Experiment: How Sweet It Is!

PURPOSE: In this activity you will see how small differences in the structure of a compound can result in obvious differences in chemical properties. This is a very important concept, since small changes in structure can have a great influence on the biological activity of the compound. A comparison will be made between the relative sweetness of various mono- and disaccharides.

CAUTION: This lab activity involves tasting various chemicals. Make sure that the containers and utensils used are designated for food use only. Be careful to avoid accidental contamination.

MATERIALS:
For a class of 25 students, the following materials are needed:

• 6 plastic or Dixie cups (4 oz)
• 25 soda straw scoops
• 25 g of the following sugars: glucose, galactose and fructose (with the formula C6H12O6)
• maltose, lactose and sucrose (with the formula C12H22O11)

PROCEDURE:

1. Examine the structures of these common sugars. Describe how they differ from each other.
2. The various sugars are located at different stations around the room. Use your straw scoop to obtain a small sample of each sugar, pour it onto the tip of your finger, then touch your finger to your tongue. DO NOT put the scoop in your mouth! For best results, rinse your mouth with water between tastings.
3. Describe the relative sweetness of each sugar tasted, as either "very sweet", "medium sweet" or "slightly (not) sweet". Throw away the straw scoops in the trash when you have finished all of the tests.
4. Compare your results with the rest of the class and prepare a table to summarize the group results.

ANALYSIS: Examine the sugars that were very sweet and those that were only slightly sweet. Are there any structural similarities or differences between the sugars in these groups that might explain the results?

SOURCES FOR THE SUGARS: All of the sugars can be purchased from a biological supply house such as Aldrich Chemical Co., P.O. Box 355, Milwaukee, WI 53201. They can also be obtained from local sources, such as a grocery store, home-made beer making supplier, health food store or a pharmacy. Sucrose is ordinary table sugar and fructose is found in Karo light syrup. Lactose is often referred to as milk sugar. Glucose, fructose and maltose are also known as dextrose, levulose, and maltobiose, respectively.


Reference: Molecules in Living Systems: A Biochemistry Module, Harper and Row, Publishers, 1973.

Experiment written by Mark Case