Monday, July 11, 2011

Class IX, BIOLOGY, "Digestion of Food"

Digestion of Food

First of all food comes in the oral cavity where the teeth crush and break the food and convert it into small particles. The tongue rolls the morsel of food and pushes it under teeth again and again so that the food is evenly divided into fine particles and the saliva secreted from the salivary glands gets mixed with the food. The saliva lubricates the food and makes the particles adhere to one another, forming a ball of food called bolus. Now the chemical digestion of food begins. Saliva contains an enzyme to digest starch in the food. The combined action of teeth, tongue and saliva pushes the bolus through the throat into the oesophagus, and then it reaches the stomach.

Definition of Digestion

Digestion is the process in which the insoluble and non-diffusible components of food are broken down and by the action of enzymes are converted into soluble and diffusible substance to be absorbed into the blood stream.

Types of Digestion

1. Mechanical digestion
2. Chemical Digestion

1. Mechanical Digestion
In mechanical digestion, the food consisting of large sized particle is broken into fine pieces by cutting, grinding, chewing and churning up, so that enzymes can act upon it efficiently and effectively. Mechanical digestion of food takes place in the mouth and stomach.

2. Chemical Digestion
In chemical digestion, the digestive enzymes mix with the food and act upon it to break it down further into simple and diffusible chemical forms. The enzymes act on carbohydrates, proteins and fats separately. Chemical digestion takes place in all the major parts of the digestive system. The digestive glands such as liver and pancreas also play very important role in this digestion.

Digestive System

All living things require food to live and carry on their life functions. Animals are unable to synthesize their food.
Digestion is the process in which the non-diffusible molecules of food are changed to diffusible ones by the action of enzymes. All the organs which take part in this process make a system which is called the digestive system.

Human Digestive System

The process of digestion takes place in the alimentary canal. It starts from the mouth and ends at the anus. The tube assumes different shapes according to their role in the process of digestion. It consists of the mouth, oesophagus, stomach, small intestine, and large intestine. Besides these organs liver and pancreas, also play important roles in digestion.


The muscles of alimentary canal produce rhythmic waves of contraction which is called peristalsis. Due to this process, food is carried through various parts of the alimentary canal.


The food of animals and human is in the solid form and may be bulky. Taking in of the food in the oral cavity and swallowing is called ingestion.

Digestion of Food in the Mouth

During mastication, the food is mixed thorougly with the saliva while the food is in the oral cavity (buccal cavity). The saliva is secreted by three pairs of salivary glands located in the buccal cavity. The saliva is continuously secreted by the salivary glands in response to the presence of food in the buccal cavity.
Saliva is alkaline and contains an enzyme ptyalin. This enzyme converts starch into sugar (maltose). The morsel of food after being chewed and thoroughly mixed with the saliva is called a bolus. It is rolled down by the swallowing action into the oesophagus which conveys it to the stomach by the wave of peristalsis. The end of stomach lined with oesophagus is called cardiac end.

Digestion of Food in the Stomach

Stomach is a thick sac like structure, in which food is stored for some time. Its wall is strong and muscular. Its inner surface has numerous glands called gastric glands. These glands secrete a juice called gastric juice. Human stomach secretes about one to two liters of this juice daily; Gastric juice contains Hydrochloric acid and two enzymes, renin and pepsin. Hydrochlroic acid changes the medium of food to acidic. This medium kills the bacteria that may be found in the food. the pepsin acts on proteins and breaks them down into peptones. Renin helps to curdle milk in infants. There is no chemical action on carbohydrates and fats present in food. the regular movements of the stomach churn up the food. the food is changed into a thick fluid called chyme. When digestion in the stomach is complete, the distal end of the stomach called the pyloric end relaxes, and allows a small amount of chyme to pass into the first part of the small intestine. Food stays in stomach for about 2-3 or 3-4 hours.

Digestion of Food in the Small Intestine

Food from stomach enters the duodenum which is the first part of the small intestine. An alkaline pancreatic juice from the pancreas and bile juice from the liver and poured into the duodenum by a common duct. Both the juices contain bicarbonates which neutralize the acidic chyme and make. It rather alkaline besides these juices other intestinal juices from the walls of the small intestine are also poured. These entire juices act on food and help in digestion of food.


It is largest gland, in the body. Its colour is reddish brown. It lies just below the diaphragm on the right side of the body under the ribs. It has five lobes, three on the right side and two on the left. The cells of the liver secrete a greenish yellow alkaline fluid which is called the bile juice. It contains bile salts and bile pigments which give greenish yellow colour to the juice. Bile contains no digestive enzymes, but it does contain bile salts which break down the large molecules of fats to small fat droplets. This process is called emulsification. This process helps in the digestion of fats. Bile juice also contains bile pigments that are by products of red blood cells, these pigments are eliminated from the body along with the faeces, and the colour of faeces is due to these pigments. Besides this, bile juice also kill the germs in the food.

Functions of Liver

1. Liver stores glycogen and regulates the level of glucose in the blood.

2. It breaks down excess amino acids. this process is called deamination.

3. It is involved in detoxification.

4. It produces and secretes bile juice which is stored in the gall bladder.

5. It metabolizes carbohydrates, fats, proteins and other compounds.

6. As a result of chemical changes a lot of heat is produced, therefore liver helps to keep the body warm.

7. It makes fibrinogen and other blood proteins.

8. It decomposes the damaged red blood cells.


It is a leaf like organ. It lies below the stomach and between the two arms of duodenum. The pancreas produces a juice which is called the pancreatic juice. This juice flows down the pancreatic duct into the duodenum. It contains three enzymes.

1. Pancreatic amylase which acts on undigested starches of the food and converts them into maltose.

2. Enzyme trypsin which breaks down proteins into peptides.

3. Lipase which splits fats into fatty acids and glycerol.

If any of the constituents of food still remain undigested, enzymes secreted by the glands in the small intestine act upon them and complete the digestion by converting peptides to amino acids, maltose and other sugars to glucose and fats to fatty acids and glycerol.

The enzymes secreted by the intestinal walls are amino-peptidases and disaccharidase,(which form glucose from maltose, lactose and sucrose). In this way food is completely digested at intestine.

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