Class 11 Biology Digestion Absorption Digestion of food

Digestion of food

  • The process of digestion is accomplished by mechanical and chemical processes.

Digestion of oral cavity

  • The buccal cavity performs two major functions, mastication of food and facilitation of swallowing.
  • The teeth and the tongue with the help of saliva masticate and mix up the food.
  • Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.
  • The bolus is then conveyed into the pharynx and then into the oesophagus by the process called deglutition.
  • The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.
  • The salivary juice contains lysozyme and salivary amylase enzymes.
  • Lysozyme present in saliva acts as an antibacterial agent that prevents infections.
  • Salivary amylase acts on the starch and splits into a disaccharide maltose.


Digestion in stomach

  • The mucosa of stomach has gastric glands, which have three major types of cells
  • mucus neck cells which secrete mucus
  • peptic or chief cells which secrete the proenzyme pepsinogen
  • parietal or oxyntic cells which secrete HCl and intrinsic factor.
  • The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the
  • The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin.
  • Pepsin converts proteins into proteoses and peptones.
  • HCl provides the acidic pH (pH 1.8) optimal for pepsins.
  • Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins.

Digestion in small intestine

  • The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine.
  • Pancreatic juice and bile are released through the hepato-pancreatic duct.
  • The pancreatic juice contains inactive enzymes:
  • trypsinogen
  • chymotrypsinogen
  • procarboxypeptidases
  • amylases
  • lipases
  • Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin.
  • The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids.
  • Bile helps in emulsification of fats and also activates lipases.
  • The intestinal mucosal epithelium has goblet cells which secrete mucus.
  • The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the succus entericus.
  • The mucus along with the bicarbonates and Brunner’s glands from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium.
  • Proteins, proteoses and peptones in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice


  • Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.


  • Fats are broken down by lipases with the help of bile into di-and monoglycerides.


  • Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides


  • The enzymes in the succus entericus act on the end products of all the reactions.
  • These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.


  • The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine.
  • The undigested and unabsorbed substances are passed on to the large intestine.
  • The functions of large intestine
  • absorption of some water, minerals and certain drugs
  • secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.
  • The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter.


Fig. Goblet cells

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