HUMAN BIOLOGY

SenSory SyStemS 271

What are chemical senses?

• Taste and smell (olfaction) are the human chemical senses.


• Taste depends on receptors in taste buds in the tongue. The
  receptors bind molecules dissolved in fluid. The five primary
  tastes are sweet, sour, salty, bitter, and umami.


• Olfaction relies on receptors in patches of epithelium in the
  upper nasal passages. Olfactory neurons send signals directly
  to the olfactory bulbs in the brain.

taKe-Home message

taste buds help make eating one of life’s pleasures. So how
do the sensory receptors in our taste buds distinguish the
tastes in different foods?
each taste category such as sweet or sour is associated
with particular “tastant” molecules. When you eat food,
however, which taste category (or combination of them) you
ultimately perceive depends on the nature of the triggering
chemical and on how it is processed by the receptor. In
each case, some event causes the receptor cell to release
a neurotransmitter that triggers nerve impulses in a nearby
sensory neuron.
For example, when you taste “salt,” the receptor cell’s
response is due to the flow of na^1 through sodium ion
channels in its plasma membrane. acidic tastant molecules
release hydrogen ions that block certain ion channels.
the blockage causes a receptor to respond with a “sour”
message.
Cells that detect bitter substances may have receptors
sensitive to as many as one hundred different trigger
tastants. this diversity probably is a survival tool. many
toxic chemicals (including plant alkaloids such as nicotine
and morphine) taste bitter, an adaptation that may help
protect us from ingesting dangerous substances. Familiar
bitter-tasting alkaloids are caffeine and quinine, the
mouth-puckering tastant in tonic water. and while many

of other individuals. For instance, one or more pheromones
in the sweat of females may account for the common obser-
vation that women of reproductive age who are in regular,
close contact with one another often come to have their
menstrual periods on a similar schedule. Many scientists
are not convinced that pheromones operate in humans,
however, and debate on the topic is always lively!

Figure 14.7 Animated!
A sensory pathway leads
from olfactory receptors
in the nose to primary
receiving centers in the
brain. (© Cengage Learning)

tasty science

“sweet” tastants are sugars, others are amino acids or

alcohols. Both bitter and sweet tastes are detected by

specific proteins inside the receptor. the taste category

called umami also is triggered by amino acids, notably

glutamate. Its name was bestowed by the Japanese

researcher who identified it.

each taste bud has receptors that can respond to

tastants in at least two—and in some cases all five—of

the taste classes. Various tastants commingle (together

with odors) into our perceptions of countless flavors.

not all taste receptors are equally sensitive.

“Bitter” ones tend to be extremely sensitive and so

can detect tiny amounts of bitter tastants—and thus

potential poisons. Sour tastants are needed in higher

concentrations before the stimulus registers. even

higher levels of sweet and salty

substances must be present for

the stimulus to register. So why

can relatively small amounts of

artificial sweeteners so readily

sweeten foods? their molecular

characteristics make them 150

times (aspartame) to more than

600 times (saccharin) as potent

as plain sucrose. Charles D. Winters/Photo Researchers/Getty Images

olfactory

nerve

tract

olfactory bulb

olfactory

nerve fibers

bone

mucus on olfactory

epithelium

olfactory receptor

cell body

SCIENCE COMES TO LIFE

14.4

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