Question

Describe the digestion of starch.

Answer 1

Digestion of starch:

(a) Digestion of starch in the oral cavity: Food is mixed with saliva and salivary amylase or ptyalin converts starch into maltose, isomaltose, and limit dextrins.

Starch \[\ce{->[Salivary][Amlylase]}\] Maltose + Isomaltose + Limit dextrin.

(b) Digestion of starch in the stomach: Gastric juice does not contain carbohydrate digestive enzyme, hence no digestion of starch takes place in the stomach.

(c) Digestion of starch in the small intestine: 40% starch is partially digested by ptyalin while the remaining 60% starch is digested by pancreatic amylase.

Starch \[\ce{->[Pancreatic][Amlylase + Cl-]}\] Maltose + Isomaltose + Limit dextrin.

The enzymes of intestinal juice act as below:
Maltose \[\ce{->[Maltase]}\] 2 Glucose
Isomaltose \[\ce{->[Isomaltase]}\] 2 Glucose
Surcrose \[\ce{->[Sucrase]}\] Glucose + Fructose
Lactose \[\ce{->[Lactase]}\] Glucose + Galactonse
Limit Dextrin \[\ce{->[Limit][Dextrinase]}\] Glucose

The humans cannot digest the cellulose due to the absence of cellulose enzyme.