Digestion Of Carbohydrates Biochemistry Notes

Biochemistry for Nurses Digestion of Carbohydrates 

Digestion Of Carbohydrates,Digestion in mouth,Digestion in stomach,Digestion in duodenum,Digestion in Small Intestine Action of Intestinal Juice...

Digestion Of Carbohydrates

    Dietary carbohydrates principally consist of the polysaccharides: Starch and glycogen. It also contains disaccharides: Sucrose (cane sugar), lactose (milk sugar) and maltose and in small amounts monosaccharides like fructose and pentoses. 

    Liquid food materials like milk, soup, fruit juice escape digestion in mouth as they are swallowed, but solid foodstuffs are masticated thoroughly before they are swallowed.

Digestion Along the GI Tract

1. Digestion in mouth:

    Digestion of carbohydrates starts at the mouth, where they come in contact with saliva during mastication. Saliva contains a carbohydrate splitting enzyme called salivary amylase (ptyalin).

Action of Ptyalin (Salivary Amylase)

    It is α-amylase, requires Cl– ion for activation and optimum pH 6.7 (range 6.6 to 6.8). The enzyme hydrolyzes α-1 → 4 glycosidic linkage at random deep inside polysaccharide molecule like starch, glycogen and dextrins, producing smaller molecules maltose, glucose and trisaccharide maltotriose. Ptyalin action stops in stomach when pH falls to 3.0.

2. Digestion in stomach:

    Practically no action. No carbohydrate splitting enzymes available in gastric juice. Some dietary sucrose may be hydrolysed to equimolar amounts of glucose and fructose by HCl.

3. Digestion in duodenum:

    Food bolus reaches the duodenum from stomach where it meets the pancreatic juice. Pancreatic juice contains a carbohydrate splitting enzyme pancreatic amylase (also called amylopsin) similar to salivary amylase. 

    Action of Pancreatic Amylase It is also an α-amylase, optimum pH 7.1. Like ptyalin it also requires Cl– for activity. The enzyme hydrolyses α 1→4 glycosidic linkage situated well inside polysaccharide molecule. Other criteria and end products of action similar to ptyalin.

4. Digestion in Small Intestine Action of Intestinal Juice

    • Intestinal amylase: This hydrolyses terminal α-1→4, glycosidic linkage in polysaccharides and oligosaccharide molecules liberating free glucose molecule.

    • Lactase: It is a β-galactosidase, its pH range is 5.4 to 6.0. Lactose is hydrolyzed to equimolar amounts of glucose and galactose.

    • Isomaltase: It catalyses hydrolysis of α-1→ 6 glycosidic linkage, thus splitting α-limit dextrin at the branching points and producing maltose and glucose.

    • Maltase: The enzyme hydrolyses the α-1→4 glycosidic linkage between glucose units in maltose molecule liberating equimolar quantities of two glucose molecules. Its pH range is 5.8 to 6.2.

    Five maltases have been identified in intestinal epithelial cells. Maltase V can act as isomaltase over and above its action on maltose.

    • Sucrase: pH range 5.0 to 7.0. It hydrolyses sucrose molecule to form equimolar quantities of glucose and fructose. Maltase III and maltase IV also have sucrase activity.