Lecture on the Digestive System - Final Part

Overview

• Recap on digestion of macromolecules: carbohydrates, lipids, proteins, and nucleic acids.
• Importance of energy from macromolecules to power reactions and as building blocks.

Carbohydrate Digestion

• Mouth: Begins with salivary amylase breaking down starch and glycogen.
  • Starch and glycogen (complex carbohydrates) start digestion here.
• Stomach: Salivary amylase is inactivated by low pH.
• Small Intestine:
  • Duodenum: Pancreatic amylase continues digestion.
  • Breakdown from polysaccharides to oligosaccharides and disaccharides.
  • Disaccharides are too large for absorption; must be monosaccharides (e.g., glucose, fructose, galactose).
  • Brush Border Cells: Enzymes make the final cut to monosaccharides for absorption.
  • Absorption methods include diffusion, facilitated diffusion, and active transport.
  • Monosaccharides absorbed into blood vessels and transported to liver.

Protein Digestion

• Mouth: No protein digestion occurs.
• Stomach:
  • Activation of pepsinogen to pepsin by stomach acid.
  • Pepsin breaks polypeptides into smaller peptides.
• Small Intestine:
  • Pancreas secretes zymogens (trypsin, chymotrypsin, carboxypeptidase) for further breakdown to dipeptides.
  • Final breakdown into amino acids by brush border enzymes.
  • Absorption into brush border cells via diffusion, active transport, etc., then into blood vessels to liver.

Lipid (Fat) Digestion

• Small Intestine: Begins significantly here with emulsification.
  • Bile emulsifies fats into smaller droplets for pancreatic lipase action.
  • Diverse lipids form micelles; enter brush border cells easily.
  • Brush Border Cells: Add proteins to create chylomicrons.
  • Chylomicrons enter lacteals in lymphatic system, then to liver.

Additional Components

• Nucleic Acids:
  • Broken down by nucleases into nucleotides.
  • Further processed by brush border cells.
• Vitamins:
  • Fat-soluble (A, D, E, K) absorbed with fats.
  • Water-soluble absorbed by diffusion; B12 requires intrinsic factor.
• Minerals:
  • Absorbed directly along the small intestine.

Water Absorption

• Most water absorption occurs in the small intestine, not the large intestine.

Large Intestine

• Anatomy:
  • Cecum, ascending, transverse, descending, and sigmoid colon.
  • Functions mainly as storage and absorption of remaining water.
  • Houses bacteria for fermentation and vitamin production.
• Bacteria:
  • Ferment cellulose, produce vitamins B and K, and contribute to gas production.
• Structure: Simple columnar epithelium, no need for increased surface area.

Defecation

• Process of feces elimination triggered by rectum stretch.
• Involuntary/autonomic control possible.

Conclusion

• Completed overview on digestive system.
• Check website for more information.

End of Lecture