Digesting facts in the lab

Yasmin Jayathirtha

As we saw in the last column, enzymes are present in all living organisms. So, though experiments using enzymes appear to be beyond the reach of a school laboratory, we can set up qualitative experiments using natural extracts to illustrate the functions we study in the biology classes. Digestion, as I noted last time, is well worth illustrating through lab work.

Carbohydrate digestion is illustrated using salivary amylase. It is good to point out that many creatures are likely to have the same or similar enzymes to digest the same foods that human beings eat. So germinating seeds will have amylase to digest the stored carbohydrates for growth. It may be an interesting project to extract amylase from various seeds and compare their activities, either across varieties (grains vs. dals) or linked to germination times. These projects will be easily doable since all the material is easily available. However, they can lend themselves to very sophisticated ideas of research work including controls, what makes a fair test, etc.

After salivary amylase begins the digestion of carbohydrate in the mouth, the next lot of digestive enzymes is found in the stomach. This is not easy to get hold of, but as we see, other organisms have similar enzymes, so let us consider protein digestion, using enzymes from other sources. Traditionally, papaya and pineapple have been used to aid digestion, tenderize meat and applied to soften corns and warts. Both of them contain proteolytic enzymes, papain in papaya and bromelain in pineapple. These enzymes break down the protein collagen and so soften meat. Packages of jelly contain the instruction: do not add fresh cut pineapple, jelly will not set. We will look at this interesting statement later.

Protein digestion
Materials required:
Hard-boiled egg
Raw papaya
Pineapple
Test tubes/watch glasses/katoris

The enzyme in papaya is found in the latex and is commercially extracted by tapping the raw fruit, i.e., making a cut and allowing it ooze out. This is then dried to make a powder. I found it simpler to cut the top off and let the ‘milk’ fl ow into a bowl of water; this meant that I got a clear solution of the latex in water. Alternatively, one can grind the papaya in a mixie and extract the juice.

The enzyme in pineapple is found in the juice and flesh, so just squeeze out the juice.

Peel the hard-boiled egg and cut pieces of the white. Put three pieces in separate containers. To one, add water, enough to cover the piece. Cover the second piece with papaya juice and the third with pineapple juice. Let the containers sit for about half an hour. At the end of this time, try to gently mash the piece of egg white. The piece in water will still be firm but the others will break up.

This experiment can be carried out under a variety of conditions:

Dependence on pH
Take three samples of boiled egg white,
1. Add 2 cm³ of the enzyme extract + 5 drops of water.
2. Add 2 cm³ of the enzyme extract + 5 drops of 1 mole/dm³ HCl
3. Add 2 cm³ of the enzyme extract + 5 drops of 1 mole/dm³ NaOH

Keep for half an hour and test for signs of digestion.

Dependence on temperature
Take four samples of boiled egg white
1. Add 2 cm³ of the enzyme, keep in the refrigerator or in an ice bath.
2. Add 2 cm³ of the enzyme, keep at room temperature.
3. Add 2 cm³ of the enzyme, keep in a water bath at 40°C.
4. Add 2 cm³ of the enzyme, keep in a water bath at 60-70°C.

Every five minutes, try to mash the egg white in each of the samples. Try for about an hour.

Jelly is made of gelatin, an animal protein and the enzyme in pineapple prevents it from setting. However, in India, jelly crystals are usually marked 100% vegetarian and are made from agar, which is a carbohydrate. However, they still say, ‘do not add fresh pineapple’ on the packet. I did wonder why, because when you check out the nutritional information, the main source is carbohydrate. I tried making the jelly, following the instructions but adding fresh cut pineapple and pineapple juice. Bromelain acts only on proteins and the jelly jelled! This is a nice illustration of the specificity of enzymes.

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