In many animals, specialized sensory organs for smell (olfaction) and taste (gustation) have evolved that detect a variety of chemical compounds. These organs contain distinct types of protein membrane receptor, and each type binds to a specific class of molecule. A large number of genes code for different chemical odor receptors, and the presence of so many receptors enables the detection of broad categories of smell and taste.

In contrast to the senses of hearing and sight, the sensory receptors of smell and taste do the “work” of detecting an odor or taste by themselves, without other structures being necessary. All that is needed is exposure to the environment and protection from damage. Both are achieved by the placement of chemoreceptors in the nasal and mouth cavities.