Digestion in a test tube!

Yasmin Jayathirtha

In the biology classroom, the most common experiments are the food tests. To check the presence of starch, the iodine test is done. For protein, a biuret test is done, and for reducing sugar, the Benedict’s test is routinely used. The tests for biochemicals are useful, but can we use them to study and illustrate processes? Some of the problems with these types of experiments are the complexity of the chemicals, processes that are completely dependent on the organization in an organism and the difficulty of working with living organisms. But we could consider those that can be replicated in test tubes.

For example, what experiments can illustrate digestion? Starch digestion is the easiest to study, since it starts in the mouth and the enzyme is available just a spitting distance away.

So, let us consider digestion in a test tube!

• Starch solution 1 per cent
• Salivary amylase (get the students to rinse their mouth out with water and spit into a beaker. A few will feel repelled but it is good to have a mix of the amylase, since there may be individual differences.)
• Test tubes
• Boiling tubes
• Droppers
• Petri dishes
• Avalakki
• Iodine solution (I₂ in KI solution)

Procedure

1. Ask the students to rinse their mouths out with water and put some Avalakki in their mouth. Let the food sit in the mouth and note the taste. Chew about 50-60 times and note the taste of the mush. What change in taste do they notice?
2. Take about 10 cm³ of starch solution in a boiling tube and add two drops of iodine solution. Add 2 cm³ of saliva solution. Keep overnight.
3. In separate petri dishes, take solid maida paste and cotton. Add about 1 cm³ of saliva to both and keep overnight.
4. Take 3 test tubes and add 5 cm³ of starch solution to each. To the first, add 2 cm³ of saliva, to the second add 2 cm³ of water and to the third add 2 cm³ of boiled saliva. Make sure that the test tubes are appropriately labelled.

Spot three rows of iodine solution on a tile. At intervals take out a drop of solution from each of the tubes and add it to the iodine solution on the tile. This will allow us to detect when and in which tube the reaction is complete. Keep overnight and test the other tubes for the presence of starch.

After the reaction is complete, test for sugar using Benedict’s solution. Add two cm ³ of Benedict’s solution to an equal amount of the contents of test tubes 1, 2, and 3 and heat in a water bath. The first test tube will show the presence of sugar, but the other two will not. Here the tube with water is the control, showing that the enzyme is needed for digestion. The result of tube 3 illustrates the fact that heating denatures enzymes.

1. The avalakki will taste sweeter after chewing.
2. The solution is colourless, showing that starch is no longer present.
3. The maida paste will have become runny, but the cotton remains unchanged. This illustrates that amylase digests starch but not cellulose, i.e., the enzymes required for digestion are specific.

When we used maida to make the starch solution, the original maida showed a positive reaction to Benedict’s solution, either because of contamination or because the maida had partially broken down. We discussed how to get rid of the sugar, maybe by washing the maida before use since sugars are soluble and starch is not. The discussion on the presence of sugar included the fact that other organisms will have their own enzymes and that when seeds germinate; the growing plant converts starch to sugar for use.

These experiments were done as a part of a project on food by 11 to 12-year- olds.

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.