Functional groups

Hydrocarbons, made up entirely of carbon and hydrogen atoms, make wonderful combustion fuels (such fuels include propane, butane, and the bulk of commercial gasoline). But can you build a living organism out of only combustion fuels? Probably not. Most large biological molecules in fact contain many types of atoms beyond just carbon and hydrogen. These additional atoms allow for functionality not typically seen in hydrocarbons.

Large biological molecules are generally composed of a carbon skeleton (made up of carbon and hydrogen atoms) and some other atoms, including oxygen, nitrogen, or sulfur. Often, these additional atoms appear in the context of functional groups. Functional groups are chemical motifs, or patterns of atoms, that display consistent “function” (properties and reactivity) regardless of the exact molecule they are found in. Biological molecules can contain many different types and combinations of functional groups, and a biomolecule’s particular set of groups will affect many of its properties, including its structure, solubility, and reactivity.

A few of the most important functional groups in biological molecules are shown in the table below. Functional groups can be classified as hydrophobic or hydrophilic based on their charge and polarity characteristics. The only hydrophobic group below is the methyl (CH$_3$start subscript, 3, end subscript) group, which is nonpolar. The remaining six functional groups in the table all have varying degrees of hydrophilic character.

One example of a strongly hydrophilic group is the carboxyl group (COOH), which can act as an acid and lose a proton to form a negatively-charged carboxylate ion (COO$^-$start superscript, minus, end superscript). Carboxyl groups are commonly found in amino acids, fatty acids, and other biomolecules. An example of a less hydrophilic group is the carbonyl group (C=O), an uncharged but polar (contains partial positive and partial negative charges) functional group. Carbonyls are found in many different biological molecules, including proteins, peptides, and carbohydrates.

In the table above, the letter R is used to represent the rest of the molecule that a functional group is attached to. For instance, R might be an ethyl (CH$_2$start subscript, 2, end subscriptCH$_3$start subscript, 3, end subscript) group, in which case the first entry in the table would correspond to ethanol. But R could also represent the bulk of a much larger molecule, such as a protein. The letter R is used throughout biology and chemistry to simplify chemical structures and highlight the most important parts (often the functional groups!) of a molecule.