The simple repeating units of polymers are often based on a nonpolar core of carbon atoms. But attached to these carbon atoms are functional groups. Functional groups are groups of one or more atoms that have particular chemical properties on their own, regardless of what they are attached to. Among the functional groups frequently encountered in biological molecules are amine (=NH), amino (–NH₂), carboxyl (–COOH), hydroxyl (–OH), ketone (=O), phosphate (–O–PO₃H₂), sulfhydryl (–SH), and methyl (–CH₃). The nitrogen, oxygen, phosphorus, and sulfur atoms in these functional groups are more electronegative than the carbon atoms, and functional groups containing these atoms are polar. The methyl group (–CH₃), on the other hand, is nonpolar.

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Because many functional groups are polar, otherwise nonpolar molecules containing these groups become polar and so become soluble in the cell’s aqueous environment. In other words, they disperse in solution throughout the cell. Moreover, because many functional groups are polar, they are also reactive. Notice in the following sections that the reactions joining simpler molecules into polymers usually take place between functional groups.