Digestion and Absorption

Digestion Overview - The Breakdown Basics

• Digestion: Mechanical & chemical breakdown of food into absorbable units.
• Types:
  • Mechanical: Chewing, churning, segmentation.
  • Chemical: Enzymatic hydrolysis.
• Key Sites:
  • Mouth: Mastication, salivary amylase (starch).
  • Stomach: Churning, pepsin (proteins), gastric lipase (fats).
  • Small Intestine: Major digestion/absorption; pancreatic & intestinal enzymes, bile (fats).

⭐ Brunner's glands in the duodenum secrete alkaline mucus to neutralize acidic chyme from the stomach.

Carbohydrate Digestion - Sweet Surrender

• Oral Cavity: Salivary α-amylase (ptyalin) begins starch breakdown.
• Stomach: Digestion halts; acid inactivates ptyalin.
• Small Intestine (Major Site):
  • Pancreatic α-amylase: Converts starch/glycogen to oligosaccharides (maltose, maltotriose, α-limit dextrins).
  • Brush Border Disaccharidases:
    • Lactase: Lactose → Glucose + Galactose
    • Sucrase: Sucrose → Glucose + Fructose
    • Maltase/Isomaltase: Maltose/α-limit dextrins → Glucose
• Absorption (Enterocytes):
  • Glucose/Galactose: Apical SGLT1 (Na+-cotransport); basolateral GLUT2.
  • Fructose: Apical GLUT5 (facilitated); basolateral GLUT2.

⭐ SGLT1 co-transports glucose and galactose with Na+ into enterocytes; its activity is crucial for oral rehydration therapy.

Protein Digestion - Amino Acid Assembly

• Stomach: Pepsinogen (chief cells) $\xrightarrow{HCl}$ Pepsin; initiates protein hydrolysis, cleaving peptide bonds.
• Small Intestine - Pancreatic Proteases (Lumen):
  • Endopeptidases: Trypsin, Chymotrypsin, Elastase break proteins into smaller peptides.
  • Exopeptidases: Carboxypeptidases A & B remove C-terminal amino acids.
  • Activation Cascade:

• Small Intestine - Brush Border Enzymes (Surface): Aminopeptidases, Dipeptidases, Tripeptidases $\rightarrow$ free amino acids, di/tripeptides.
• Absorption (Enterocytes):
  • Amino acids: Na+-dependent cotransporters (e.g., for neutral, basic, acidic AAs).
  • Di/Tripeptides: PepT1 (H+-cotransporter); intracellularly hydrolyzed to AAs.

⭐ Enteropeptidase (enterokinase), a brush border enzyme, is essential for activating trypsinogen to trypsin, which then activates other pancreatic proenzymes.

Lipid Digestion - Fatty Facts Unfurled

• Key Enzymes: Lingual lipase (mouth), gastric lipase (stomach), pancreatic lipase & colipase (SI).
• Bile Salts (Liver): Emulsify fats $ ightarrow$ smaller droplets. Critical for lipase action.
• Pancreatic Lipase: TGs $ ightarrow$ 2-Monoacylglycerol (2-MAG) + 2 Free Fatty Acids (FFAs).
• Micelle Formation: 2-MAG, FFAs, cholesterol, fat-soluble vitamins + bile salts $ ightarrow$ transport to enterocyte.
• Absorption:
  • LCFAs & 2-MAG: Enter enterocyte $ ightarrow$ re-esterified to TGs.
  • SCFAs/MCFAs: Directly to portal blood.
• Chylomicrons: TGs + ApoB-48 $ ightarrow$ packaged in enterocyte $ ightarrow$ lymph (lacteals) $ ightarrow$ blood.

⭐ Abetalipoproteinemia: ApoB-48/ApoB-100 deficiency $ ightarrow$ no chylomicrons $ ightarrow$ severe fat malabsorption.

Vitamin & Mineral Absorption - Tiny Titans' Transport

• Fat-Soluble (A,D,E,K): Absorbed with dietary fats (micelles) in small intestine.
• Water-Soluble:
  • B12 (Cobalamin): Binds Intrinsic Factor (IF); absorbed in terminal ileum. 📌 B12-IF: Terminal Team.
  • Folate: Jejunum (active transport).
  • Vit C (Ascorbic Acid): Na⁺-dependent active transport; ileum.
• Minerals:
  • Iron ($Fe^{2+}$): Duodenum (DMT1 transporter). 📌 Iron: Duodenal Doyen.
  • Calcium ($Ca^{2+}$): Duodenum, jejunum (Vit D-dependent active transport).

⭐ Pernicious anemia: autoimmune parietal cell destruction → IF deficiency → B12 malabsorption in terminal ileum.

High‑Yield Points - ⚡ Biggest Takeaways

• Carbohydrate digestion: Salivary amylase (mouth), pancreatic amylase & brush border enzymes (small intestine). Absorption: SGLT1 (glucose/galactose), GLUT5 (fructose).
• Protein digestion: Pepsin (stomach), pancreatic proteases & brush border peptidases (small intestine). Absorbed as amino acids, di/tri-peptides.
• Fat digestion: Pancreatic lipase (major); requires bile salts for emulsification. Chylomicrons for lymphatic absorption.
• Iron absorption: Duodenum (Fe2+ form); regulated by hepcidin. Vitamin C enhances.
• Vitamin B12 absorption: Requires intrinsic factor; occurs in terminal ileum.
• Calcium absorption: Duodenum/jejunum; Vitamin D dependent.