Digestion - Nutrition in Human Beings

The food that we eat cannot be used by the cells in the body in the form in which it is eaten. Conversion of complex food material into smaller substances so that it can be absorbed by the cells is called digestion. Taking in of food is termed ingestion.

Digestive System

Alimentary canal is a long continuous tube constituted made by mouth, pharynx, oesophagus, stomach, small intestine, large intestine, and rectum. The glandular organs, salivary glands, liver and pancreas and the alimentary canal form the digestive system.


The process of digestion requires enzymes present in the digestive juices secreted by the organs of digestive system. They convert complex substances into simpler ones. Enzymes are chemicals which speed up chemical reactions taking place in cells. Almost all enzymes are complex proteins and remain unchanged during the chemical reaction. They can, therefore, be used repeatedly.

Process Involved in Nutrition

The entire process of nutrition includes the following steps: ingestion, digestion, absorption, assimilation and egestion.

A. Ingestion and Digestion

The process of taking in food through the mouth is called ingestion. The digestion of food starts from the mouth and ends in the small intestine.

1. Mouth: Carbohydrates, such as starch, are broken down or digested to form sugar. Saliva contains an enzyme salivary amylase that breaks down starch into sugar. It also helps in lubricating the food and making it easier for swallowing.

2. Oesophagus: There is no digestion in this part, also called gullet. The oesophagus or the food pipe by the contraction of muscles in its wall pushes the food into the stomach. Muscle movement is termed peristalsis and helped food travel down the alimentary canal.

3. Stomach: The stomach is a highly muscular organ. The gastric glands present in its walls secrete gastric juice containing hydrochloric acid (HCl) and enzymes like pepsinogen. HCl activates pepsinogen into pepsin and kills bacteria. Proteins are broken into smaller fragments called peptones by the enzyme pepsin.

4. Small intestine: The food moves from the stomach to duodenum, which is the upper part of the small intestine. Emulsification of fat (fat is broken into fat droplets) takes place with the help of the bile juice secreted by the live and stored in gall bladder. Bile does not have any digestive enzymes but it creates an alkaline medium which is essential for the action of pancreatic enzymes.

The pancreatic juice contains three enzymes:

  • Trypsin - converts peptones and proteoses to smaller peptides.
  • Amylase - converts starch into maltose.
  • Lipase - converts fats into fatty acids and glycerol.

The digestion of proteins into the end products amino acids, carbohydrates into glucose, and fats into fatty acids and glycerol is completed in the small intestine.

The inner surface of the small intestine contains thin finger-like projections called villi, which increase the surface area for absorption of digested food into the blood capillaries lining the villi. The blood then carries the absorbed food to different parts of the body and undigested food is pushed into the large intestine.

5. Large intestine: This part of the body absorbs water from the undigested food and solid waste is lubricated to form the faeces. The faeces pass on to the lower part of the large intestine, called the rectum, and are thrown out of the body through the anus.

B. Absorption

Blood capillaries in the villi pick up digested food and take it to all cells.

C. Assimilation

The absorbed food supplied to cells is used to release energy and also to build up the cell components. This is called assimilation.

D. Egestion

The process by which the undigested food material or waste is released from the body is called egestion.