Life on Earth is carbon-based, as all macromolecules that make up living organisms contain carbon atoms. All organic compounds have a carbon backbone. Each carbon atom is tetravalent and can bond with four other atoms, making it an extraordinarily flexible component of biological molecules. Because carbon’s valence electrons are stable, it rarely becomes an ion. As the carbon chain increases in length, structural modifications such as ring structures, double bonds, and branching side chains become possible.

Carbon-carbon bonds form the basis of the carbon skeleton. Hydrogen atoms readily bond to the carbon atom. Molecules that contain only hydrogen and carbon are called hydrocarbons. Hydrocarbons usually form long chains or have branches protruding at various points. Changing the number of bonds changes the properties of the molecule. For example, a fatty acid with a long hydrocarbon tail with one or more double bonds will behave differently than a fatty acid with no double bonds.

Isomers

Molecules having the same chemical formula but different structural arrangements are called isomers. One example of isomers can be seen in two different molecules that share the chemical formula C₆H₁₄. Hexane has a single linear chain of carbon atoms, while isohexane has a branch point on the second carbon atom. Some isomers are mirror images of each other, which are called enantiomers.

Functional Groups Build on Carbon Skeletons

The unique properties of biological molecules are conferred by functional groups bonded to the carbon skeleton, such as amino (–NH₂) or methyl groups (–CH₃). Functional groups can alter the structural and chemical properties of the molecule. The properties of functional groups influence many fields of study, such as synthetic drug design.