The primary way that plants obtain nitrogen compounds is through the nitrogen cycle— a series of reactions involving several different types of bacteria, including nitrogen-fixing bacteria and denitrifying bacteria. During nitrogen fixation, symbiotic bacteria that live in association with the roots of certain plants (such as legumes like peas) are able, through a series of enzyme reactions, to make nitrogen available for plants. Nitrogen is crucial to all organisms because it is an important element of proteins and nucleic acids. (For more about plants, see the chapter “Plant Structure, Function, and Use.”)