Five-carbon molecules can contain only single carbon-carbon bonds, or they can have one or more double bonds, or one or more triple bonds. Isoprene, with two double bonds, is a noteworthy member of the 5-carbon group because it is released into the atmosphere by living and decaying vegetation. Photo-oxidation of isoprene released by plants is a major source of the carbon monoxide in the atmosphere. The molecular structure of each of the 5-carbon compounds is so distinctive that each infrared signature is easily recognized. See SPECTRA, chapter B.