The Roles of Different Enzymes in the Digestion Within the Human Alimentary Canal : a Guide for Students

INTRODUCTION

DIGESTION is the process by which complex food materials are broken down into simple food materials. In Human alimentary canal mainly three parts are involved in digestion and these are a) Buccal cavity, b) Stomach and c) Small intestine.

Mainly three types of foods are digested in Human alimentary canal, these are :
a) Carbohydrate b) Protein and c) Fat.

DIGESTION IN BUCCAL CAVITY

A. Digestion of Carbohydrate in buccal cavity :

1. Both mechanical and chemical digestion of complex carbohydrate starts in the buccal cavity. The mechanical process which breaks the food
materials and mixes with saliva is called chewing. The product of which is called Bolus.

2. Human Saliva contains inorganic and organic chemical compounds. The most important amylolytic enzyme secreted in buccal cavity by the salivary
glands (Parotid gland, Submaxillary gland and sublingual gland) is Ptyalin or salivary amylase which is a weak enzyme.

3. Mode of action of Ptyalin is given below :

A. Boiled starch + Ptyalin = Soluble starch ;

B. Soluble starch + Ptyalin = Erythrodextrin + Maltose ;

C. Erythrodextrin + Maltose + Ptyalin = Acrodextrin + Maltose ;

D. Acrodextrin + Maltose + Ptyalin = Isomaltose + Maltose.

4. Although Ptyalin reacts with starch in the buccal cavity, but the action of Ptyalin also occurs in the stomach before the concentration of HCl becomes
adequate.

B. Digestion of Protein and fat in buccal cavity :

1. Due to absence of proteolytic and lipolytic enzyme in saliva, protein and fat digestion do not occur in the buccal cavity. But, Lingual lipase is found recently in
the saliva.

Note : Lysozyme is an enzyme present in the saliva that does not take part in the digestion but kills the germs (mainly bacteria) of food. Saliva also contains an
enzyme called Kallikrein that acts on plasma protei to produce Bradykinin.

DIGESTION IN STOMACH

A. Digestion of Carbohydrate in stomach :

1. Due to Peristalsis movement in the oesophagus, the bolus enters into the stomach. The wall of the stomach contains numerous gastric glands which
secrete gastric juice.

2. Mode of secretions of stomach cells are given below :