Olfaction (smell) and gustation (taste) are called chemical senses because they work by detecting molecules rather than physical forces like pressure, vibration, or light. Every other sense responds to a form of energy: your eyes detect photons, your ears detect sound waves, and your skin detects pressure and temperature. Smell and taste are fundamentally different. They respond to the chemical structure of substances in your environment, using specialized receptors that bind to specific molecules and convert that chemical interaction into electrical signals your brain can interpret.
What Makes a Sense “Chemical”
Your body classifies sensory receptors by the type of stimulus they respond to. Photoreceptors in the eye respond to light. Mechanoreceptors in the ear and skin respond to physical forces. Thermoreceptors respond to temperature changes. Chemoreceptors respond to chemicals. Smell and taste both rely on chemoreceptors, which is why they share a category that no other major sense belongs to.
There is actually a third chemical sense most people don’t know about: the trigeminal chemosensory system. This is the system responsible for the burn of chili peppers, the cooling sensation of menthol, and the sting of ammonia. Together, these three systems in the nose and mouth are all dedicated to detecting chemicals in the environment, but olfaction and gustation are the two most commonly discussed.
How Smell Detects Airborne Molecules
Your sense of smell detects airborne molecules called odorants. When you inhale, these molecules travel to a patch of tissue called the olfactory epithelium high inside your nasal cavity. There, they dissolve in a thin layer of mucus that coats the surface. This mucus layer is essential: it acts as a solvent that carries odorant molecules to the receptor cells embedded in the tissue. If the mucus dries out or is disrupted, your ability to smell drops significantly.
Once dissolved, odorant molecules bind to receptor proteins on the surface of olfactory neurons. Humans have roughly 400 different types of these receptor proteins, and each neuron typically expresses just one type. A single odorant molecule can activate multiple receptor types, and a single receptor type can respond to multiple odorants. This combinatorial system is what allows you to distinguish thousands of different smells from a relatively limited set of receptors. The pattern of which receptors fire, and how strongly, creates a unique neural signature for each scent.
The sensitivity of this system is remarkable. Some compounds can be detected at concentrations measured in parts per trillion. Methyl mercaptan, the chemical added to natural gas so you can smell a leak, has a detection threshold around 0.00007 parts per million. Hydrogen sulfide (the rotten egg smell) is detectable at similarly tiny concentrations. Your nose is, in many cases, more sensitive than laboratory instruments.
How Taste Detects Dissolved Molecules
Your sense of taste works on a similar principle but detects molecules dissolved in saliva rather than air. When you eat or drink, chemicals from food dissolve in saliva and wash over taste buds on your tongue and the back of your mouth. Inside each taste bud, specialized receptor cells respond to five primary categories of tastant: sweet, salty, sour, bitter, and umami (savory).
The detection mechanisms differ by taste quality. Sweet, bitter, and umami tastants bind to receptor proteins on the cell surface, triggering a chain reaction inside the cell that ultimately releases calcium from internal stores and generates an electrical signal. Sour taste works differently: acid molecules (hydrogen ions) flow directly into the cell through a dedicated ion channel, changing the cell’s electrical charge. Salty taste is different still. Sodium ions from salt enter the cell through their own channels, and that influx alone is enough to trigger signaling without any of the internal chemical cascades the other tastes require.
What all five have in common is that the process starts with a chemical molecule physically interacting with a receptor or channel on the taste cell. No molecule, no signal. This is what makes taste a chemical sense.
How They Differ From Other Senses
The distinction becomes clearer when you compare chemical senses to the alternatives. Vision starts when photons of light hit pigment molecules in your retina, changing their shape and triggering an electrical signal. The stimulus is electromagnetic radiation, not a chemical in the environment. Hearing starts when sound waves vibrate tiny hair cells in your inner ear. Touch relies on pressure deforming nerve endings in your skin. In every case, the stimulus is a form of physical energy.
Smell and taste instead require direct molecular contact. A specific chemical compound has to physically reach a receptor and bind to it. This is why you can’t smell something through a sealed glass jar, even though you can see it perfectly well. Light passes through the glass, but the molecules cannot.
Why the Brain Treats Them as Partners
Although smell and taste use different receptors in different locations and detect different types of molecules (airborne versus dissolved), your brain frequently combines their signals into a single experience: flavor. What most people call “taste” when eating is actually a blend of true taste from the tongue and smell from the nose, particularly retronasal smell, which is the scent of food traveling from the back of your mouth up into the nasal cavity as you chew.
Recent neuroimaging research has shown that taste signals and retronasal smell signals converge in overlapping regions of the insular cortex, a part of the brain tucked deep between the temporal and frontal lobes. The brain creates a shared neural code for flavor in this area, which is why losing your sense of smell (as many people experienced during COVID-19) makes food taste bland even though the taste buds on your tongue are functioning normally.
What Happens When Chemical Detection Fails
Because both senses depend on molecules reaching and binding to receptors, anything that disrupts that chain causes problems. Nasal congestion blocks odorants from reaching the olfactory epithelium. Dry mouth reduces the saliva needed to dissolve tastants. Damage to the mucus lining of the nose impairs smell not because the neurons are broken, but because odorants can no longer dissolve and reach them.
COVID-19 provided a dramatic illustration. The virus didn’t typically infect olfactory neurons directly. Instead, it attacked the supporting cells and blood vessel cells surrounding them, disrupting the environment the neurons need to function. For taste, the virus may have interfered with a protective component of saliva called sialic acid, which normally shields the proteins that carry tastant molecules into taste pores. Without that protection, the gustatory molecules degraded before they could reach their receptors. In both cases, the chemical detection machinery was intact but couldn’t access the chemicals it was designed to detect.
This vulnerability is the flip side of being a chemical sense. Vision can be impaired by physical damage to the eye, but light itself is never blocked by mucus or saliva. Chemical senses depend on a more fragile chain of events: the right molecule, in the right solvent, reaching the right receptor, in the right condition.
One Unique Feature of Smell
Olfaction has a neurological quirk that sets it apart from every other sense, including taste. Signals from your olfactory neurons travel directly to the brain’s cortex without first passing through the thalamus, a relay station that all other sensory information must cross. Smell signals go straight from the nose to the olfactory bulb and then to cortical areas including the temporal lobe, the hypothalamus, and the amygdala (the brain’s emotional processing center). This direct wiring is thought to explain why smells can trigger vivid emotional memories in a way that sights and sounds rarely do.
Taste signals, by contrast, follow the more conventional route through the thalamus before reaching the gustatory cortex. So while both senses detect chemicals, they process that chemical information through very different neural pathways once it leaves the receptor.