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Why Do Onions Make People Cry?

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Have you ever wondered why onions make people cry? This lesson provides two explanations. One is a matter of survival and the other is a matter of the precise chemical reactants involved in this process.

What Makes Us Cry?

What do bittersweet movies, sad songs, and onions have in common? They make us cry, of course! But why? Well, you'll have to ask the screenwriters and songwriters about movies and songs, but this lesson will tell you why onions make you cry! It has to do with survival and with various chemical compounds that interact with our eyes and our brain.

Defensive Mechanisms

There are two ways we can answer the question: Why do onions make people cry? One way of looking at it is through a general lens. Why do onions do this to us, generally speaking? And then, we can look at it through a magnifying lens. What are the specifics of what actually triggers those tears?

Generally speaking, it may be that onions make people cry as a defensive mechanism. Some plants smell really bad and others have really sharp thorns. Onions make us cry. Those could all act as defensive mechanisms for the plants. They may be trying to ward off anyone or anything that may want to eat them, like herbivores. Of course, in the case of onions, they are doing a terrible job when it comes to humans because they are gleefully eaten all over the world.

The Chemistry

So the matter of survival is the easy answer as to why onions make people cry; the general view of things so to speak. As for our magnifying lens, we need to look at the science behind what actually causes us to tear up. That is to say, the small but important details behind the defensive mechanism.

Onions belong to the plant genus Allium, to which garlic (among other plants) also belong. When growing, onions absorb a chemical element called sulfur from the earth. Sulfur is then used by onions to make something called amino acid sulfoxides. These are compounds that are important precursors to the distinct smells and tastes of Allium vegetables.

As soon as you cut, break, or crush an onion, you break apart its cells. Once the cells are broken, they release various enzymes; protein molecules that speed up biochemical reactions. One of these enzymes is called alliinase. It converts a specific amino acid sulfoxide, called S-1-propenyl-L-cysteine sulfoxide, into a sulfenic acid called 1-propenyl sulfenic acid. Sulfenic acid is a sulfur-based acid compound that is generated from sulfur that plants absorb from the ground. This acid is unstable and is quickly converted by an enzyme called lachrymatory-factor synthase into syn-propanethial-S-oxide, a tear-inducing and volatile sulfur compound.

The latter compound moves into the surrounding air as a vapor, where it comes into contact with our eyes. Once it comes into contact with our eyes, a small amount of it uses the water naturally found in our protective tear film to form sulfuric acid. This is a very strong acid.

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