... like I'm 5 years old
You slice into an onion, and within moments your eyes sting and begin to water. The onion is not making you emotional. Cutting it starts a chemical reaction that releases an invisible, airborne irritant. When that substance reaches your eyes, they produce tears to dilute it and wash it away.
An intact onion keeps certain ingredients stored apart inside its cells. Your knife breaks those cells open, allowing the ingredients to mix. Enzymes—proteins that accelerate chemical reactions—quickly transform sulfur-containing compounds into a volatile substance called propanethial S-oxide, also known as the onion’s lachrymatory factor. “Lachrymatory” simply means tear-producing.
Because the substance evaporates easily, it travels upward through the air. It meets the moist surface of your eyes and irritates sensitive nerve endings there. Your nervous system responds automatically by instructing the lacrimal glands above your eyes to release extra tears. This is a protective cleaning reflex, much like blinking when dust approaches.
Cooking prevents much of this drama because heat changes the shape of the enzymes, stopping them from efficiently producing the irritant. The Library of Congress explanation of onion tears describes this reaction and identifies propanethial S-oxide as the immediate irritant.
Imagine an onion as a cupboard containing two sealed bottles. Cutting the onion smashes both bottles, their contents mix, and an irritating mist escapes. Your eyes turn on their windshield washers—your tears—to clear the mist away.
... like I'm in College
Picture the onion beneath your knife as a tightly organized chemical warehouse. Its cells contain sulfur-based molecules known as S-alk(en)yl-L-cysteine sulfoxides, which contribute to the characteristic flavor and aroma of onions. The cells also contain an enzyme called alliinase, but cellular compartments normally prevent the enzyme and its principal substrates from freely mixing.
The knife destroys those internal barriers. Alliinase encounters a precursor commonly called 1-PRENCSO, or isoalliin, and breaks it down. This produces pyruvate, ammonia and a short-lived sulfur compound called 1-propenesulfenic acid. If you want the broader background, how enzymes catalyze reactions explains how biological catalysts accelerate transformations without being consumed.
A second enzyme, lachrymatory-factor synthase, redirects the unstable sulfenic acid into (Z)-propanethial S-oxide. This highly volatile molecule leaves the damaged tissue, spreads through the surrounding air and reaches the ocular surface. Irritation then activates a reflex involving sensory nerves and the lacrimal glands, producing the familiar flood of tears.
For years, scientists believed the tear-producing molecule formed spontaneously after alliinase began the reaction. In 2002, Japanese researchers reported that lachrymatory-factor synthase specifically catalyzes its formation. Their landmark report in Nature revised the accepted explanation and suggested that reducing this enzyme’s activity might produce onions that retain more of their characteristic flavor without causing as many tears.
Imagine building an onion cell from Lego bricks. First, construct a large room representing the cell. Inside it, build two smaller storage areas. Put yellow sulfur-containing bricks in one area and a blue alliinase figure in the other. As long as the walls remain standing, the blue figure cannot reach the yellow pieces, so very little happens.
Now bring down a Lego knife. It crashes through the outer wall and the internal storage areas, scattering their contents across the same baseplate. The blue alliinase figure immediately takes apart the yellow precursor bricks and rebuilds them into several products, including a small, unstable sulfur structure.
A second figure, lachrymatory-factor synthase, grabs that unstable structure. Acting like a specialist builder on an assembly line, it rapidly rearranges the pieces into a new model: propanethial S-oxide. This model has imaginary wings, representing volatility, so it lifts away from the onion and travels toward a nearby Lego person.
The flying structure lands on the figure’s transparent eye tiles. An eye-sensor brick detects irritation and sends a message along a Lego wire to a tear factory. The factory releases clear blue tiles that flow across the eyes, diluting and removing the intruder.
In real chemistry, the pieces are atoms joined into molecules rather than plastic bricks. The same small selection of atomic “bricks” can be rearranged into substances with very different properties, as explored in how atoms form molecules. Your tears are therefore the final stage of a microscopic assembly line accidentally switched on by your knife.
... like I'm an expert
At the molecular level, onion lachrymogenesis begins when tissue disruption abolishes subcellular compartmentation. In the conventional model, trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide, or 1-PRENCSO, is stored in the cytoplasmic region, while substantial alliinase activity is sequestered within vacuoles. Cutting permits rapid enzyme-substrate contact, activating sulfur secondary metabolism that remains largely restrained in intact tissue.
Alliinase, a C–S lyase, cleaves 1-PRENCSO to generate pyruvate, ammonia and (E)-1-propenesulfenic acid. This sulfenic-acid intermediate is highly reactive and sits at a metabolic branch point. Some material can feed pathways leading to thiosulfinates and downstream flavor compounds, while lachrymatory-factor synthase, or LFS, catalyzes its conversion into (Z)-propanethial S-oxide.
LFS therefore influences more than tear production. Experimental silencing of the LFS gene sharply decreased lachrymatory-factor synthesis while redirecting sulfur metabolites toward thiosulfinates and other volatile products. The resulting change demonstrates that pungency, aroma and lachrymation emerge from competing branches of a connected reaction network rather than from one universal “onion chemical.” The relevant LFS-silencing research documents this altered metabolite profile.
Structural studies identify a specialized LFS active-site environment. Arg71 and Glu88 are essential for activity, while mechanistic models also assign a proton-transfer role to Tyr102. These residues facilitate conversion of the sulfenic-acid substrate into the lachrymatory sulfine. Once volatilized, the product contacts the tear film, stimulates ocular sensory pathways and triggers reflex lacrimation.