Digestion in Human Beings

Class 7 Science · Chapter 9: Life Processes in Animals · NCERT Curiosity section 9.1.1

Learn

At lunch, Ishaan chews a piece of chapati for a whole minute for a dare. "Hey, it's turning sweet!" he says. Kabir is puzzled. Food contains carbohydrates, proteins and fats that are too complex for the body to use directly. They must be broken down into simpler forms: digestion. This happens in a long tube from mouth to anus, the alimentary canal.

In the mouth, teeth crush and chew food into small pieces (mechanical digestion). Saliva moistens it and breaks starch into sugar, which is why chapati turns sweet. The tongue pushes the soft lump into the food pipe (oesophagus), whose walls contract and relax in waves to push food down. Brush twice a day and rinse after meals to keep teeth healthy.

The stomach churns food and adds digestive juice, acid and mucus. The juice breaks down proteins; the acid helps and also kills harmful bacteria; the mucus protects the stomach lining. Food leaves as a semi-liquid. Much of this was first learnt in 1822, when a doctor could watch digestion through a patient's wound that never fully closed.

The small intestine, about 6 m long, is the longest part. The liver sends bile (mildly basic), which neutralises acid and breaks fat into droplets; the pancreas sends a basic juice that digests carbohydrates, proteins and fats; the intestine wall adds its own juice. Thousands of tiny finger-like projections then absorb the simple nutrients into the blood.

Undigested food passes to the large intestine (about 1.5 m, wider but shorter), which absorbs water and salts, making semi-solid stool. Helpful bacteria there break down fibre; fibre-rich and fermented foods like curd and buttermilk keep it healthy. Stool is stored in the rectum and removed through the anus: egestion.

Explore: does saliva change starch?

1. Test each tube with iodine and sort it.

Explore: the journey of food.

1. Put the parts in order.

Remember

• Digestion: mouth (chewing, saliva on starch) → food pipe → stomach (acid, protein) → small intestine (bile, pancreatic juice, absorption).
• The large intestine absorbs water; stool leaves through the anus (egestion).

Quick check

Where are most digested nutrients absorbed into the blood?

1. small intestine
2. stomach
3. large intestine
4. food pipe

Think about it

Why might someone with very little stomach acid fall ill more often from bad food?

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.