16.2 DIGESTION OF FOOD

16.2 DIGESTION OF FOOD:
  • The process of digestion is accomplished by mechanical and chemical processes. 
1. Digestion in buccal (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 thoroughly. 
  • 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 swallowing or deglutition. 
  • The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis. 
  • The gastro-oesophageal sphincter controls the passage of food into the stomach. 
The saliva:
  • The saliva secreted into the oral cavity contains electrolytes (Na+, K+, Cl–, HCO3–) and enzymes, salivary amylase and lysozyme. 
  • The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase. 
  • About 30 per cent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose. 
  • Lysozyme present in saliva acts as an antibacterial agent that prevents infections.
2. Digestion in the stomach
  • The mucosa of stomach has gastric glands. 
  • Gastric glands have three major types of cells namely -
(i) mucus neck cells which secrete mucus; 
(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and
(iii) parietal or oxyntic cells which secrete HCl and intrinsic factor (Castle's intrinsic factor essential for absorption of vitamin B12). 
  • The stomach stores the food for 4-5 hours. 
  • The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme. 
  • The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach. 
  • Pepsin converts proteins into proteoses and peptones (peptides). 
  • The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid. 
  • 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. 
  • Small amounts of lipases are also secreted by gastric glands.
3. Digestion in the small intestine
  • Various types of movements are generated by the muscularis layer of the small intestine. 
  • These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion. 
  • 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 :
  • The pancreatic juice contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. 
  • Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice. 
  • The mucus alongwith the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. 
  • Sub-mucosal glands (Brunner’s glands) also help in this. 
  • Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice as given below:
NCERT Class XI Biology: Chapter 16 - Digestion And Absorption ...
  • The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes. 
  • Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles
  • Bile also activates lipases. 
The intestinal juice or succus entericus:
  • The intestinal mucosal epithelium has goblet cells which secrete mucus. 
  • The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus. 
  • This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. 
Process of Digestion: Digestion Process in Mouth, Stomach, Intestines
  • The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms. 
  • These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.
  • The breakdown of bio-macromolecules mentioned above occurs in the duodenum region of the small 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. 
  • No significant digestive activity occurs in the large intestine. 
  • The functions of large intestine are: 
  • (i) absorption of some water, minerals and certain drugs; 
  • (ii) 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. 
  • It is temporarily stored in the rectum till defaecation.
Rgulation of digestion:
  • The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts. 
  • The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva. 
  • Gastric and intestinal secretions are also, similarly, stimulated by neural signals. 
  • The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS. 
  • Hormonal control of the secretion of digestive juices is carried out by local hormones produced by the gastric and intestinal mucosa.

Biology Champ | Gastro-Intestinal Hormone

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