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How is food digested in the human body?

Class 7 ScienceNCERT chapter: Life Processes in Animalsइसे हिंदी में पढ़ें

Exam-ready answer

Digestion is the breaking down of complex food into simple, soluble substances. In the mouth, teeth chew food and saliva converts starch into sugar. The stomach adds hydrochloric acid and digestive juices that act on proteins. In the small intestine, bile, pancreatic juice and intestinal juice complete digestion, and villi absorb the digested food into the blood.

Explanation

Food passes through the alimentary canal: mouth, food pipe (oesophagus), stomach, small intestine, large intestine, rectum and anus. In the mouth, teeth cut and grind food, and the tongue mixes it with saliva from the salivary glands. Saliva begins digestion by converting starch into sugar. The food is then pushed down the food pipe by wave-like movements of its walls.

The stomach is a thick-walled bag. Its inner lining releases mucus, hydrochloric acid and digestive juices. The acid kills many bacteria that enter with food and makes the medium acidic, which the digestive juices need to begin breaking down proteins. The mucus protects the stomach lining from the acid. The partly digested food then moves into the small intestine, a highly coiled tube about 7.5 metres long.

In the small intestine, bile from the liver helps digest fats, and pancreatic juice from the pancreas acts on carbohydrates, fats and proteins. Intestinal juice completes the job: carbohydrates become simple sugars like glucose, fats become fatty acids and glycerol, and proteins become amino acids. Finger-like villi absorb these into the blood. The large intestine absorbs water and some salts, and undigested waste leaves through the anus.

Points to remember

  1. Digestion: breakdown of complex food into simple, soluble substances the body can absorb.
  2. Alimentary canal: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.
  3. Saliva converts starch into sugar; stomach acid and juices begin the digestion of proteins.
  4. Bile from the liver and pancreatic juice from the pancreas act on food in the small intestine.
  5. Villi of the small intestine absorb digested food; the large intestine absorbs water and some salts.

Exam tip

Draw and label the human digestive system, then make a table of each organ, the juice it receives and the food that juice acts on. End with the final products: glucose, fatty acids and glycerol, and amino acids.

Questions students also ask

Why does a piece of chapati taste sweet after chewing it for a while?
Chapati contains starch. Saliva in the mouth converts this starch into sugar, so after chewing for some time the chapati begins to taste slightly sweet. This shows that the digestion of carbohydrates starts in the mouth.
What is the role of villi in the small intestine?
Villi are tiny finger-like projections on the inner wall of the small intestine. They greatly increase the surface area for absorption. Digested food passes through the walls of the villi into the blood vessels inside them and is carried to all parts of the body.
What are absorption and assimilation?
Absorption is the passing of digested food through the walls of the intestine into the blood. Assimilation is the use of this absorbed food by body cells to build new parts of the body and to get energy.

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