Note: There are many aliphatic (hydrocarbon chains) and aromatic groups. The methyl group and benzyl groups are shown as examples. Notice also that many of the examples have more than one functional group. The letter R stands for the remainder of the molecule. Finally, note that a carbon atom double-bonded to an oxygen atom, C=O, called a carbonyl group, is present in aldehydes, ketones, and carboxylic acids, including amino acids. Carbonyl groups are common in biochemicals.