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: