Life Processes

Functions of stomach
(i) The muscular walls of the stomach help in mixing the food thoroughly with more digestive juices.
(ii) The contraction of muscular walls also cause to break down comparative large food pieces into smaller pieces to expose larger surface of food for the action of gastric enzymes.
(iii) The exit of food from the stomach is regulated by a sphincter muscle which releases it in a small amounts into the small intestine for proper digestion.
(iv) Gastric glands present in the wall of the stomach release hydrochloric acid, pepsin enzyme and mucus.
(a) Hydrochloric acid kills bacteria and also makes the medium acidic for the action of pepsin enzyme.(b) Pepsin enzyme help in digesting proteins.(c) Mucous protects the inner lining of the stomach from the action of the HCl acid under normal conditions.

Activity to show action of saliva on starch
Procedure:
(i) 1 mL of 1% starch solution is taken in two test tubes and labelled as A and B.
(ii) 1 mL of saliva is added to test tube A. Nothing is added to test tube B (control).
(iii) The test tubes are left undisturbed in safe place and similar conditions for about 30 minutes.
(iv) After about 30 minutes, a few drops of dilute iodine solution is added to each test tube.
Observations:
(a) In test tube A there no change in iodine solution i.e., solution remain brownish that is the colour of iodine solution. It means that starch is not present.
(b) In test tube B, the solution turns blue black. It proves presence of starch.
Conclusion:
Saliva has enzyme which can digest starch and convert into other carbohydrate (sugars).
Conclusion
In test tube A the saliva acts on the starch and break them down. whereas in test tube  B where no saliva is present the starch is not broken down.

Dental caries: Dental caries is the tooth decay that causes gradual sofetening of enamel and dentine. 
It is caused when bacteria act on sugars and produce acids that softens or demineralises the enamel. It happens when masses of bacterial cells together with food particles stick to the teeth to form dental plaque. As plaque covers the teeth, saliva cannot reach the teeth surface to neutralise the acid.

Precaution:
to prevent tooth decay we should brush our teeth after eating to remove the plaque before the bacteria can produce acids.
If left untreated microorganisms may invade the tooth pulp, causing inflammation and infection.