Carbohydrate Metabolism

The commonly consumed foods, such as cereals, sugar, tubers, and milk, contain carbohydrates in three major forms, starch, lactose, and sucrose.

As soon as the food enters the mouth, salivary amylase enzymes break down the starch into smaller polymers. As the food particles travel down the digestive system into the small intestine, pancreatic amylases break it down into disaccharides, such as maltose or dextrin.

The digestive enzymes secreted by brush-border cells of the small intestine metabolize these disaccharides into glucose

Lactose, the carbohydrate found in milk, is catabolized by beta-galactosidase, an enzyme secreted by the brush-border cells, into its monomers -  glucose and galactose.

Similarly, sucrose, a carbohydrate found in fruits, is hydrolyzed by the enzyme sucrase, also expressed on the brush-border cells, into its monomers -  glucose, and fructose.

These monomers are then transported across the intestinal epithelium into the bloodstream and finally get absorbed by the body cells. 

Once inside a cell, glycolytic pathways metabolize glucose into pyruvate, coupled with the production of energy molecules - ATP and NADH. The other two monomers, galactose, and fructose are converted to glucose-6-phosphate and then enter glycolysis.