Complex food stuffs like carbohydrates, fat and protein need to be digested or hydrolysed before assimilation. The final product of carbohydrate digestion is monosacharides, that of fats are fatty acids and glycerol and of protein are amino acids. These food stuffs are digested in different parts of the alimentary canal in different pH media.

(1) Carbohydrate Digestion:

Hydrolysis of carbohydrate starts in the buccal cavity. The food here is masticated thoroughly with the help of teeth, tongue, cheek muscles and saliva. Saliva is alkaline and has starch splitting enzyme the ptyaline or amylase. It converts insoluble starch into soluble sugars like maltose and dextrin.

Starch Ptyalin — Maltose, Maltotriose and dextrin.


In stomach, the medium is acidic and action of ptyalin in stopped byHCl. However 5% of starch is hydrolysed by HCl.

In duodenum, the medium is again alkaline due to bile. Here the partially digested starch comes in contact with the pancreatic and intestinal juices. The pancreatic juice contains amylopsin that acts upon starch and glycogen and converts them into maltose (disaccharide). The intestinal amylase (disaccharidases like sucrose, lactase, maltase, etc.)Now act upon the disaccharides and complete the digestion of starch. Their actions can be summerised as follows:

Starch amylase — Maltose+ Maltotriose +dextrin

Maltose – Maltase — Glucose


Lactose – lactase — Glucose +Galactose

Sucrose – Sucrase — Glucose + fructose.

(2) Fat digestion:

Fats are hydrolysed to form fatty acid, glycerol and glycerides: A small part of fat is digested in the stonmach by the action of gastric lipase in acidic medium.


Fat + HCl – Gastric / Lipase — Fatty acid, Glycerol

(50%, mono-and diglycerides 50%)

Alongwith chyme fat, fatty acids and glycerols enters the duodenum and bile salts with their detergent action emulsify fat into small globules called emulsions. Thus, pancreatic and intestinal lipases get more surface area to act.

Fat + Bile — Emulsified fat


Emulsified fat lipase –Fatty acid — Glycerol (mono-and diglycerides)

Fat is not completely digested. A portion of fat remains suspended in the form of very small droplets (mono-and diglycerides) and diffuse into lacteals present in the villi.

(3) Protein Digestion:

Protein digestion is initiated in the stomach by the enzyme pepsin in acidic medium.


(4) Pepsin attacks the peptide bonds of protein molecules and breaks them into metaproteins proteoses, peptones and polypeptides. The proteoses are the largest fragments of protein molecules. Peptones are smaller and polypeptides are the smallest containing very few amino acids. Pepsin acting for longer time can break protein into amino acids. But due to the short stay of food in stomach semeidigested protein enter the intestine.

In young mammals milk, protein occurs in the stomach. The chief protein of milk caseinogens. An enzyme rennin curdles milk and caseinogens is converted to solid caseinogens is converted to solid casein. The Ca++ present in the milk combines with casein to change it into calcium caseinate. Protein digestion in the stomach can be summarized absent follows:

(a) Pepsinogen — Activator / HCl — Pepsin

Protein — Pepsin / +H2O — Proteoses


Proteoses — Pepsin / +H2O — Peptones

Peptones — Pepsin / +H2O — Polypeptides

(b) Proennin — HCl — Rennin

Caseinogen — Rennin – Casein

Casein – Rennin / HCI — Paracasein

Paracasein + Ca++ — Calcium paracaseinate.

Calcium paracaseinate + Pepsine — Protoeoses and peptones.

In the deuodenum, the semidigested proteins become alkaline to be completely digested into amino acids. Protoeses, peptones and polypeptides are further broken by the enzyme trypsin and chymotrypsin coming from the pancreas. These enzymes are active in the alkaline medium which is maintained by the bile.

Enzyme enterokinase, peptidases etc.present in the succus entericus and the process of intestinal digestion. The following steps of reaction occur in the small intestine:

Trypsinogen – Enterokinase — Trypsin

Chymotrypsinogen +Trypsin — Chymotrypsin

Proteoses >

Peptones > — Chymotrypsin — Peptides / pH 8-9 + Amino acids

Polypeptides >

Peptides – Peptidases / pH 8-9 — Amino acids