Download The Structure and Function of Macromolecules

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UNIT ONE: THE CHEMISTRY OF LIFE
Chapter Five: The Structure and Function of Macromolecules
(Text from Biology, 6th Edition, by Campbell and Reece)
The Structure and Function of Macromolecules (Chapter Five)
POLYMER PRINCIPLES
Most Macromolecules are Polymers
The four main classes of macromolecules are carbohydrates, lipids, proteins
and nucleic acids. The large molecules in carbohydrates, proteins, and
nucleic acids are polymers,
polymers long molecules consisting of similar or identical
building blocks linked by covalent bonds. These blocks are small molecules
called monomers.
monomers
Monomers are connected by a condensation reaction,
reaction also known as a dehydration reaction,
reaction where a
water molecule is lost to allow the two monomers to bond together. One monomer loses a hydroxyl
group (-OH), while the other loses a hydrogen (-H). Enzymes help to speed up these dehydration
reactions.
Hydrolysis is the process that reverses the dehydration reaction and breaks polymers back into
monomers. By adding a water molecule to the bond, a hydrogen atom will attach to one monomer and
the hydroxyl will attach to the other monomer. Digestion works through hydrolysis: enzymes work to
speed up hydrolysis and break apart large polymers into monomers that can be absorbed into the
bloodstream.
An Immense Variety of Polymers Can Be Built From a Small Set of Monomers
There is an amazing number of different combinations of polymers that result from the approximately
40 to 50 common monomers. The variation in the linear sequence the units follow result in unique
macromolecules from small molecules common to all life.
CARBOHYDRATES – FUEL AND BUILDING MATERIAL
Carbohydrates include both sugars and their polymers. Monosaccharides are single sugars (also
known as simple sugars) and disaccharides are double sugars. Polysaccharides are carbohydrates.
Sugars, the Smallest Carbohydrates, Serve as Fuel and Carbon Sources
Monosaccharides generally have molecular formulas that are a multiple of CH2O. Glucose is the most
common monosaccharide. A sugar has a carbonyl group and multiple hydroxyl groups. Depending on
the location of the carbonyl group, the sugar is either an aldose or a ketose. Another criterion for
grouping sugars is the size of the carbon skeleton, which can be from three to seven carbons long.
Those with three carbons are trioses, five carbons are pentoses, and six carbons are hexoses.
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UNIT ONE: THE CHEMISTRY OF LIFE
Chapter Five: The Structure and Function of Macromolecules
(Text from Biology, 6th Edition, by Campbell and Reece)
Simple sugars can also differ in the spatial
arrangement of their parts around asymmetric
carbons. Glucose can be drawn in a linear carbon
skeleton, but this is not really an accurate
representation. In aqueous solutions, most sugars
form rings.
Monosaccharides are major nutrients in the cell. They fuel cellular work and the carbon skeletons work
as raw material for other types of small organic molecules. A disaccharide consists of two
monosaccharides joined by a glcyosidic linkage,
linkage a covalent bond formed between two
monosaccharides by a dehydration reaction. Two glucoses bonded together result in maltose, while
glucose and fructose bond together to create a sucrose. Lactose is created from galactose and
glucose.
Polysaccharides, the Polymers of Sugars, Have Storage and Structural Roles
Polysaccharides
Polysac charides are macromolecules, composed of a few hundred to a few thousand monosaccharides
joined by glycosidic linkages. Some polysaccharides store energy for the cells, while others are building
materials for structures.
Storage Polysaccharides
Starch is a storage polysaccharide in plants and consists entirely of alpha glucose molecules. Amylose,
the simplest form of starch consists of 1-4 glycosidic linkages. Amylopectin, a more complex type of
starch, is branched and has 1-6 linkages at the branch points. Starch represents stored energy which
can be released by breaking the bonds between the glucose monomers. Most animals can hydrolyze
plant starch. Starch is usually helical.
Animals store energy in glycogen,
glycogen a polymer even more branched than amylopectin. Humans and
other vertebrates store glycogen mainly in liver and muscle cells.
Structural Polysaccharides
Cellulose is created from beta glucose molecules. Because of the slightly different ring structure in
beta glucose, when they bond, every other glucose monomer is upside down in respect to the other.
Cellulose are grouped into microfibrils in plant cells, which are a strong building material for plants. It
is the most abundant organic compound on Earth. Most animals do not have the beta enzyme
necessary to break down cellulose. Some microbes are able to break down cellulose – cows have
cellulose-digesting bacteria in the first compartment in its stomach.
Chitin is the carbohydrate used by arthropods to build exoskeletons. While pure chitin is leathery, it
becomes hardened when encrusted with calcium carbonate.
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UNIT ONE: THE CHEMISTRY OF LIFE
Chapter Five: The Structure and Function of Macromolecules
(Text from Biology, 6th Edition, by Campbell and Reece)
LIPIDS – DIVERSE HYDROPHOBIC MOLECULES
Lipids have little or no affinity for water and have no monomers. They consist mostly of hydrocarbons.
Fats Store Large Amounts of Energy
A fat consists of glycerol and fatty acids. Glycerol is an alcohol with three carbons, each with its own
hydroxyl group. A fatty acid has a long carbon skeleton, with a carboxyl group at the end of a long
hydrocarbon chain. The nonpolar carbon-hydrogen bonds in the hydrocarbon chains are the reason why
fats are hydrophobic.
To make a fat, three fatty acids join together and bond to the glycerol through ester linkage (bond
between hydroxyl and carboxyl group). The resulting Triacylglycerol has three fatty acid tails linked to
one glycerol head. The terms saturated fats and unsaturated fats refer to the nature of the bonds
between the carbon and hydrogen in the fatty acid tails. If there are no double bonds, then there are
as many hydrogens as possible bonded to the carbon skeleton – this structure is a saturated fatty
acid.
acid An unsaturated fatty acid has one or more double bonds and will have a kink in its tail wherever
there is a double bond.
Most saturated fats are animal fats – these are solid at room temperature. Fat from plants and fishes
are generally unsaturated and are liquid at room temperature – they are oils.
Fats are used for energy storage. A gram of fat stores more than twice as much energy as a gram of a
polysaccharide. Since animals must carry energy stores with them, it is more economic to have fat for
energy storage, since it takes up less space.
Phospholipids are Major Components of Cell Membranes
Phospholipids are similar to fats, but they only have two fatty acids
tails. The third hydroxyl group of glycerol is instead joined to a
phosphate group. When phospholipids are added to water, they selfassemble into micelles: a phospholipid droplet with the hydrophobic
tails inside and the hydrophilic heads facing the water.
Steroids Include Cholesterol and Certain Hormones
Steroids are characterized by a carbon skeleton of four fused rings. One steroid, cholesterol,
cholesterol is a
common component of animal cell membranes.
PROTEINS – MANY STRUCTURES, MANY FUNCTIONS
Proteins are the workhorses of the cell and are used for structural support, storage, transport of
substances, signaling, movement, and defense. They are also used as enzymes. All proteins are
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UNIT ONE: THE CHEMISTRY OF LIFE
Chapter Five: The Structure and Function of Macromolecules
(Text from Biology, 6th Edition, by Campbell and Reece)
constructed out of the same set of 20 amino acids. Polymers of amino acids are called polypeptides
polypeptides.
eptides
Proteins consist of one or more polypeptides folded and coiled into specific shapes.
A Polypeptide is a Polymer of Amino Acids Connected in a Specific Sequence
Amino acids are organic molecules possessing both carboxyl and amino groups.
The general formula for an amino acid: The alpha carbon is in the center, bonded
to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.
The R group is also known as the side chain.
The physical and chemical properties of the side chain
determine how a particular amino acid will behave.
When two amino acids are positioned so that the
carboxyl group of one is adjacent to the amino group
of the other, an enzyme can begin the dehydration
reaction that will form the peptide bond (type of
covalent bond).
When this process is repeated over and over, a
polypeptide will result. At one end of the chain is a
free amino group, and the other end has a free
carboxyl group.
A Protein’s Function Depends on Its Specific Conformation
A polypeptide is not quite the same as a protein. A functional protein is one or more polypeptides
twisted, folded, and coiled into a uniquely shaped molecule. The amino acid sequence determines
what three-dimensional shape the protein will take. Some proteins are globular while others are
fibrous.
Four Levels of Protein Structure
When a cell creates a polypeptide, the chain automatically folds to achieve the shape it needs to carry
out its function. This shape is held together by a variety of different bonds between parts of the chain.
Primary Structure.
Structure The primary structure of a protein is the sequence of amino acids. Even a slight
change in the order of amino acids can affect the protein’s ability to function. Frederick Sanger was
the pioneer in determining the primary structure of proteins.
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UNIT ONE: THE CHEMISTRY OF LIFE
Chapter Five: The Structure and Function of Macromolecules
(Text from Biology, 6th Edition, by Campbell and Reece)
Secondary Structure. Most proteins have segments of
their chain repeatedly coiled or folded – these coils
and folds are referred to as the secondary structure.
structure
They are the result of hydrogen bonds at regular
intervals along the polypeptide backbone. This is
limited to the atoms of the backbone, not the side
chains. One main type of secondary structure is the α
helix,
helix a coil held together by hydrogen bonding
between every fourth amino acid. The other main type
of secondary structure is the β pleated sheet,
sheet where
two or more regions of polypeptide chain lie parallel to
each other. Hydrogen bonds between parts of the
backbone in the parallel regions hold the structure
together. Pleated sheets make up the inner part of
many globular proteins and are seen in some fibrous
proteins.
Tertiary Structure. A protein’s tertiary structure
consists of irregular contortions because of
interactions between side chains. A hydrophobic
interaction occurs when amino acid with hydrophobic
side chains become grouped into the core – the water
molecules bond to each other and to hydrophilic parts
of the protein. When the nonpolar amino acid side
chains are brought together, van der Waals
interactions help hold them together.
Strong covalent bonds called disulfide bridges form where amino acids with sulfhydryl groups are
brought close together. Ionic bonds can also occur between side chains.
Quaternary Structure. Proteins that consist of two or more polypeptide chains also have quaternary
structure
structure,
ure where polypeptides can be coiled into a triple helix or bunched into a roughly spherical
shape. Collagen is a fibrous protein while hemoglobin is a globular protein.
What Determines Protein Conformation?
pH, salt concentration, temperature, and other aspects of a protein’s environment can affect what
happens to a protein. Changes in its environment can cause a protein to become denatured and
biologically inactive. Denaturation agents can disrupt the bonds that hold the protein together.
Excessive heat can also overpower the weak interactions that stabilize the shape of the protein. Some
proteins can return to normal after conditions are fixed.
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UNIT ONE: THE CHEMISTRY OF LIFE
Chapter Five: The Structure and Function of Macromolecules
(Text from Biology, 6th Edition, by Campbell and Reece)
The Protein-Folding Problem
Biologists have discovered that chaperonins are protein molecules that help other proteins fold
correctly. They work to keep the new polypeptide away from other influences that could affect the
polypeptide’s development.
Determining the Structure of a Protein
X-ray crystallography depends on the diffraction of an X-ray beam by the individual atoms in the
crystal of the protein. A model can then be built of the protein.
NUCLEIC ACIDS – INFORMATIONAL POLYMERS
The amino acid sequence of a polypeptide is programmed by genes.
genes Genes consist of DNA, which is a
polymer belonging to the class of compounds known as nucleic acids.
acids
Nucleic Acids Store and Transmit Hereditary Information
The two types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
(RNA) They
enable living organisms to reproduce their complex components from one generation to the next. DNA
provides directions for its own replication.
A DNA molecule consist of hundreds or thousands of genes. DNA molecules are copied and passed
when cells reproduce by dividing. Genes along the length of DNA direct mRNA (messenger RNA) to
produce a polypeptide.
A Nucleic Acid Strand is a Polymer of Nucleotides
Monomers of nucleic acids are nucleotides.
nucleotides They are each composed of a nitrogenous base, a pentose,
and phosphate group.
A pyrimidine has a six-membered ring of carbon and nitrogenous atoms – they are cytosine, thymine,
and uracil. Purines are larger and have a six-membered ring fused to a five-membered ring – adenine
and guanine.
The pentose connected to the nitrogenous base is ribose in the nucleotides of RNA and deoxyribose in
DNA. Because deoxyribose does not have an oxygen atom on one of its carbons, it receives the name
deoxyribose.
A phosphate group is attached to the number five carbon of the sugar.
In a nucleic acid polymer, or polynucleotide,
polynucleotide nucleotides are joined by covalent bonds called
phosphodiester linkages between the phosphate of one nucleotide and the sugar of the other.
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UNIT ONE: THE CHEMISTRY OF LIFE
Chapter Five: The Structure and Function of Macromolecules
(Text from Biology, 6th Edition, by Campbell and Reece)
Inheritance is Based on Replication of the DNA Double Helix
The RNA molecules of cells consist of a single polynucleotide chain. However, DNA molecules have
two polynucleotides that spiral around an imaginary axis to form a do ubl e hel ix. According to the
base-pairing rules, Adenine always pairs with Thymine, and Guanine always pairs with Cytosine
(ATGC). The two strands are complementary.
We Can Use DNA and Proteins As Tape Measures of Evolution
Genes and their products document the hereditary background of an organism. Since DNA molecules
are passed through generations, related individuals have greater similarities in their DNA than
unrelated individuals do. Thus, two species that appear closely related based on fossil and anatomical
evidence also share a greater proportion of their DNA and protein sequences than do more distantly
related species.