IM

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.