Digestion of Carbohydrates

 Digestion of Carbohydrates:

Human food contains three types of carbohydrates, namely, polysaccharides, Oligosaccharides and disaccharides, which need digestion. The polysaccharides include starch, glycogen and cellulose. The Oligosaccharides include dextrins. The disaccharides are sucrose ( cane sugar) and  lactose ( milk sugar). These are digested by Carbohydrases to monosaccharides glucose, fructose and galactose. 

Carbohydrates: Pryalin ( salivary amylase) present in saliva converts cooked starches in food into a sugar called maltose. This conversion occurs in the mouth. All sugars are converted to simple monosaccharides like glucose by the action of enzymes ( sucrase, maltose, lactase) in the small intestine.Glucose is absorbed through the capillaries of villi in the small intestine. It is then carried to liver by portal vein it is stored as glycogen.

1):Buccal Cavity ( Mouth):

The food receives saliva in the buccal cavity. The salive contains enzymes salivary amylase,or ptyalin, and lysozyme. Ptyalin is activated by chloride ions present in the saliva itself. Pytalin splits starch and glycogen first into dextrins and then into double sugars maltose and isomaltose and small dextrins called " limit dextrins'. Salivary digestion in oral cavity is restricted because of the shorter time the food is retained here. However, about 30% of the starch in the food is hydrolysed in the cavity .Lysozyme kills microbes.

Starch and Glycogen➡️➡️➡️Maltose + 
Isomaltose + 'Limit' Dextrins

Mastication mixes the food with saliva and breaks it into small particles with larger total surface area. This facilities the action of pytalin. Water and mucin of saliva help the teeth in mastication by moistening the food.
It can be divided into two parts:

1): Vestibule: Vestibule, an outer part. It lies between lips and inner lining of cheeks externally and gums and teeth internally.

2): Cavity of mouth an inner party. It is bounded by teeth and mastoid bone at the sides, palate above and tongue below.

Palate forms the roof of mouth cavity. It consists of hard palate which is in front and soft palate which  is behind. Uvula is a conical process which hangs from the middle of soft palate. Two folds of mucous membranes called anterior and posterior pillars of fauces lie on either side of uvula. Tonils lie in between these folds.

The important structure of  mouth are:

  1): Tongue   2): Teeth   3): Salivary glands

1): TONGUE: 

Tongue lies in the floor of the mouth and it is attached to hyoid bone. Tongue contains.

1): A root at which blood vessels and nerves pass.

2): A tip which is pointed when the tongue is protruded and rounded when the tongue is in the mouth.

3): Two margins which are in contract with lower teeth. 

4): An upper surface which contains a small elevation called dorsum.

5): A lower surface which contains a soft ligamentous structure called frenulum 

The two important structure of tongue are:

1): Taste buds which are on the lateral aspects of tongue.

2): Three types of  papillae present on the upper surface. They are: Circumvallate papillae, Fungiform papillae and Filiform papillae 

2): Teeth:

Teeth are concerned with mastication. Depending on the age at which they arise, teeth can be classified into two types:

1) Permanent teeth     the teeth of adult life

2) Temporary or milk teeth_  the teeth of childhood. 

Permanent teeth: They are 32 in number and 16 are present in each jaw. Each half of the upper and lower jaw contains 8 teeth. They are : 2 incisors, 1 canine, 2 premolars and 3 molars.

Temporary teeth:  They are 20 in number and each jaw has 10 teeth. Each half of the jaw has 2 incisors, 1 canine and 2 molars.

Eruption of teeth: Even at birth, all the permanent and temporary teeth are buried in the alveolar process of jaws. The first tooth appears in a child at the 7th month. Later, the full set of temporary teeth is completed at the age of two years. 

    The first permanent tooth to appear is the first molar which appears at the age of 6 years. The set of permanent teeth is completed at the age of 12 years. But the third molar ( wisdom tooth) appears between 17 and 25 years.

Structure of tooth: Tooth contains:

1):  A crown _ which projects above the gum.

2): A neck _ which is surrounded by gum.

3): A root_ which lies beneath the gum.

Tooth is made of : 

1): Dentine _ the main mass and it is a hard material. 

2): Pulp _ the central cavity which contains connective tissues, blood vessels and nerves.

3): Enamel _ the covering , a part of which projects above the gums.


There are three pairs of salivary glands in the mouth. They are parotid, submandibular and sublingual glands.

Parotid glands : One on each side is present below and in front of each ear. Each gland has a duct called Stenson's duct. This duct opens on  the inner side of cheek opposite to the second upper molar tooth.

Submandibular glands( Submaxillary glands): They are smaller than parotid glands. One on each side lies under the angle of jaw. Each gland has a duct called Wharton's duct which opens near the mid line under the tongue.

Sublingual glands: They are the smallest salivary glands which lie under the tongue. They pour the secretions directly into the mouth through several small openings. 


It is a mixed secretion of all the three pairs of salivary glands. It is an alkaline fluid containing water to the extent of 99%. The solid contents of saliva are:

1): mucin which is a glycoprotein

2): pytalin, an enzyme which converts starch into maltose. Also it contains salts of sodium, potassium, calcium and mangansium.


1): It converts cooked starch into a soluble sugar called maltose.

2): It acts as a solvent for food and helps in its swallowing. 

3): It moistens, lubricates and cleans the mouth.

4): It excretes organic and inorganic substances and some drugs.

 2): Stomach:  

Salivary digestion of carbohydrates continues in the stomach till pytalin is destroyed by HCI of the gastric juice. This takes about half an hour. The gastric juice itself has no carbohydrase.

3): Small Intestine: 

The food meets two juice in the  small intestine: pancreatic juice and intestinal juice.

(i) Pancreatic Juice: It contains a carbohydrase named pancreatic amylase.This enzymes hydrolyses more starch and glycogen to dextrins and the latter to double sugars maltose and isomaltose, and " limit" dextrins.

 Starch and Glycpgen ➡️➡️➡️➡️ Maltose + 
Isomaltose + ' Limit' Dextrins 

( ii) ) Intestinal Juice: It contains six carbohydrases : intestinal amylase ( traces only), maltose, isomaltase, " limit" dextrinase, sucrase and lactase.

1): Product of carbohydrate digestion like glucose and other simple sugars occurs through capillaries of villi.

2): Products of protein digestion like aminoacids are also absorbed through capillaries of villi.

3): Products of fat metabolismmetabolism like fatty acids and glycerol are absorbed through lacterals of villi.


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