Carbohydrate Digestion and Absorption

Carbohydrate Digestion and Absorption

Overview of the Digestive System

• Oral Cavity: Contains teeth and salivary glands (including the parotid gland).
• Esophagus: Transports food to the stomach.
• Stomach: Churns food and passes it to the duodenum.
• Duodenum: The first part of the small intestine, where pancreatic and gallbladder secretions enter.
• Small Intestines: Jejunum and ileum are responsible for nutrient absorption.
• Pancreas and Liver/Gallbladder: Secrete digestive substances into the duodenum via a duct.
• Enterocyte: A cell of the small intestine involved in absorption.
• Portal System: Transports absorbed nutrients to the liver.

Carbohydrate Ingestion

• Three major carbohydrates ingested:
  • Starch
  • Glycogen
  • Cellulose
• Polysaccharides: Many sugars in a long chain.
• Oligosaccharides: Two or more sugars, shorter chains than polysaccharides.
• Monosaccharides: Single sugars that can be absorbed.

Polysaccharides Breakdown

• Polysaccharides must be broken down into monosaccharides for absorption.

Types of Carbohydrates

• Starch: Plant-based carbohydrate.
• Cellulose: Also plant-based, but indigestible.
• Glycogen: Animal carbohydrate, stored glucose in skeletal muscle.

Molecular Structure of Sugars

• Basic sugar molecule (glucose, galactose, fructose) is a six-carbon ring.
  • Formula: C6H{12}O_6
• Starch and glycogen are long chains of these rings with branches.

Chemical Bonds

• Alpha 1-4 Glycosidic Bond:
  • Binds monosaccharides in starch and glycogen.
• Beta 1-4 Glycosidic Bond:
  • Found in cellulose; humans lack the enzyme to break this bond.

Digestion in the Oral Cavity

• Salivary glands, especially the parotid gland (near the ear), release alpha-amylase.
  • Alpha-Amylase: Molecular scissors that chop alpha 1-4 glycosidic bonds.
  • Cannot break beta 1-4 glycosidic bonds (cellulose).

Cellulose

• Cellulose remains undigested and becomes fiber.
• Starch and glycogen are digestible due to alpha 1-4 glycosidic bonds.

Alpha Amylase Limitations

• Alpha-amylase breaks 1-4 glycosidic bonds but not terminal ones.

Movement Through the Digestive System

• The polysaccharide moves through the esophagus and relatively unchanged through the stomach.
• Alpha 1-6 Glycosidic Bond:
  • At branching points in starch and glycogen.
  • Alpha-amylase cannot break alpha 1-6 glycosidic bonds.

Duodenum

• As substances move into the duodenum, fats, proteins, and acid trigger enteroendocrine cells.

Enteroendocrine Cells and CCK

• Enteroendocrine Cells: Line the duodenum and release cholecystokinin (CCK) into the bloodstream.
  • Cholecystokinin (CCK):
    • Triggers gallbladder contraction (important for fat digestion).
    • Stimulates the pancreas to release pancreatic juices, including alpha-amylase.

Pancreatic Alpha-Amylase

• Pancreatic alpha-amylase is more potent than salivary amylase.
• Further digests alpha 1-4 glycosidic bonds.

Result

• Results in disaccharides and trisaccharides (maltose, maltotriose, sucrose, lactose) and alpha-dextrin.

Disaccharides and Trisaccharides

• Maltose: Two glucose molecules.
• Maltotriose: Three glucose molecules.
• Sucrose: Glucose and fructose.
• Lactose: Glucose and galactose.
• Alpha-Dextrin: Contains alpha 1-6 glycosidic bond.

Intolerance

• Some individuals lack enzymes to break down certain disaccharides.
• Undigested disaccharides remain in the digestive tract, pulling water and causing diarrhea.
• Example: Lactose intolerance due to lack of lactase.

Brush Border Enzymes

• Brush border cells in the small intestines produce enzymes that break down disaccharides.
  • Maltase: Breaks down maltose and maltotriose.
  • Sucrase: Breaks down sucrose.
  • Lactase: Breaks down lactose.
  • Alpha-Dextrinase (Isomaltase): Breaks down alpha-dextrin (alpha 1-6 glycosidic bond).

Monosaccharides

• These enzymes result in the release of monosaccharides:
  • Glucose.
  • Fructose.
  • Galactose.

Absorption of Monosaccharides

Sodium-Potassium Pump

• The sodium-potassium ATPase pump maintains a sodium gradient.
  • Moves three sodium ions out of the cell and two potassium ions into the cell, consuming ATP.
  • Creates a sodium deficiency inside the cell, facilitating sodium influx.

Transport Mechanisms

• Glucose and Galactose:
  • Transported via the Sodium-Glucose Transporter (SGLT).
  • Secondary active transport, piggybacking on the sodium concentration gradient.
• Fructose:
  • Transported via GLUT5 (glucose transporter 5).
  • Facilitated diffusion down its concentration gradient.

Movement into Bloodstream

• All three monosaccharides (glucose, fructose, galactose) move into the bloodstream via GLUT2.
  • GLUT2: Facilitated diffusion down their concentration gradients.

Summary of Digestion and Absorption

1. Initial Breakdown: Salivary amylase breaks alpha 1-4 glycosidic bonds.
2. Further Digestion: Pancreatic alpha-amylase further breaks down bonds in the small intestine, triggered by CCK release.
3. Brush Border Enzymes: Maltase, sucrase, lactase, and alpha-dextrinase break disaccharides into monosaccharides.
4. Absorption:
   • Fructose via GLUT5.
   • Glucose and galactose via SGLT.
5. Transport to Liver: All three monosaccharides enter the bloodstream via GLUT2 and are transported to the liver via the portal system.
6. Liver Metabolism: Liver converts galactose and fructose into glucose, which can be used for ATP production (glycolysis) or stored as glycogen.