(a) With the help of an activity, explain the action of saliva on the food we eat. (b) Why is bile juice important in the process of digestion?

(a) Activity to demonstrate the action of saliva on starch (Activity 5.3):

1. Procedure:

• Take $1\text{ mL}$ of $1\%$ starch solution in two test tubes, labeled A and B.
• Add $1\text{ mL}$ of saliva to test tube A and leave both test tubes undisturbed for $20\text{--}30\text{ minutes}$.
• Add a few drops of dilute iodine solution to both test tubes.

1. Observation:

• Test tube B (without saliva) turns blue-black, indicating that starch is present.
• Test tube A (with saliva) does not change colour (remains yellowish-brown), indicating the absence of starch.

1. Conclusion:

• Saliva contains an enzyme called salivary amylase which breaks down starch (a complex carbohydrate) into simple sugars. Since starch was digested in test tube A, it did not give a positive blue-black test with iodine.

(b) Importance of bile juice in digestion:

Bile juice, secreted by the liver and stored in the gallbladder, is essential for two reasons:

1. Alkaline Medium Creation: The food arriving from the stomach is highly acidic. Bile juice neutralizes this acid and makes the food alkaline, which is a prerequisite for pancreatic enzymes (like trypsin and lipase) to function.
2. Emulsification of Fats: Fats are present in the small intestine as large globules, making it difficult for lipase enzymes to act on them. Bile salts break these large globules down into smaller globules, vastly increasing the surface area and efficiency of enzyme action.