Grade Level: 6th - 8th; Type: Biology
This project is about exploring why onions make people cry and what can be done to prevent this reaction. The goal is to learn about enzymes in onions and lachrymal glands in the eye.
- What happens to your eyes when you cut an onion?
- How does cutting an onion affect the eyes? How can you prevent this from happening?
- What is a gas? An enzyme?
Cutting onions with a knife causes individual cells to be ripped open. Enzymes inside the cells, called allinases produce a gas called propanethiol S-oxide. This gas is very reactive and produces sulfuric acid with the water in your eyes, which makes them tear up. Your lachrymal glands located by your tear ducts produce tears to wash the sulfuric acid away. Chilling the onion slows down the enzymatic production of propanethiol S-oxide because these reactions do not occur as easily at cold temperatures. This is why it is easier on the eyes to cut cold onions. Cutting the onions under water prevents the gas from reaching your eyes. Wearing goggles does the same thing.
- Kitchen knife
- Cutting board
- Basin of water
- Sport goggles
- Electronic Timer
- Peel an onion that has been left at room temperature for several hours. Start a timer just before your actually start cutting the onion. Begin to chop the onion into big pieces. Make a note of how much time elapses before your eyes start to tear up. Make a note about how your eyes were affected by the onion.
- Peel a second onion that has been stored in the refrigerator for several hours. Start a timer just before your actually start cutting the onion open. Begin to chop the onion into big pieces. Make a note of how much time elapses before your eyes start to tear up. Make a note how your eyes were affected by the onion. Did your eyes start to tear more or less quickly than in step #1? Was the experience more or less intense?
- Fill a basin of water in the kitchen sink. Peel a third onion and set your timer as you did in steps #1 and #2. Using your knife, try to cut the onion in big pieces under water, making sure the onion stays submerged. Be careful that the onion doesn’t slip out of your hands when you are cutting. Did your eyes tear up? Was the experience more or less intense? Have someone take a picture of the onion being cut under water.
- Build a tall, relatively narrow Lego structure.
- Put on a pair of swimming goggles. Repeat everything you did in step one, wearing the goggles. Have someone take a picture of you cutting the onion as you wear the goggles.
- Write up your experiment. Compare the length of time it took for your eyes to start tearing up and how intensely they stung. Did your eyes sting more when you were cutting the room temperature onion or the cold onion? How about the onion that was under water?
Terms/Concepts: Lachrymal glands,Enzymes (It is sufficient for 6 graders to be able to define “enzyme.” Older students may wish to explore the allinase enzyme family.
Imani, S. et al. “Plant biochemistry: an onion enzyme that makes the eyes water”. Nature, 419, Oct. 17, 2002: 685.
Wikipedia: Lachyrimal Gland
About.com: Why do Onions Make You Cry?