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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
Skeletons may be arranged in rings.
Cyclohexane
Benzene
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
Male lion
Testosterone
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 (water-loving)
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
GLYCOGEN
CELLULOSE
STARCH
Glucose
monomer
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
Carboxyl (acid)
group
Amino
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)
Serine (Ser)
Hydrophobic
Hydrophilic
Aspartic acid (Asp)
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
Amino acid
Dipeptide
Amino acid
Peptide
bond
Dehydration
reaction
Amino
group
Carboxyl
group
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
Pleated sheet
Polypeptide
(single subunit
of transthyretin)
LE 3-14d
Levels of Protein Structure
Amino acids
Hydrogen
bond
Alpha helix
Pleated sheet
Polypeptide
(single subunit
of transthyretin)
Transthyretin, with
four identical
polypeptide subunits
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)
Sugar
Phosphate
group
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