Four kinds of molecules are characteristic of living things: proteins, carbohydrates, lipids, and nucleic acids. With the exception of the lipids, these biological molecules are polymers (poly, “many”; mer, “unit”) constructed by the covalent bonding of smaller molecules called monomers. The monomers that make up each kind of biological molecule have similar chemical
structures:
• Proteins are formed from different combinations of 20 amino acids, all of which share chemical similarities.
• Carbohydrates can form giant molecules by linking together chemically similar sugar monomers (monosaccharides) to form polysaccharides.
• Nucleic acids are formed from four kinds of nucleotide monomers linked together in long chains.
• Lipids also form large structures from a limited set of smaller molecules, but in this case noncovalent forces maintain the interactions between the lipid monomers.
Polymers with molecular weights exceeding 1,000 grams per mole are considered to be macromolecules. The proteins, carbohydrates, and nucleic acids of living systems certainly fall into this category.
