DIGESTION AND ABSORPTION

 

Digestive glands :

• The digestive glands associated with the alimentary canal includes-
  • Salivary gland
  • Liver
  • Pancreas.
  
  
• There are three pairs of salivary gland present in the buccal cavity.
  • Parotid gland (below internal ear)
  • Sub-maxillary / submandibular (below lower jaw)
  • Sub-lingual (below tongue)
  
  
• All salivary glands produce saliva into the buccal cavity.

Liver :

• Largest gland of the body weighing about 1.2 to 1.5 kg in adult.
• Located below diaphragm and has two lobes.
• Structural and functional unit of liver is the hepatic lobules.
• Hepatic lobules consist of hepatic cells arranged in the form of cords.
• Each lobule is covered by a thin connective tissue sheath called Glisson’s capsule.
• The bile secreted by the hepatic cells passes through the hepatic ducts and stored in the gall bladder in concentrated form.
• Bile from the gall bladder is transported by cystic duct.
• Cystic duct along with hepatic duct forms the common bile duct.
• Bile duct joined with pancreatic duct to form hepato-pancreatic duct which open into the duodenum.
• Hepato-pancreatic has a swelling portion called ampulla of Vater; the opening is guarded by sphincter of Oddi.

Pancreas :

• Pancreas is a compound myxocrine gland (both exocrine and endocrine) elongated organ situated between the limbs of ‘U’ shaped duodenum.
• The exocrine aciner cells secrete pancreatic juice containing enzymes.
• The endocrine Islets of Langerhans secrete hormones like insulin and glucagon.

DIGESTION OF FOOD :

• Digestion is accomplished by mechanical and chemical process.

In the buccal cavity :

• Buccal cavity performs two major functions;
  • Mastication of food.
  • Facilitation of swallowing.
  
  
• The teeth and tongue with the help of saliva masticate and mix up the food.
• The saliva composed of ;
  • Electrolytes (Na⁺, K⁺, Cl^- HCO^-₃)
  • Enzyme- salivary amylase or ptyalin.
  • Lysozyme.
  
  
• About 30% of starch is hydrolyzed into disaccharide (maltose) by salivary amylase in optimum pH 6.8).
• Lysozyme acts as antibacterial agent preventing infections.
• Mucus in the saliva helps in lubricating and adhering the masticated food particle into a bolus.
• The bolus is then passed into oesophagus through pharynx by swallowing or deglutition.
• By peristalsis the bolus from the oesophagus passed into the stomach.

In the stomach :

• The mucosa of stomach has gastric glands.
• Gastric glands have three major types of cells namely –
  • Mucus neck cells – secretes mucus.
  • Peptic or chief or zymogen cells – secretes proenzymes pepsinogen.
  • Parietal or oxyntic cells – secretes HCl and castles intrinsic factor (factor essential for absorption of vitamin B₁₂)
  
  
• The stomach stores the food for 4-5 hours.
• The food mixed with the acidic gastric juice and form chyme.
• Pepsinogen converted into active pepsin in presence of HCl.
• Active pepsin converts proteins into proteose and peptones (peptides).
• Mucus and bicarbonate ions play important role in lubrication and protection of mucosal epithelium from excoriation by HCl and active enzymes.
• HCl provides the acidic pH of stomach (pH1.8)
• Rennin is an enzyme present in gastric juice helps in digestion of milk proteins.
• Small amount of lipases are present in gastric juice helps in digestion of fats.

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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