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Digestion of Food

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Digestion of Food - Lesson Summary

Digestion is the process of converting complex food into simpler and absorbable molecules. Digestion begins in the oral cavity where the teeth masticate the food and the tongue mixes it with saliva. Saliva is rich in electrolytes and enzymes such as salivary amylase and lysozyme. Salivary amylase hydrolyses starch into maltose, whereas lysozyme kills microbes. Mucus lubricates the food and sticks the particles together to form a bolus. Thereafter, the bolus passes into the oesophagus by deglutition. Peristalsis helps the bolus to pass from the oesophagus into the stomach through gastro-oesophageal sphincter. The bolus in the stomach mixes with the gastric juices secreted by the gastric glands.

These glands are stimulated by the gastrin hormone and inhibited by the enterogastrin hormone and consist of mainly three types of cells – mucus neck cells which secrete mucus, peptic or chief cells which secrete pepsinogen and parietal or oxyntic cells which secrete hydrochloric acid and intrinsic factor. The churning of the stomach walls mixes these gastric gland secretions with the bolus and forms chyme. The proenzyme pepsinogen present in the chyme is converted into active pepsin by HCl.      

The chyme from the stomach passes through the pyloric sphincter and enters the duodenum, the first part of the small intestine. Here, the chyme mixes with pancreatic juice and bile. Bile contains bile pigments such as bilirubin and biliverdin, bile salts, cholesterol and phospholipids.  On the other hand, pancreatic juice consists of inactive enzymes such as trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. The enterokinase secreted by the intestinal mucosa, acts on inactive trypsinogen to form active trypsin and also converts inactive chymotrypsinogen into active chymotrypsin. The proteolytic enzymes trypsin, chymotrypsin and carboxypeptidase act upon the proteins, proteoses and peptones and convert them into smaller peptides.

Similarly, the pancreatic amylase hydrolyses the carbohydrates into simple sugars; lipases break down fats into diglycerides and monoglycerides while nucleases break down nucleic acids into nucleosides and nucleotides. The succus entericus secreted by the walls of the intestine contains enzymes such as disaccharidases, dipeptidases, lipases and nucleosidases, which further breakdown to simpler molecules to be absorbed in the jejunum and the ileum of the small intestine.
 
These undigested substances called faeces, enter the caecum of the large intestine through the ileo-caecal valve. The colon absorbs water, minerals and certain drugs in the faeces, from where it enters the rectum and is temporarily stored till defecation.

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