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Garlic is often called "the stinking rose." That's because it contains molecules composed of the same atomsthat lurk in burnt matches and rotten eggs. But if you put a whole clove of garlic right up to your nose, you won't smell much. The molecules that create the garlic smell are not actually present in natural garlic. They are synthesized in a reaction that occurs when garlic is cut or crushed. When a knife slices through garlic, cell membranes rupture, releasing an enzyme called allinase. Allinase can chemically change a tiny, odorless molecule called alliin into allicin. Allicin is the pungent, sulfur-containing molecule that can alienate friends who get too close and add zest to bland food. During any chemical reaction, atoms rearrange themselves into new substances. But all chemical reactions do not proceed in the same way. Some transformations occur spontaneously and explosively while others are hard to initiate and proceed as slowly as a rusting fence. The ease and speed with which molecules change or undergo chemical reaction depend on how often they collide and the energy needed to get the reaction started. Molecules are surrounded by negatively charged electrons which repel the electrons in other molecules. For each reaction, molecules must move fast enough to overcome these repulsive forces. This energy barrier is like crossing a mountain. Just as an ocean wave may have enough energy to knock you off your feet, a molecular collision must have enough energy to break chemical bonds. If the reactant molecules have enough energy, they can climb the energy mountain, react, and fall down the other side as products. Usually you can give molecules extra energy and make them collide more often by heating them up. But alliin and allinase only need to meet at room temperature to create allicin. The secret is in the structure of the allinase enzyme. One section of the molecule is called the active site. In a typical enzyme, the active site looks like a dent or crevice in the side of the molecule. The shape of the crevice in allinase exactly matches the molecular shape of alliin. The enzyme and substrate fit together like a key fits a lock. At the active site, alliin is stretched and twisted until chemical bonds holding it together snap and allicin is formed. Allicin, which no longer fits the enzyme, drifts away, leaving allinase unchanged and the whole process starts again. The enzyme facilitates the reaction by reducing the energy needed to break chemical bonds. Alliin and allinase have found a low-energy tunnel through the energy mountain. After ingestion, the odoriferous sulfur molecules circulate in the bloodstream and escape from your body through exhaled air and perspiration--as any nose will tell you.
  • Why don't some people like the smell of garlic? What smells are unpleasant to you?
  • How does garlic get into your breath? What steps can you take to minimize garlic breath?


living cells. Enzymes are found in plants and animals. The enzyme catalase splits ordinary hydrogen peroxide molecules into water and oxygen gas. Can catalase break down other molecules? Do all plant or animal products contain catalase? Let's find out.


  • several plastic cups
  • 3% hydrogen peroxide
  • potato, bread, apple, turnip, cheese, vinegar, and milk
  1. Cut off the end of a small potato to expose the inner surface. Do not peel off the potato skin. Put the end piece of potato into a cup and cover it completely with hydrogen peroxide. What happens? When you cut the potato, cells are disrupted, releasing catalase. The enzyme immediately begins reacting with hydrogen peroxide, producing water and bubbles of oxygen gas. Does catalytic activity occur at both the peeled and unpeeled areas of the potato? Explain.
  2. Cut another slice of potato and remove the skin. Cut this slice of potato into four pieces of equal size. Place each piece in a separate cup. Now pour hydrogen peroxide into the first cup. Pour vinegar into the second cup. Pour water into the third cup. Pour milk into the fourth cup. What happens? You'll find that catalase is highly specific for the hydrogen peroxide molecule. Other molecules do not have the correct shape to react with this particular enzyme. You can try this with other liquids.
  3. Now pour hydrogen peroxide into four cups. Add a small piece of bread to the first cup. Add a small piece of apple to the second. Add a small piece of cheese to the third. Add a small piece of turnip to the fourth cup. What did you observe? Try this activity with other foods.


    1. Do all foods contain catalase? What other foods do you think contain catalase?
    2. What evidence did you see that a reaction was taking place?


  • Bonar, A. (1985) The Macmillan treasury of herbs. New York: Macmillan.
  • Doolittle, R. (1985, Oct) Proteins. Scientific American, pp. 88-99.
  • Grosser, A. (1981) The cookbook decoder. New York: Beaufort Books.
  • Harris, L. (1975) The book of garlic. New York: Holt Rinehart and Winston.
  • Hausman, P. (1989) The healing foods. Emmaus, PA: Rodale Press.
  • Kowalchik, C. (1987) Rodale's illustrated encyclopedia of herbs. Emmaus, PA: Rodale
  • Scott, D. (1994, Apr) Designer catalysts. ChemMatters, pp. 13-15.
  • Tocci, S. (1987) Biology projects for young scientists. New York: Grolier.
  • VanCleave, J. (1990) Biology for every kid. New York: John Wiley & Sons.
  • VanCleave, J. (1989) Chemistry for every kid. New York: John Wiley & Sons.

Additional sources of information

Chemist or biochemist
County extension office