1. Summary of Digestion of Carbohydrates
Mouth: limited breakdown of starch and glycogen occurs;
brief period of contact.
Salivary amylase (an α-1,4 glucosidase);
Does not breakdown α-1,6-glucosides.
Oligosaccharides and some maltose are the products.
2) Stomach: no significant digestive enzymes present.

2. Summary of Digestion of Carbohydrates
3) Small Intestine: Responsible for most of carbohydrate digestion.
a) Lumen: Pancreatic amylase ( its substrate specificity is similar to
that of salivary amylase; hydrolyzes α-1,4 linkages);
Secreted by the pancreatic duct into the duodenum.
Quantitatively more important than the salivary enzyme.
Products are: maltose (a disaccharide), maltotriose (a trisaccharide),
and the α-limit dextrins.
The α-limit dextrins contain approx. 8 glucose units with one or more
α-1,6 branch points. They will be further digested to maltose, maltotriose,
and glucose on the luminal surface of the epithelial cells.

3. Summary of Digestion of Carbohydrates
b) Brush Border of the Mucosal Epithelium
Final hydrolysis of di- and oligosaccharides to monosaccharides.
Catalyzed by enzymes on the surface of the small intestinal epithelial cells.
Excess capacity for digestion of starch and sucrose.

5. Enzymes of carbohydrate digestion
Product
Natural Substrate
Specificity
Enzyme
Glucose
Amylose
α-(1 4) Glucose
Amylase
Isomaltose,
α-dextrin
α-(1 6) Glucose
Isomaltase
Maltose, maltriose
Maltase
Glucose , Fructose
Sucrose
α-(1 2) Glucose
Sucrase
Glucose , Galactose
Lactose
β-(1 4) Galactose
Lactase

6. Monosaccharide absorption
Different sugars have different mechanisms for absorption:
Glucose and galactose are transported into the mucosal cells by an active, energy-requiring process that involves specific transport protein (GLUT-1) and requires concurrent uptake of sodium ions.
Fructose uptake requires a sodium-independent monosaccharide transporter (GLUT-5) for its absorption.
All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2.