Definition-Digestion is a process of enzymatic hydrolysis in which covalent bonds of the food are broken down to form monomeric units.

The component of the normal diet includes carbohydrates, lipids, proteins, vitamins, minerals and water. Out of these constituents water, vitamins, minerals, free monosacharides and free amino acids can be absorbed interest of body absent such. However, complex carbohydrates, lipids and proteins are high molecular weight substances. These substances can not be absorbed till these are structurally simplified into absorbable molecular forms. This chemical conversion of these molecules into their simpler forms is called digestion. In mammals digestion is extracellular absent it occurs in the lumen of various parts of the alimentary canal.

Digestion in mouth cavity- In mouth cavity, food is chewed to break them into smaller particles. Saliva secreted from salivary gland helps in the process. It is slightly acidic in nature and contains an enzyme called salivary amylase or ptyalin. Carbohydrate digestion starts in the buccal cavity by amylase. Gradual reduction of carbohydrate or starch occurs through the following steps:

Strach – Ptyalin — Erythrodextrin


Erythrodextrin — Ptyalin — Archodextrin.

Archodextrin — Ptyalin — Maltose

No other food stuff is hydrolysed in the mouth cavity.

Digestion in the stomach: – As the bolus enters the stomach starch digestion stops. The gastric glands secret pepsinogen and HCl. Pepsinogen is activated by HCl to become active pepsine that starts hydrolysis of protein. Pepsin changes proteins into proteoses, peptones and polypeptides.


Pepsinogen – Activator / HCl — Pepsin

Protein — Pepsin / +H2O — Proteoses

Proteoses – Pepsin /+H2O — Peptones

Peptones – Pepsin / +H2O — Polypeptides


In young mammals milk, protein, caseinogens is digested by the help of an enzyme called rennin. It is secreted from the fundus absent prorennin which converts to active rennin in the presence of HCl. It curdles milk and caseinogens is precipitated to solid casein. The Ca++ present in milk changes it to calcium caseinogens so that pepsin can reduce it into proteoses and peptones.

Proennin – HCl — Rennin

Caseinogen – Rennin — Casein

Casein — Rennin / HCl — Paracasein


Paracasein + Ca++ — Calcium paracaseinate.

Calcium paracaseinate — Pepsine — Protoeoses and peptones.

Inside the stomach due to hydrolysis of protein and grinding action of its muscular wall the food is converted to a paste like pulp called chyme that passes into the small intestine.

Digestion in Intestine:



Being stimulated by acidic chime the mucous cells of duodenum secrets cholecystokinin and secretin hormones into blood which induce the gall bladed and pancreas respectively to release their contect into the duodenum through their ducts. Bile from gall bladder affects alkaline medium in the intestine for the ferments to work to their optimum level.Pancreatic juice contains trypsinogen, chymotrypsinogen, pancreatic amylase, lipase, maltase, sucrose, lactase, etc. enzymes. Brunner’s glands and crypts of Leiberkuhn among villi also secret intestinal juice or succus entericus which have enterokinase, peptidases, etc. Brush border of the mucous lining also secrets some of the enzymes. Enterokinase activates trypsinogen to trypsin which changes chymotrypsinogen into chymotrypsin. Now chymotrypsin hydrolyses proteoses, peptones and polypeptides interest opeptides and amino acids.

(a) Carbohydrate:

Carbohydrates are attacked by the pancreatic and intestinal amylases. In active amylopsin secreted by pancreas becomes amylase in alkaline medium. The starch molecules are converted into maltoses, sucroses and lactoses.maltase, sucrose and lactase break them to monosaccharides.


Starch — amylase / pH 8-9 — Maltose, sucrose and lactose

Maltose – Maltase — Glucose

Lactose – lactase — Glucose +Galactose

Sucrose – Sucrase — Glucose + Fructose.

The cellulose present in food remains undigested. But in herbivorous mammals, it is digested in the caecum by the symbiotic becteria and protozoans present there.

(b) Fat Digestion:

A small amount of fat is digested in the stomach by gastric lipase present in small quantities and active in acidic medium. Almost all the fat is digested in the small intestine by pancreatic lipase. The bile salts emulsify fat into a soapy substances so that small droplets are formed. Hence lipase now gets much more surface area to act on fat. Fat is converted into fatty acids and glycerol absent the final product of fat digestion.

Fat — Lipase / pH 8-9 — Fatty acids + Glycerol