Why Onions Make You Cry
Learners can trace the chemical defense pathway from a damaged onion cell to the eye's tear response, and explain why this defense evolved.
A complete interactive classroom, not just a preview.
Start when you are ready to enter this Stage's 12 scenes and explore, respond, and learn as you go.
Why does cutting an onion make you cry, and what is the chemical chain of events that causes it?
- cell-compartments
- Onion cells keep reactants (alliin, alliinase) in separate compartments until damage mixes them.
- enzyme-reaction
- Alliinase converts alliin into a sulfenic acid intermediate within seconds of cutting.
- lfs-pathway
- Lachrymatory-factor synthase rapidly converts the intermediate into syn-propanethial-S-oxide (PFSO), the volatile irritant.
- travel-to-eye
- PFSO evaporates and diffuses through air to reach the eyes of anyone nearby.
- tear-response
- PFSO activates TRPA1 pain/irritant receptors in the corneal nerve endings, triggering tear glands and the familiar cry.
- defense-evolution
- The pathway evolved to deter herbivores, not to inconvenience humans; the tear response in humans is a side effect.
The onion is 'releasing' something it already had stored as a gas.
Show that the irritant is newly synthesized by an enzyme reaction after cells are cut; it does not pre-exist as a gas inside the onion.
Sulfur 'smells bad,' so the sulfur itself is what stings your eyes.
Distinguish the smell-bearing sulfur compounds from the specific irritant PFSO, showing that volatile sulfur chemistry and eye irritation are separate outputs of the same damaged-cell cascade.
An onion cries because it feels pain.
Make clear that plants do not feel pain as animals do; the response is a chemical defense deployed passively when cells rupture.
- detailed organic synthesis mechanisms
- other Allium species comparisons beyond brief mention
- role of sulfur in human nutrition
- industrial or culinary applications
- Learner draws or describes the pathway: cut cell → alliinase meets alliin → sulfenic acid → LFS → PFSO vapor → eye → TRPA1 → tears.
- Learner explains why the onion produces this chemical (deterring herbivores), not to make humans cry.
- Learner identifies at least one piece of evidence that distinguishes observation from inference in the model.
- Apply the 'compartmentalized reactants + activating enzyme' logic to predict why crushing garlic or cutting other damaged plant tissues produces different sensory effects.
Curious general learners ages 13+ with basic high-school biology. No prior chemistry background assumed.
- 01A Familiar Stingslide
- 02Predict the Causeinteractive
- 03Inside an Onion Cellslide
- 04Watch the Reaction Unfoldinteractive
- 05Test the 'Pre-Made Gas' Ideainteractive
- 06From Cell to Eyeinteractive
- 07Eyes On: An Irritant-Detector Gameinteractive
- 08Why Does the Onion Do This?slide
- 09Repair the Mental Modelinteractive
- 10Transfer: What About Garlic?interactive
- 11What We're Still Unsure Aboutslide
- 12A New Questionslide
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