Digestive System Lecture Notes

Overview of Digestion

• Digestion involves breaking down macromolecules: carbohydrates, lipids, proteins, and nucleic acids.
• Energy from these macromolecules powers chemical reactions in the body.

Carbohydrate Digestion

• Starts in the Mouth:
  • Salivary Amylase begins breaking down carbohydrates (e.g., starch and glycogen).
  • Starch and glycogen are complex carbohydrates.
• Stomach:
  • Salivary amylase stops working in the acidic environment.
• Small Intestine:
  • Pancreatic Amylase continues digestion in the small intestine (duodenum).
  • Polysaccharides are broken down into oligosaccharides and then disaccharides.
  • Brush Border Enzymes finalize digestion to monosaccharides (e.g., glucose, fructose, galactose) for absorption.
• Absorption:
  • Monosaccharides absorbed by diffusion, facilitated diffusion, or active transport.
  • Transported to liver via blood.

Protein Digestion

• Stomach:
  • Pepsinogen activated to Pepsin by stomach acid, begins protein digestion.
  • Proteins become oligopeptides.
• Small Intestine:
  • Pancreatic enzymes (zymogens like trypsin, chymotrypsin, carboxypeptidase) continue digestion to smaller peptides and amino acids.
  • Brush Border Enzymes complete digestion to single amino acids.
• Absorption:
  • Amino acids absorbed similarly to carbohydrates.
  • Transported to liver for processing.

Lipid Digestion

• Emulsification:
  • Bile emulsifies fats into smaller droplets (emulsification droplets).
• Enzymatic Action:
  • Pancreatic Lipase acts on emulsified fats to produce diverse lipids (e.g., micelles).
• Absorption:
  • Micelles cross brush border cells and are packaged into Chylomicrons.
  • Transported via lacteals to lymphatic system and then to liver.

Digestion of Other Components

• Nucleic Acids:
  • Nucleases from pancreas break down DNA/RNA into nucleotides.
  • Further processed and absorbed in brush border.
• Vitamins and Minerals:
  • Fat-soluble vitamins (A, D, E, K) absorbed with fats.
  • Water-soluble vitamins absorbed by diffusion.
  • Minerals absorbed directly; some require specific conditions (e.g., B12 with intrinsic factor).

Large Intestine

• Anatomy:
  • Composed of cecum, ascending, transverse, descending, sigmoid colon, rectum, and anus.
  • Lacks villi; main function is water absorption and forming feces.
• Function:
  • Bacterial flora ferment fibers, produce vitamins B and K.
  • Movement of waste facilitated by muscular contractions.
• Feces Composition:
  • Contains undigested fiber, mucus, bacteria, epithelial cells.
  • Used medically to assess health.

Defecation

• Triggered by rectum stretching.
• Voluntary and involuntary control mechanisms.
• Important to maintain regular bowel movements for health.

This concludes the lecture on the digestive system. Further information can be found on the class website.