The journey begins in the mouth

In Grade 6 you met the processes every living being needs to survive: nutrition, respiration, excretion and reproduction. Together they are called life processes. This chapter looks closely at two of them: nutrition (how animals get and use food) and respiration.

Animals eat in very different ways. Bees and sunbirds suck nectar from flowers. Human babies and many young animals drink their mother's milk. Snakes like the python swallow their prey whole. Some water animals filter tiny bits of food floating around them.

Food gives animals energy. But food is made of big, complex parts — carbohydrate, protein and fat — that the body cannot use as they are. They must first be broken down into simpler forms.

This happens in a long tube called the alimentary canal, which runs from the mouth to the anus. As food moves along it, digestive juices made at different places break it down, and the simple parts are absorbed and carried around the body.

The journey starts in the mouth. Teeth crush and chew food into small pieces. This first breaking into fine pieces is called mechanical digestion.

Saliva does a different job. Have you noticed your mouth 'watering' when you think of your favourite food? That water is saliva. Chew a piece of chapati or boiled rice for 30 to 60 seconds and it starts to taste sweet. Chapati and rice contain starch (a carbohydrate), and saliva contains a digestive juice that breaks starch into sugar.

Activity 9.1 proves it. Put plain boiled rice in test tube A and chewed boiled rice in test tube B, add water to both, then add iodine solution. Tube A turns blue-black, because iodine turns blue-black with starch. Tube B shows little or no colour change, because saliva has already broken its starch into simple sugars.

Breaking complex food into simpler forms inside the body is called digestion. In the mouth, food is only partly digested; the rest happens further along.