Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 50.4 The Senses of Taste and Smell Rely on Similar Sets of Sensory Receptors
Taste and smell are both chemosensory systems that detect molecules in the environment. Though processed by separate pathways, both senses use related receptor types and are essential for feeding, mating, navigation, and memory. In many animals, these chemical senses are critical for survival.
1. Chemoreception and Sensory Functions
- Gustation (taste): detects chemicals (tastants) dissolved in liquid
- Olfaction (smell): detects airborne chemicals (odorants)
- In aquatic animals, taste and smell are not distinguished
- Chemical sensing aids in locating food, avoiding danger, social signaling, and orientation
2. Taste in Insects and Mammals
- Insects: taste receptors on sensory hairs of feet and mouthparts; smell via antennae
- Example: DEET blocks mosquito receptors that detect humans
- Mammals: taste receptors are modified epithelial cells in taste buds on papillae
- Each taste bud detects sweet, sour, salty, bitter, or umami
- All tongue regions detect all five tastes
3. Taste Receptor Mechanisms
- Sweet, umami, bitter: detected by GPCRs (G protein-coupled receptors)
- Sweet and umami use specific GPCR pairs
- Bitter uses >30 receptors, each sensing multiple compounds
- Sour: detected via TRP ion channels
- Salty: detected by sodium channels specific to Na⁺
- Each taste cell expresses only one receptor type; taste depends on the activated afferent neuron
4. Taste Reprogramming Experiment
- Mice normally can’t detect PBDG, a bitter compound to humans
- Researchers inserted the human PBDG receptor into sweet or bitter taste cells
- Mice with it in bitter cells avoided PBDG
- Mice with it in sweet cells preferred PBDG
- Conclusion: taste depends on neuron identity, not chemical identity
5. Smell in Humans
- Olfactory receptor cells: neurons in the nasal epithelium
- Cilia bind odorants via specific GPCRs
- Signal pathway: odorant → GPCR → G protein → adenylate cyclase → cAMP → Na⁺/Ca²⁺ channels → depolarization → action potentials
- Humans: ~380 olfactory receptor genes; mice: ~1,200
- Each cell expresses one OR gene; same-receptor cells project to same olfactory bulb region
6. Odor Discrimination and Processing
- Odors identified via combinatorial receptor activation
- Brain integrates signals into unified perceptions, modulated by context
- Complex odors are not the sum of parts, but new, holistic perceptions
7. Taste–Smell Interaction and Perception
- Taste and smell interact in the brain
- Flavor relies heavily on smell
- Blocking smell (e.g. during a cold) reduces flavor perception
- Olfactory pathways connect to memory and emotion centers
In a Nutshell
Taste and smell detect environmental chemicals using related receptor mechanisms. Taste cells respond to distinct tastants, while olfactory neurons detect a vast array of odorants via a large family of GPCRs. Both systems contribute to survival behaviors, and their signals are processed and integrated in the brain to form our complex perceptions of flavor and scent.