Animals receive information about chemicals through chemoreceptors - they are responsible for smell and taste. The sense of smell is called olfaction . In the nose of vertebrates there is mucus that covers the epithelium. Odors must diffuse through this mucus to reach receptors on the olfactory hairs. If an odorant molecule binds to the receptor, a G protein is activated.
G protein activates the enzyme adenylate cyclase.
Adenylate cyclase causes an increase of a second messenger. The second messenger binds to a sodium channel protein in the plasma membrane. The sense of taste is called gustation . The tongue of vertebrates and the skin of fishes have taste buds - a cluster of many taste sensors. Taste buds are embedded in the epithelium.