Role of Digestive Enzymes

Role of Digestive Enzymes The process of digestion converts the ­solid food into absorbable and assimilable forms. This is accomplished by mechanical and chemical processes. Digestion in the Buccal Cavity The smell, sight and taste as well as the ­mechanical stimulation of food in the mouth, triggers a reflex action which ­results in the secretion of saliva. The ­mechanical digestion starts in the mouth by grinding and chewing of food. It is called mastication. The saliva contain water, electrolytes (Na + , K + , Cl – , HCO – ) , salivary amylase (ptyalin), antibacterial agent lysozyme and a lubricating XI Std Biology-Zoology Chapter- cholesterol and phospholipids but has no enzymes. Bile helps in emulsification of fats. Bile salts reduce the surface tension of fat droplets and break them into small globules. Bile also activates lipases to digest lipids Proteins and partially digested proteins in the chyme on reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice.The pancreatic juice contains enzymes such as trypsinogen, chymotrypsinogen, carboxypeptidases, pancreatic amylases, pancreatic lipases and nucleases. ­Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the enzyme chymotrypsinogen in the pancreatic juice. Digestion in the Small Intestine The bile, pancreatic juice and intestinal juice are the secretions released into the small intestine. Movements generated by the muscularis layer of the small intestine helps in the thorough mixing of the food with various secretions in the intestine and thereby facilitate digestion. The bile contains bile pigments (bilirubin and biliverdin) as the break down products of hemoglobin of dead RBCs, bile salts, Figure . The Stomach and Gastric Secretions Oesophagus Cardiac sphincter Pyloric sphincter Small intestine Gastric rugae Stomach Epithelium Pepsinogen Pepsin HCI Cl - H + . Pepsinogen and HCI are secreted. . HCI converts pepsinogen to pepsin. . Pepsin digests proteins into peptones. Chief cell Parietal cell Bicarbonates in the saliva make the pH . to . . If the bicarbonates level in saliva is reduced the saliva becomes acidic and the tooth enamel may get dissolved. What would happen if HCl is not secreted in the stomach? XI Std Biology-Zoology Chapter- Trypsin hydrolyses proteins into polypeptides and peptones, while chymotrypsin hydrolyses peptide bonds associated with specific amino acids. The pancreatic amylase converts glycogen and starch into maltose. Lipase acts on emulsified fat (triglycerides) and hydrolyses them into free fatty acid and monoglycerides. Monoglycerides are further hydrolysed to fatty acid and glycerol. Nucleases in the pancreatic juice break the nucleic acid into nucleotides and nucleosides. The secretions of the Brunner's gland along with the secretions of the intestinal glands constitute the intestinal juice or succus entericus . The enzymes in the intestinal juice such as maltase, lactase, sucrase (invertase), peptidases, lipases, nucleotidases and nucleosidases act on the breakdown products of bile and pancreatic digestion. Maltose Maltase glucose + glucose Sucrose Sucrase glucose + fructose Lactose Lactase glucose + galactose Dipeptides, Tripeptides Peptidase amino acids Nucleotides Nucleotidase Nucleoside + Phosphoric acid Nucleoside Nucleosidase Sugar + ­Nitrogen base Diglycerides and monoglycerides