key concept3.1Macromolecules Characterize Living Things

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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:

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.