerides, are lipids whose
main function is energy storage
– Polymers of fatty acids (usually three
molecules) and one glycerol molecule
– Formed by dehydration reactions
• Saturated fatty acids
– Contain the maximum number of
hydrogens
– Have no double bonds between carbons
• Unsaturated fatty acids
– Contain fewer than the maximum possible
hydrogens
– Have double bonds between carbons
• Oils are liquid fats
3.9 Phospholipids, waxes, and steroids are lipids
with a variety of functions
– Phospholipids
– Contain two fatty acid groups and the
element phosphorus
– Are a major component of cell membranes
• Waxes
– Consist of a single fatty acid linked to an
alcohol
– Form waterproof coatings
• Steroids
– Have backbones bent into rings, as in
cholesterol
– Are often hormones or the basis of
hormones
CONNECTION
3.10 Anabolic steroids pose health risks
• Anabolic steroids are natural and synthetic
variants of the male hormone testosterone
– Build up bone and muscle mass
– Can cause serious health problems
PROTEINS
3.11 Proteins are essential to the structures and
activities of life
• A protein is a polymer constructed from amino
acid monomers
• The structure of the protein determines its
function
• The seven major classes of protein are
– Structural: hair, cell cytoskeleton
– Contractile: producers of movement in
muscle and other cells
– Storage: sources of amino acids, such as
egg white
– Defense: antibodies, membrane proteins
– Transport: carriers of molecules such as
hemoglobin, membrane proteins
– Signaling: hormones, membrane proteins
– Enzymes: regulators of the speed
biochemical reactions
Animation: Structural Proteins
Animation: Storage Proteins
Animation: Transport Proteins
Animation: Receptor Proteins
Animation: Contractile Proteins
Animation: Defensive Proteins
Animation: Enzymes
Animation: Hormonal Proteins
Animation: Sensory Proteins
Animation: Gene Regulatory Proteins
3.12 Proteins are made from amino acids linked
by peptide bonds
•
Protein diversity is based on different
arrangements of a common set of 20 amino acid
monomers
•
Each amino acid contains
– An amino group
– A carboxyl group
– One of twenty functional ("R") groups
•
The three groups and a hydrogen atom are
bonded to a central "alpha" carbon
LE 3-12a
Amino
group
Carboxyl (acid)
group
• The structure of the R group determines the
specific properties of each amino acid
• An amino acid may be hydrophobic or
hydrophilic, depending on the characteristics of
the R group
LE 3-12b
Leucine (Leu)
Hydrophobic
Serine (Ser)
Aspartic acid (Asp)
Hydrophilic
• Cells link amino acids together by dehydration
synthesis
• The bonds between amino acid monomers are
called peptide bonds
• Dipeptides are two amino acids long;
polypeptides are from several to more than a
thousand amino acids long
LE 3-12c
Carboxyl
group
Peptide
bond
Amino
group
Dehydration
reaction
Amino acid
Amino acid
Dipeptide
3.13 A protein's specific shape determines its
function
• A protein consists of one or more polypeptide
chains spontaneously folded into a unique
shape
LE 3-13
Groove
Groove
• The folding of a polypeptide creates grooves
that enable other molecules to bind to it
• In denaturation, chemical or physical
changes can cause proteins to lose their
shape and thus their specific function
3.14 A protein's shape depends on four levels of
structure
• Primary structure: the unique sequence of
amino acids forming the polypeptide
• Secondary structure: the coiling or folding of
the chain, stabilized by hydrogen bonding
– May be alpha helix or pleated sheet (which
dominates the silk protein of a spider's web)
• Tertiary structure: the overall three-dimensional
shape of the polypeptide
• Quaternary structure: the association of two or
more polypeptide chains (subunits)
LE 3-14a
Levels of Protein Structure
Amino acids
LE 3-14b
Levels of Protein Structure
Amino acids
Hydrogen
bond
Alpha helix
Pleated sheet
LE 3-14c
Levels of Protein Structure
Amino acids
Hydrogen
bond
Alpha helix
Polypeptide
(single subunit
of transthyretin)
Pleated sheet
LE 3-14d
Levels of Protein Structure
Amino acids
Hydrogen
bond
Alpha helix
Polypeptide
(single subunit
of transthyretin)
Transthyretin, with
four identical
polypeptide subunits
Pleated sheet
Animation: Protein Structure Introduction
Animation: Primary Protein Structure
Animation: Secondary Protein Structure
Animation: Tertiary Protein Structure
Animation: Quarternary Protein Structure
• Collagen is an example of a protein with a
quaternary structure
– Three subunits wound into a helix
– Structure provides great strength to long
fibers
TALKING ABOUT SCIENCE
3.15 Linus Pauling contributed to our understanding of
the chemistry of life
•
Felt that the study of individual parts must come first,
then putting the parts together
•
Began his career by studying chemical bonding
•
First described the alpha helix and pleated sheet
protein structures
•
Discovered how abnormal hemoglobin causes sickle
cell disease
•
Won two Nobel prizes, for chemistry and for peace (for
helping produce a nuclear test ban treaty)
NUCLEIC ACIDS
3.16 Nucleic acids are information-rich polymers
of nucleotides
• There are two types of nucleic acid-DNA and
RNA
• Nucleic acids are polymers of nucleotide
monomers composed of
– A five-carbon sugar
– A phosphate group
– A nitrogenous base-adenine (A), thymine
(T), cytosine ( C), and guanine (G) in DNA;
A, G, C, and uracil (U) in RNA
LE 3-16a
Nitrogenous
base (A)
Phosphate
group
Sugar
• Nucleotide monomers are formed into a
polynucleotide with a sugar-phosphate
backbone and attached nitrogenous bases
LE 3-16b
Nucleotide
Sugar-phosphate
backbone
• Hydrogen bonding between nitrogenous
bases creates the final structure of the
nucleic acid
– RNA usually consists of a single
polynucleotide strand
– DNA is a double helix
• Two polynucleotides are twisted around
each other
• Nitrogenous bases protruding from the
backbone pair with each other, A with T
and G with C
LE 3-16c
Base
pair
– Specific sequences of DNA make up genes
that program the amino acid sequences of
proteins