Chemoreception: Receptors in Animals and Plants

Chemoreception is the ability to detect and respond to chemical signals, and it is shared by virtually every living organism on Earth. Animals use it to smell food, taste danger, and track mates. Plants use it to detect pathogens, sense soil nutrients, and even warn neighboring plants of an insect attack. Though animals and plants diverged roughly a billion years ago, both kingdoms independently evolved elaborate receptor systems to read their chemical environments. The mechanisms look very different on the surface, but the underlying logic is strikingly similar: a receptor on or inside a cell binds a specific molecule, triggering a cascade of internal signals that change the organism’s behavior or physiology.

How Mammals Smell

The mammalian sense of smell begins in the nose, where specialized olfactory neurons each display one type of odorant receptor on their surface. When an airborne molecule drifts in and locks onto one of these receptors, the receptor activates an enzyme that produces a small signaling molecule called cAMP. That molecule, in turn, opens ion channels in the neuron’s membrane, generating an electrical signal that travels to the brain. This chain of events, from receptor binding to enzyme activation to ion channel opening, happens in milliseconds and gives mammals the ability to distinguish thousands of different odors.

What is surprising is how inefficient the process appears at the single-molecule level. A detailed study of the mammalian olfactory signaling chain found that when a single odorant receptor binds its most effective odor molecule one time, it produces on average much less than one downstream signaling event.¹ The system compensates through sheer numbers: millions of receptor molecules working in parallel, with the brain summing up their collective activity. This architecture allows mammals to detect vanishingly faint odors even though individual receptor events are unreliable.

Insects Use a Completely Different Design

If you assumed insect smell works the same way as mammalian smell, the past few years of research have proven otherwise. Insect odorant receptors are not the same class of protein that mammals use. Instead, they form ligand-gated ion channels, meaning the receptor itself is the channel. When an odor molecule binds, the channel opens directly, letting ions rush in without the intermediate signaling steps that mammalian neurons require.

Recent structural studies have revealed the architecture of these insect receptor complexes in remarkable detail. Each functional unit consists of one odorant-sensing subunit (called OR) and three copies of a conserved co-receptor subunit called Orco. When an odor molecule binds to the single OR subunit, it shifts one of the channel’s pore-forming segments outward, creating an asymmetric opening that lets ions flow through.² The three Orco subunits act as a scaffold; they do not bind the odorant themselves but hold the structure in place so that one binding event on the lone OR subunit is enough to open the pore.³

This design has practical consequences for how insects navigate the world. Because the receptor is the channel, the response can be extremely fast: there is no waiting for an enzyme to make a second messenger. For a mosquito homing in on a human through a shifting plume of body odor, speed matters. Female mosquitoes integrate olfactory cues with temperature, humidity, and visual information to find hosts, and odorant receptors on their antennae are the first step in that process.⁴ The skin bacteria that convert sweat metabolites into volatile compounds are what give each person their characteristic scent, and mosquito ORs are tuned to detect precisely those compounds.

Taste Receptors and the Flavor Map

Taste perception in vertebrates relies on a different family of receptors than olfaction, though both are proteins embedded in cell membranes. Two major receptor families handle most of what you experience on your tongue. The T1R family detects sweet and umami (savory) stimuli, while the T2R family handles bitter compounds.⁵ Individual differences in taste sensitivity, including why some people find certain vegetables intolerably bitter while others barely notice, can often be traced to genetic variants in these receptor genes.

The sweet receptor is particularly well characterized. It is a pair of proteins, T1R2 and T1R3, that work together as a single unit. Knockout mice missing either protein lose their ability to respond to sugars, artificial sweeteners, and sweet-tasting amino acids alike. Behavioral attraction to sweets is nearly abolished, and the nerve signals from the tongue are greatly diminished.⁶ This tells us that there is essentially one molecular gateway for sweetness in mammals, and without it, the entire category of sweet taste disappears.

How Fish Smell Underwater

Chemoreception in aquatic environments faces a different physical challenge than in air. Odorant molecules must dissolve and diffuse through water, which they do more slowly than in a gas phase. Fish compensate with highly sensitive olfactory systems tuned to water-soluble molecules. Amino acids, for instance, serve as important food-related olfactory cues in fish, triggering attractive responses that help them locate prey.⁷

This aquatic olfactory system is also vulnerable to environmental disruption. Anthropogenic acidification of both freshwater and ocean ecosystems has been shown to impair the olfactory abilities of fish and other aquatic organisms, leading to measurably impaired behavioral responses.⁸ In freshwater, the problem seems to be that acidification chemically alters the odor cues themselves and reduces receptor sensitivity. In ocean environments, elevated CO₂ appears to interfere with neurotransmitter function in the brain, disrupting how the fish interprets what it smells rather than what it detects.

Experiments with larval clownfish illustrate how dramatic the effect can be. Larvae raised in normal seawater could distinguish between chemical cues that help them find suitable reef habitat. Those raised in water simulating projected ocean acidification levels became attracted to stimuli they would normally avoid. At even lower pH levels, they stopped responding to olfactory cues entirely.⁹ For species that rely on chemical navigation to find home reefs, this kind of sensory disruption could threaten population survival.

Clearing the Signal

Detecting a chemical is only useful if the system can reset and detect the next one. In insect antennae, enzymes near the olfactory receptors are thought to break down odorant molecules after they have delivered their signal, preventing the receptor from being permanently saturated. Studies on the fruit fly have identified a carboxylesterase enzyme that modulates both the physiological and behavioral responses to pheromones, suggesting these degrading enzymes play a real role in shaping how quickly and precisely insects track chemical plumes.¹⁰ Without this kind of signal termination, a moth trying to follow a pheromone trail would be blinded by the first strong whiff and unable to track changes in concentration.

Plants Detect Pathogens at the Cell Surface

Plants do not have noses or tongues, but they are covered in chemical receptors. One of the most critical jobs of these receptors is pathogen detection. Sitting in the outer membrane of plant cells are pattern recognition receptors (PRRs), proteins that scan the environment for telltale molecular signatures of microbes. These signatures, known as microbe-associated molecular patterns, include fragments of bacterial cell walls, fungal sugars, and other molecules that plants cannot make themselves.¹¹

When a PRR binds one of these microbial signatures, it activates what researchers call pattern-triggered immunity: a defensive response that can include strengthening cell walls, producing antimicrobial compounds, and alerting neighboring cells. All plant PRRs identified so far are membrane-associated receptor kinases or receptor-like proteins, reflecting the fact that most plant-infecting microbes colonize the spaces between and around cells rather than penetrating deeply into them.¹² This is a striking parallel to the innate immune receptors in animals, which also detect conserved microbial signatures, even though the two systems evolved independently over the course of a billion years of separate evolution.¹³

Beyond microbial detection, plant receptor kinases also recognize damage-associated signals from the plant’s own injured cells. This danger-triggered immunity works alongside pathogen sensing, giving the plant a two-layered alarm system: one layer watches for foreign molecules, the other watches for signs of tissue damage.¹⁴

Plants That Scream in Glutamate

When a caterpillar chews through a leaf, the damaged cells release glutamate, the same amino acid that serves as the major excitatory neurotransmitter in the human brain. In plants, glutamate acts as a wound signal. Glutamate receptor-like (GLR) ion channels in the plant detect the released glutamate, which triggers a wave of calcium ions that propagates through the plant’s vascular system to distant, undamaged organs.¹⁵ When that calcium wave arrives at a far-off leaf, it induces the accumulation of jasmonate, a defense hormone that ramps up the production of toxins and anti-feeding compounds.

This system bears an almost eerie resemblance to neurotransmitter signaling in animals: a chemical released by damaged tissue is detected by ion-channel receptors and converted into a propagating electrical and chemical signal. But the plant has no neurons, no synapses, and no nervous system. It has co-opted a fundamentally similar molecular toolkit for a completely different body plan. Recent work has also shown that plants can desensitize their GLR channels through a calcium/calmodulin-dependent mechanism, preventing the signaling system from staying permanently activated after a wound.¹⁶ Signal termination, it turns out, is as important for plants as it is for insects tracking pheromone trails.

Sensing Nutrients in the Soil

Plant roots live in a chemical landscape as complex as any an animal navigates by smell. Nitrate, the primary nitrogen source for most plants, is not evenly distributed in soil. Roots need to find it, grow toward it, and adjust their architecture depending on how much is available. The transporter protein NRT1.1 plays a dual role in this process: it both imports nitrate into root cells and acts as a sensor that adjusts root growth in response to nitrate availability.¹⁷

The mechanism is clever. NRT1.1 does not just transport nitrate; it also transports auxin, the hormone that controls where and how roots grow. When nitrate is absent, NRT1.1 moves auxin away from a root tip, suppressing growth in that direction. When nitrate is present, it blocks NRT1.1’s auxin-transport activity, allowing auxin to accumulate and the root to grow toward the nutrient-rich zone. NRT1.1 functions as a dual-affinity transporter, switching between high- and low-affinity modes depending on whether a specific amino acid residue gets phosphorylated, which allows it to respond across a wide range of nitrate concentrations.¹⁸ The plant effectively “smells” the soil by coupling nutrient uptake directly to growth-hormone routing.

Plant Hormone Perception

Animals typically sense external chemicals through membrane-bound receptors, but plants have evolved an additional trick: some of their most important chemical receptors sit inside the cell and work by controlling protein destruction. The auxin receptor TIR1, for example, is part of the cell’s protein-recycling machinery. When auxin enters a cell and lands in a binding pocket on TIR1, it acts like molecular glue, stabilizing the interaction between TIR1 and a group of transcription repressors. That interaction tags the repressors for destruction, which releases genes from repression and allows the cell to respond.¹⁹ Crystal structures of TIR1 show that the auxin molecule sits in a surface pocket at the interface between receptor and target, physically bridging the two proteins. It is a fundamentally different strategy from ion channels or enzyme cascades: the signal works by accelerating the removal of a brake rather than pressing an accelerator.

Airborne Conversations Between Plants

Plants release volatile organic compounds (VOCs) into the air, and neighboring plants can detect and respond to them. A plant being eaten by herbivores may emit VOCs that prime its neighbors to ramp up their own defenses before the insects arrive. Despite the large number of VOCs that plants emit and perceive, only a small number of the receptors involved have been identified so far.²⁰ The receptors that have been characterized are often phylogenetically distinct but structurally similar, and researchers have highlighted the involvement of a signaling pathway called KAI2 (originally known for its role in detecting smoke-derived compounds that trigger seed germination after fire) in expanding the repertoire of plant VOC perception.

The field is still in early days. How plants translate different VOC signals into different physiological responses when using a limited set of receptors remains one of the open questions. It is possible that combinatorial coding, similar to how the mammalian brain distinguishes odors by the pattern of activated receptor types, plays a role, but that hypothesis is far from proven in plants.

Fungal Chemoreception and Mating

Fungi occupy an interesting middle ground between plants and animals in terms of chemoreception. They are not photosynthetic like plants, and they lack the nervous systems of animals, yet they depend heavily on chemical signaling. In many fungal species, mating requires one cell to detect pheromones secreted by a compatible partner. The yeast Saccharomyces cerevisiae uses G protein-coupled receptors on its cell surface to detect mating pheromones, triggering a signaling cascade that reorients cell growth toward the pheromone source.²¹ This is the same general class of receptor that mammals use for olfaction, reinforcing the idea that G protein-coupled receptors are an ancient and versatile solution to the problem of chemical sensing. An analysis of signaling systems across prokaryotes and eukaryotes suggests that the building blocks of chemosignaling, including receptors, G proteins, and downstream enzymes, trace back to bacterial ancestors and were co-opted and elaborated as eukaryotic life diversified.²²

Olfactory Receptors in Unexpected Places

One of the more surprising developments in chemoreception research is the discovery that olfactory receptors are not confined to the nose. In mammals, genes originally identified as odorant receptors turn out to be expressed in the liver, kidney, gut, and other organs, where they appear to play metabolic roles that have nothing to do with smelling. A receptor called OR1A1, for instance, responds to a compound called (–)-carvone (found in spearmint oil) in liver cells. When activated, it triggers a chain of events that reduces triglyceride synthesis and lipid accumulation. These effects disappear when OR1A1 is knocked out, confirming the receptor is directly responsible.²³

The implications are still being worked out, but the finding suggests that the distinction between “sensory” and “metabolic” receptors may be a false one. The same molecular hardware that detects odors in the nose might regulate fat storage in the liver or glucose handling in the gut. Researchers are exploring whether olfactory receptors in various tissues could become drug targets for metabolic diseases and even cancers, though that work remains early-stage.

Building Artificial Noses from Biological Parts

If biological olfactory receptors are exquisitely sensitive to specific chemicals, why not borrow them for technology? That is exactly what several research groups have done. One team embedded mosquito odorant receptors and their Orco co-receptors into an artificial lipid membrane on a chip, creating a sensor that could detect the volatile compound octenol (a molecule found in human breath and sweat that attracts mosquitoes). The sensor produced measurable ion current changes when exposed to octenol, and the researchers installed it on a mobile robot that could navigate toward the gas source.²⁴

A more recent approach encapsulated olfactory receptors inside a hydrogel matrix and mounted the assembly on a flexible carbon nanotube transistor. The porous hydrogel lets odorant molecules diffuse in and bind to the embedded receptors, causing conductance changes in the underlying transistor that can be read electronically. Built on a flexible plastic film, the device functions as a bendable bioelectronic nose capable of detecting odorants in both gas and liquid phases.²⁵ Applications could range from food-safety monitoring to environmental sensing to medical diagnostics, where detecting specific volatile biomarkers in breath or body fluids could flag disease.

These devices highlight a theme running through all of chemoreception: the receptor is the bottleneck. Whether the downstream machinery is a neuron, a calcium wave in a plant, or a transistor on a chip, what matters first is that a molecule binds to a receptor with the right specificity and affinity. Evolution has spent billions of years optimizing that initial binding event. Engineering is now learning to plug it into entirely new outputs.