HSEB Notes on Physiology of digestion
Lesson : Digestion
Class : 12
Man is omnivorous. It means man depends on plant materials and animal products. The food contains carbohydrate, protein and fats. The food is digested at different parts of alimentary canal.
Digestion in mouth
In mouth, the food is mixed with the saliva from salivary gland. Saliva contains the enzyme called salivary amylase. Salivary amylase acts on carbohydrate and changes some of carbohydrate into maltose, isomaltose and limit dextrine. Food remains in mouth for a short time so digestion is not complete.
Enzyme substrate products
Salivary amylase + starch ------------- maltose + isomaltase+ limit dextrine
Digestion in stomach
In the stomach, the gastric juice is secreted by gastric gland. Gastric juice contains pepsiongen, HCl and water. Peptic cells or zymogenic cells secrete pepsinogen. Parietal cells secrete HCl. Prorenin is found in infants.
Functions of HCl 1. converts inactive pepsinogen into active pepsin
2. provides acidic medium
3. stops growth of Bacteria
Pepsin changes inactive prorenin into active rennin. Pepsin acts on protein and changes it into peptones and proteoses. Rennin acts on milk casein. Milk casein is changed into paracasein. Paracasein combines with Calcium ion to form Calcium paracaseinate. It is then acted by pepsin and changed into peptones and proteoses. In the stomach, gastric lipase is also secreted. It acts on fats to change into fatty acids and glycerol.
Enzyme substrate products
Pepsin + protein --------- peptones + proteoses
Rennin + milk casein ------- paracasein
Paracasein + Ca ++ -- --- Calcium paracaseinate
Pepsin + Calcium paracaseinate --- peptones + proteoses
Lipase + fats -------------------- fatty acids and glycerol
Digestion in duodenum
The partially digested food called chime is passed on to the duodenum slowly. It is mixed with bile juice and pancreatic juice. The bile juice contains bile pigments and bile salts. The pancreatic juice contains enzymes like trypsinogen, amylase and lipase etc. The inactive trypsinogen is converted into active trypsin by enterokinase. Trypsin then acts on protein, peptones and proteoses and change into polypeptides. Amylase acts on starch and changes into maltose. Lipase acts on emulsified fat and change into fatty acids and glycerol.
The functions of bile 1. it neutralizes acids
2. it provides alkaline medium
3. it emulsifies fats
Enzyme substrate products
Trypsin + protein, proteoses, peptones ------ polypeptides
Amylase + starch ----------------------------- maltose
Lipase + emulsified fat ---------------------- fatty acids, glycerol
Digestion in jejunum and ileum
Here, intestinal juice is produced by crypts of Lieberkuhn. The intestinal juice contains aminopeptidase, dipeptidase, maltase, lactase, sucrase etc. the aminopeptidase acts on polypeptides and changes into smaller peptides. Dipeptidase acts on dipeptides and changes into aminoacids. Maltase acts on maltose. It is changed into glucose. Lactase acts on lactose to change that into galactose and glucose. Sucrase acts on sucrose and changed into fructose and glucose.
Enzyme substrate products
Aminopeptidase + polypeptides -------------------- smaller peptides
Dipeptidase + smaller peptides --------------- aminoacid
Maltase + maltose ------------------------- glucose
Lactose + lactose --------------------------- glucose and galactose
Sucrase + sucrose -------------------------- glucose and fructose
Absorption
Absorption takes place in the ileum mainly. For this, the internal lining of ileum is provided with large no. of finger like projections called villi. The villi increase the absorptive surface. These are provided with blood vessels and lymph. Glucose, aminoacid etc are absorbed into the blood vessel. Blood takes them to liver for storage of glucose. The fatty acids and glycerol come into the lymph which is taken into adipose tissue for storage.
Assimilation
Glucose, fatty acids and glycerol are used for the liberation of energy in the body. Aminoacids are used to produce protoplasm in the body for growth and repair of damaged tissue.
Egestion
Undigested materials are passed into large intestine. Water is absorbed here. Then these are carried down. Lastly undigested materials are discharged out from anus. It is called egestion.
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