The following information can be presented as an interesting sidelight of how enzymes trigger chemical reactions, as an example of how cells can compartmentalize different compounds, and/or as a discussion of how organisms have evolved means of defense.
A number of plants, including onions and garlic, produce compounds that protect them against grazing predators, such as insects, a number of bacteria, and animals. In both garlic and onions, a compound stored inside cells interacts with an enzyme stored between cells only when the bulb is chewed or cut. In onions, the compound produced when the enzyme comes in contact with the intercellular chemical is called a lachrymator because it produces tears. What happens is that the lachrymator becomes airborne, combines with the fluids in the eye, and forms sulfuric acid. The formation of sulfuric acid in our eyes is very painful, stings the eyes, and produces tears. Onions are not offensive until they are cut or grazed. The rupture of the cell membrane triggers a chemical reaction that has evolved as a mechamism to protect these plants.