The senses of taste and smell are related because they use the same types of receptors and are stimulated by molecules in solutions or air.

Taste stimuli are molecules taken in from the environment. The primary tastes detected by humans are sweet, sour, bitter, salty, and umami.

The senses of smell and taste combine at the back of the throat. When you taste something before you smell it, the smell lingers internally up to the nose causing you to smell it. Both smell and taste use chemoreceptors, which essentially means they are both sensing the chemical environment. This chemoreception in regards to taste, occurs via the presence of specialized taste receptors within the mouth that are referred to as taste cells and are bundled together to form taste buds. These taste buds, located in papillae which are found across the tongue, are specific for the five modalities: salt, sweet, sour, bitter and umami. These receptors are activated when their specific stimulus (i.e. sweet or salt molecules) is present and signals to the brain.

Tastants produce signal molecules received by receptors, which are then processed by the brain to identify smells and tastes. In humans, there are five primary tastes; each taste has only one corresponding type of receptor. Thus, like olfaction, each receptor is specific to its stimulus (tastant). Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant. A salty tastant (containing NaCl) provides the sodium ions (Na⁺) that enter the taste neurons, exciting them directly. Sour tastants are acids which belong to the thermoreceptor protein family. Binding of an acid or other sour-tasting molecule triggers a change in the ion channel which increases hydrogen ion (H⁺) concentrations in the taste neurons; thus, depolarizing them. Sweet, bitter, and umami tastants require a G-protein-coupled receptor. These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them.

Key Points

• Humans can taste sweet, sour, bitter, salty, and umami; umami is the savoriness of certain foods that are commonly high in protein.

• The senses of smell and taste are directly related because they both use the same types of receptors.

• If one’s sense of smell is not functional, then the sense of taste will also not function because of the relationship of the receptors.

• Taste results when molecules are dissolved in fluid and reach the gustatory receptors on the tongue; the signals are sent to the brain to determine which flavor (bitter, sour, sweet, salty, umami ) is being consumed.

• Taste buds are found on the tongue and contain clusters of gustatory receptors on bumps called papillae; fungiform papillae each contain one to eight taste buds; they also have receptors for pressure and temperature.

• The ability to smell and taste declines with age.

Key Terms

umami: one of the five basic tastes, the savory taste of foods such as seaweed, cured fish, aged cheeses and meats

receptor: a protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions

tastant: any substance that stimulates the sense of taste

papilla: a nipple-like anatomical structure

odorant: any substance that has a distinctive smell, especially one added to something (such as household gas) for safety purposes

chemoreceptor: a sensory cell or organ responsive to chemical stimuli

thermoreceptor: a nerve cell that is sensitive to changes in temperature

G protein–coupled receptor: a receptor protein located in the cell membrane that binds extracellular substances and transmits signals from these substances to an intracellular molecule