42
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. Each kind of biological molecule is made up of monomers with 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 that are held together by covalent bonds.
Polymers containing thousands or more atoms are called macromolecules. The proteins, carbohydrates, and nucleic acids of living systems certainly fall into this category. Although large lipid structures are not polymers in the strictest sense, it is convenient to treat them as macromolecules (see Key Concept 3.4). Green plants have the living world’s most abundant protein (rubisco; see Chapter 10), most abundant carbohydrate (cellulose in plant cell walls), and most abundant lipid (monogalactosyl diglyceride in leaves).
focus your learning
Isomers are molecules having the same composition but different structures.
Monomers are chemically linked via condensation reactions to form polymers.