Activities and Experiments

To study the action of pepsin on proteins

Boil egg white in distilled water till a cloudy suspension is obtained. Cool it and divide the mixture into five equal portions in separate test-tubes. Label them A to E.

Add different substances to each of the five tubes as shown in the table below and explain the observations:

Test-tube	Additions	Observation	Inference
A	Only pepsin	No change	Pepsin did not act under neutral pH
B	Only Hydrochloric acid (few drops)	No change	HCl does not digest proteins
C	Pepsin + HCI	Suspension becomes clear	Pepsin acts in acidic conditions
D	Pepsin + Sodium bicarbonate (alkaline)	No change	Pepsin does not act in alkaline conditions
E	Previously boiled pepsin + HCI	No change	Pepsin is denatured by high temperature and does not act

To study the digestive action of saliva on starch

Rinse mouth 2 to 3 times to remove any food particles and collect saliva in three test-tubes labelled A, B and C. To B add half the quantity of 2% starch solution. To C add boiled saliva. Leave them for 5 minutes.

At the end of five minutes,

a) Addition of iodine to A gives no blue colour. Why?

The action of salivary enzymes on starch converts it to sugar. Since there is no starch in the test-tube, There is no blue colour.

b) Addition of Benedict's reagent to B gives red precipitate. Why?

Benedict's reagent gives red colour precipitate with sugar. In B also the salivary enzyme has converted starch to sugar and the latter gives the red precipitate.

c) Additon of iodine to one portion gives blue colour and addition of Benedict's reagent to the other portion does not produce any red precipitate. Why?

C contained boiled saliva. Heating denatures the enzyme present in saliva. Hence there is no action on the starch. The solution therefore gives blue-black colour with iodine. Since sugar has not been formed, there is no red precipitate with Benedict's solution.