by • 24/07/2011 • Old Pattern Biology NotesComments (0)733


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.


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.


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.


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.


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.

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