Cutting through an onion can make even the most unemotional of people start to well up. It seems peculiar that whole onions don’t irritate your eyes and do not have the same characteristic smell associated with chopped onions – so what is happening when you cut through the onion that causes us to cry?
Well, when you cut into an onion you break up the cells within the onion flesh, crushing and slicing though the cell walls. This mixes all the cell contents together, causing various chemicals to mix: sulphoxide-containing amino acids in the onion to mix with the enzymes alliinase and lachrymatory factor synthase. These enzymes break down the amino acids to release small acidic compounds into the air which can get into your eyes and act as “lachrymatory agents” – chemicals which can cause your eyes to tear and sting.
But the real question here is why onions have evolved this rather complex chemical process, simply to make us cry when we cut them? Simply put, the answer is “animal plant warfare”.
Onions can’t run away when an animal decides to eat them and so they have developed defence mechanisms against the would-be animal onion eaters. The onion plant evolved certain secondary metabolites (meaning, biological molecules not directly required for life) that cause a tearing and burning sensation when released to try and discourage animals from eating them.
There is an ironic twist to this evolutionary tale. Normally, species evolve to be better at counteracting another’s defensive mechanisms. So, in theory, you would expect that animals would evolve ways to withstand the onions tear-inducing chemical assault, allowing them to eat more raw onions. But in this battle between plant and animals, the animals have evolved to be more sensitive to onions rather than less! We animals have evolved certain proteins (in particular one known as TRPA1) that acts as a receptor for the onion’s irritating molecules – causing the body to feel more pain and irritation as a warning to stay away from nasty toxins. Those lucky onions will live to survive another day (unless you’re a crafty human who decides to cook them first)!
But don’t worry; a team of researchers in 2008 genetically modified a group of onion plants that will make cooking French onion soup significantly less painful. The scientists suppressed (‘turned off’) a gene that makes one of the enzymes crucial to the formation and release of the irritating agent. So let’s hope that one day we’ll all be able to buy “no more tears” onions!
Answer by Kane Heard
. S. Imai, N. Tsuge, M. Tomotake, Y. Nagatome, H. Sawada, T. Nagata and H. Kumagai, Nature, 2002, 419, 685.
. S. Wöll, S. H. Kim, H. J. Greten and T. Efferth, Nat. Prod. Bioprospect., 2013, 3, 1-7.
. K. Kang, S. R. Pulver, V. C. Panzano, E. C. Chang, L. C. Griffith, D. L. Theobald and P. A. Garrity, Nature, 2010, 464, 597-600.
. C. C. Eady, T. Kamoi, M. Kato, N. G. Porter, S. Davis, M. Shaw, A. Kamoi and S. Imai, Plant Physiol., 2008, 147, 2096-2106.
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