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Macromolecules include carbohydrates, proteins, lipids, and nucleic acids. They
are larger, with molecular weights ranging from hundreds of Daltons (sucrose) to billions
(some nucleic acids). These molecules all contain carbon atoms, and so belong to a group of
what are known as organic chemicals. Third, they are held together largely by covalent
bonds, which gives them important structural stability and forms the basis of some of their
functions. And finally, carbohydrates, proteins, lipids, and nucleic acids are all unique to
the living world. These molecular classes do not occur in inanimate nature. You won’t find
proteins in rocks—and if you do, you can be sure they came from some living organism.
Most of these biological molecules are large polymers (poly, “many”; mer, “unit”) constructed
by the covalent bonding of smaller molecules called monomers (Table 3.1). The monomers
that make up each kind of biological molecule have similar chemical structures. Thus
chains of chemically similar sugar monomers (saccharides) form the different
carbohydrates; the thousands of different proteins are formed from combinations of a mere
20 amino acids, all of which share chemical similarities.
The Building Blocks of Organisms
MONOMER COMPLEX POLYMER (MACROMOLECULE)
Amino acid Polypeptide (protein)
Monosaccharide (sugar) Polysaccharide (carbohydrate)
Nucleotide Nucleic acid
Each functional group has specific chemical properties, and when it is attached to a larger
molecule, it confers those properties on the larger molecule.
Isomers are molecules that have the same chemical formula—the same kinds and numbers
of atoms—but the atoms are arranged differently. (The prefix iso-, meaning “same,” is
encountered in many biological terms.) Of the different kinds of isomers, we will consider
two: structural isomers and optical isomers.
Structural isomers differ in how their atoms are joined together. Consider two simple
molecules, each composed of four carbon and ten hydrogen atoms bonded covalently, both
with the formula C4H10. These atoms can be linked in two different ways, resulting in two
different forms of the molecule:
The different bonding relationships in butane and isobutane are distinguished by their
structural formulas, and the two molecules have different chemical properties.
Optical isomers occur when a carbon atom has four different atoms or groups of atoms
attached to it. This pattern allows two different ways of making the attachments, each the
mirror image of the other (Figure 3.2). Such a carbon atom is called an asymmetrical
carbon, and the two resulting molecules are optical isomers of each other. You can envision
your right and left hands as optical isomers. Just as a glove is specific for a particular hand,
some biochemical molecules that can interact with one optical isomer of a carbon compound
are unable to “fit” the other.
Both carbohydrates and proteins can play structural roles, supporting and protecting
tissues and organs. However, only the nucleic acids specialize in information storage. These
macromolecules function as hereditary material, carrying the traits of both species and
individuals from generation to generation.
The functions of macromolecules are directly related to their three-dimensional shapes and
to the sequences and chemical properties of their monomers.
Polymers are constructed from monomers by a series of reactions called condensation
reactions (sometimes called dehydration reactions; both terms refer to the loss of water).
Condensation reactions result in covalently bonded monomers. These reactions release a
molecule of water for each covalent bond formed (Figure 3.4A). The condensation reactions
that produce the different kinds of polymers differ in detail, but in all cases, polymers form
only if energy is added to the system. In living systems, specific energy-rich molecules
supply this energy.
The reverse of a condensation reaction is a hydrolysis reaction (hydro, “water”; lysis,
“break”). Hydrolysis reactions digest polymers and produce monomers. Water reacts with
the covalent bonds that link the polymer together, and the products are free monomers. The
elements (H and O) of H2O become part of the products (Figure 3.4B). The linkages
between monomers thus can be formed and broken inside living tissues.
The functions of proteins include structural support, protection, transport, catalysis,
defence, regulation, and movement. The monomeric subunits of proteins are the 20 amino
acids. Among the functions of macromolecules listed earlier, only two—energy storage and
information storage—are not usually performed by proteins