Explain the process of digestion in man.

Explain the process of digestion in man.

The main objective of digestion is to convert complex food molecules in to simple. Which can easily absorbed by the body cells. This process include ingestion, digestion, absorption, assimilation and egestion or defaecation.

1. 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

2. 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

3. 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

4. Digestion in Jejunum and Ileum: 

Here, intestinal juice is produced by crypts of Lieberkuhn. The intestinal juice contains amino peptidase, dipeptidase, maltase, lactase, sucrase etc. the amino peptidase acts on polypeptides and changes into smaller peptides. Dipeptidase acts on dipeptides and changes into amino acids. 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

Amino peptidase + polypeptides -------------------- smaller peptides

Dipeptidase + smaller peptides --------------- amino acid

Maltase + maltose ------------------------- glucose

Lactose + lactose --------------------------- glucose and galactose

Sucrase + sucrose -------------------------- glucose and fructose

5. 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, amino acid 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.

6. Assimilation: 

Glucose, fatty acids and glycerol are used for the liberation of energy in the body. Amino acids are used to produce protoplasm in the body for growth and repair of damaged tissue.

7. 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.