The Effect of pH on Enzyme Activity- Salivary Amylase and Starch Digestion
The ability of the enzyme amylase, found in human saliva, to hydrolyze or break down starch molecules will be demonstrated in this lab. The activity of the enzyme in various pH solutions will also be demonstrated.
MATERIALS (for a class of 30)
- 10 test tubes
- 100 ml of 1 % starch (add one gram of soluble starch in a paste to boiling water)
- 15 rubber bands
- 50 ml of iodine test solution (Lugol's iodine or 1 g iodine to 100 ml 1.0M KI) divided into 5 dropper bottles.
- 50 ml of 2.0 M HCl divided into 5 dropper bottles
Iodine is toxic and an irritant and will stain clothing. Handle acids with care, avoid contact with skin or eyes.
Obtain a sample of saliva (1-2 ml). This can be done by chewing a clean rubber band and drooling into a test tube. It is nearly worth doing this lab just to watch students do this! Place 10 drops (no bubbles) in each of two test tubes. Add 10 drops of 1% starch to each tube and 2 drops of 2.0M HCl to one of the tubes. Allow the tubes to stand for approximately 15 minutes. It is even better to warm them in your hand during this period. Then add 1-2 drops of iodine solution. A black or dark blue color indicates the presence of starch, the yellow iodine color means no starch remains, having been hydrolyzed by the amylase. Ask students about the activity of amylase in the stomach.
Tube: Results of Iodine Test (+or-) After 15 minutes
Starch+Acid+Saliva Starch+Saliva Starch only
All substances can be poured down the sink.
The enzyme amylase will catalyze the hydrolysis of starch to maltose when the pH is near 7.0. But when the HCl is added to the solution the amylase will be denatured which results in the enzyme being deactivated. The iodine serves as an indicator for the presence of starch. Iodine (I2) will reach with iodide ion to produce the I3- ion. This ion will form a dark blue complex with the starch molecule.
REFERENCE: John Whitsett, LaCrosse Central HS, LaCrosse, WI 54601