It is likely that all living organisms respond to some of the chemicals in their environment. Animals need to detect food sources at a distance, and to test the nature of those sources when they have reached them. Often they use chemical methods for communication between different individuals, and especially for sexual interactions.

The chemical senses are conventionally divided into smell, or olfaction, and taste, or gustation. For humans and other mammals the distinction seems fairly clear: we use our sense of smell to detect air-borne chemicals arising from a distant source, and we use our sense of taste to sample solid or liquid material in our mouths. For fish and other aquatic animals, where odorant substances are necessarily carried in water, the logic of distinguishing between olfaction and taste is less clear. Even in ourselves, the olfactory receptors are essential components of the sensations we receive from food or drink in the mouth, as anyone whose nasal passages are blocked during a cold will know.

In this chapter we concentrate on the transduction mechanisms in the receptors mediating taste and olfaction in mammals, and by way of contrast we also have a quick look at how insects do it.

Taste mechanisms in mammals

Soluble tastant molecules are detected by taste receptor cells on the tongue (see Roper, 1992; McLaughlin & Margolskee, 1994; Lindemann, 1996). The sensitive cells are grouped in taste buds (fig. 16.1).