Overview
This lecture covers the digestion and absorption of carbohydrates, explaining the breakdown of complex polysaccharides into absorbable monosaccharides and their transport into the body.
Types of Dietary Carbohydrates
- Main digestible carbs in foods are amylopectin and amylose.
- Cellulose (from plants) cannot be digested due to the lack of the enzyme cellulase and acts as dietary fiber.
Carbohydrate Digestion: Enzymes and Locations
- Digestion starts in the oral cavity with salivary alpha-amylase (also called ptyalin), which breaks alpha 1-4 glycosidic bonds.
- Salivary amylase converts amylopectin and amylose into maltose, malto-triose, and alpha-limit dextrins.
- Only about 10-15% of carbohydrate digestion is due to salivary amylase; most digestion happens after swallowing.
- Pancreatic amylase (from the pancreas) continues breaking alpha 1-4 bonds in the duodenum, producing the same products as salivary amylase.
Brush Border Enzymes and Final Carbohydrate Breakdown
- Brush border enzymes on small intestine enterocytes (microvilli) include lactase, maltase (glucoamylase), sucrase, and isomaltase.
- Lactase breaks lactose into glucose and galactose.
- Maltase splits maltose and malto-triose into glucose.
- Sucrase splits sucrose into glucose and fructose and can also help break down maltose/malto-triose.
- Isomaltase specifically breaks alpha 1-6 glycosidic bonds in alpha-limit dextrins, aiding complete glucose release.
Absorption of Monosaccharides
- Fructose enters enterocytes via facilitated diffusion using GLUT5 transporter.
- Glucose and galactose enter by secondary active transport using SGLT (sodium-glucose transporter) with sodium.
- All three monosaccharides exit the enterocyte into blood via GLUT2 transporters on the basolateral membrane.
Transport to the Liver
- Monosaccharides are absorbed into the hepatic portal vein (combining several veins) and transported to the liver for metabolism (e.g., glycogenesis, glycolysis).
Key Terms & Definitions
- Amylopectin — Branched polysaccharide with alpha 1-4 and alpha 1-6 glycosidic bonds.
- Amylose — Linear polysaccharide with only alpha 1-4 glycosidic bonds.
- Cellulose — Indigestible plant polysaccharide; source of fiber.
- Alpha 1-4 glycosidic bond — Linkage between carbon 1 of one glucose and carbon 4 of another.
- Alpha 1-6 glycosidic bond — Branch point linkage between carbon 1 and carbon 6 of glucose units.
- Salivary amylase — Oral enzyme that starts starch digestion.
- Pancreatic amylase — Pancreatic enzyme that continues starch breakdown.
- Brush border enzymes — Enzymes on intestinal microvilli for final carb digestion.
- GLUT5 — Transporter for fructose absorption.
- SGLT — Sodium-glucose cotransporter for glucose/galactose absorption.
- GLUT2 — Basolateral monosaccharide transporter to blood.
Action Items / Next Steps
- Review the structure and function of carbohydrate enzymes and transporters.
- Study further details of liver metabolism of monosaccharides in upcoming liver lectures.