Lecture 9: Chemical Digestion in the Small Intestine

Key Concepts

• Digestion of Carbohydrates

  • Carbohydrates are broken down into monosaccharides for absorption.
  • Major enzymes:
    • Pancreatic amylase: Converts starch into maltose.
    • Maltase: Breaks maltose into two glucose monomers.
    • Sucrase: Breaks sucrose into glucose and fructose.
    • Lactase: Breaks lactose into glucose and galactose.
  • Digestion starts in the mouth with salivary amylase, halts in the esophagus and stomach, resumes in the duodenum.
  • Lactose Intolerance: Lack of lactase leads to symptoms including diarrhea, gas, bloating, and cramps. Solutions include lactase supplements or lactose-free products.

• Digestion of Proteins

  • Begins in the stomach with hydrochloric acid and pepsin turning proteins into peptides.
  • Pancreatic enzymes and brush border enzymes play a key role.
  • Important enzymes:
    • Aminopeptidase: Removes amino acids from the amino end.
    • Carboxy peptidase: Removes amino acids from the carboxyl end.
    • Dipeptidase: Splits dipeptides into amino acids.

• Digestion of Lipids

  • Starts in the mouth but predominantly occurs in the small intestine.
  • Bile: Emulsifies triglycerides.
  • Pancreatic lipase: Splits into fatty acids and monoglycerides, forming micelles for absorption.
  • Inside cells, reassembled into triglycerides and form chylomicrons.

• Digestion of Nucleic Acids

  • Broken down by pancreatic nucleases:
    • Ribonuclease (RNA digestion)
    • Deoxyribonuclease (DNA digestion)
  • Further broken down by brush border enzymes into nucleotides.

Absorption Mechanisms

• Carbohydrates

  • Glucose and galactose: Absorbed via secondary active transport with sodium (symporters).
  • Fructose: Absorbed through facilitated diffusion.

• Proteins

  • Amino acids: Absorbed through active transport or secondary active transport.
  • Dipeptides and tripeptides: Use symporters with hydrogen ions.

• Lipids

  • Short-chain fatty acids: Diffuse directly across membranes.
  • Long-chain fatty acids: Emulsified by bile, absorbed, reassembled into chylomicrons, and enter lacteals.

Transport and Regulation

• Small Intestine Regulation

  • Enteric reflexes respond to chyme presence, increasing motility.
  • Vasoactive intestinal polypeptide (VIP): Stimulates intestinal juice production.
  • Segmentation and peristalsis are influenced by nervous system signals.

• Nutrient Transport

  • Monosaccharides and amino acids enter blood capillaries, travel via the hepatic portal vein to the liver.
  • Chylomicrons enter lacteals, travel through the lymphatic system, and enter the bloodstream at the left subclavian vein.
  • Chyle: Lipid-rich fluid in lacteals.

• Absorption Processes

  • Include diffusion, facilitated diffusion, osmosis, active transport, and secondary active transport.
  • ATP or concentration gradients are necessary for active transport.

This lecture outlines the digestive processes occurring in the small intestine, including the breakdown of carbohydrates, proteins, lipids, and nucleic acids, as well as the absorption and transportation of nutrients in the body.