Overview
This lecture covers the special functions of the gastrointestinal (GI) tract, focusing on the processes of digestion and absorption for carbohydrates, proteins, and fats.
General Concepts of Digestion and Absorption
- Digestion reduces food size to allow for absorption and utilization by the body.
- Digestion occurs throughout the GI tract, from mouth to large intestine, in varying degrees.
- Absorption primarily occurs in the small intestine, where nutrients move into enterocytes and then to the blood or lymph.
- Villi and microvilli in the intestine increase surface area for absorption.
- Enzyme activity decreases with age, reducing digestive efficiency in older individuals.
- Two types of digestion: luminal (in the gut lumen) and membranous (at the enterocyte membrane).
- Nutrient transport across the enterocyte membrane occurs via simple diffusion, active transport, facilitated diffusion, and pinocytosis.
Carbohydrate Digestion and Absorption
- Carbohydrates are digested to glucose, fructose, and galactose.
- Luminal digestion uses amylase (mouth, stomach, intestine) to break down carbs into smaller sugars.
- Membranous digestion uses enzymes (glucoamylase, isomaltase, sucrase, lactase, trehalase) on the enterocyte membrane.
- Glucose and galactose enter enterocytes via SGLT1; fructose via GLUT5.
- All three use GLUT2 to move from enterocyte to bloodstream.
Protein Digestion and Absorption
- Proteins are digested to amino acids, dipeptides, and tripeptides.
- Luminal digestion uses endopeptidases (pepsin, trypsin, chymotrypsin, elastase) and exopeptidases (carboxypeptidase A & B).
- Membranous digestion uses enterokinases to activate trypsin, which cascades activation of other proteases.
- Final products enter enterocytes mainly via simple diffusion; large proteins may be absorbed by pinocytosis.
Lipid Digestion and Absorption
- Lipids are digested to cholesterol, free fatty acids, monoglycerides, and glycerol.
- Triglycerides, phospholipids, and cholesterol are the major dietary fats.
- Gut motility forms fat droplets, which are hydrolyzed by lipase, phospholipase A2, and cholesterol ester hydrolase.
- Emulsification by bile salts and phospholipids increases enzyme efficiency.
- Micelles (bile salt + lipid products) deliver fatty acids, monoglycerides, and cholesterol to enterocytes.
- Fatty acids and monoglycerides enter enterocytes via fatty acid binding proteins and are repackaged as chylomicrons.
- Chylomicrons enter lymphatic lacteals; glycerol enters the blood directly via simple diffusion.
- 95% of cholesterol is reabsorbed; LDL ("bad") cholesterol promotes atherosclerosis, HDL ("good") cholesterol is protective.
Key Terms & Definitions
- Digestion — Breakdown of large food particles into absorbable molecules.
- Absorption — Movement of nutrients from the intestinal lumen into enterocytes and then into blood or lymph.
- Luminal Digestion — Enzymatic breakdown of nutrients in the gut lumen.
- Membranous Digestion — Final enzymatic breakdown at the surface of enterocytes.
- Enterocyte — Intestinal epithelial cell involved in nutrient absorption.
- Villus/Villi — Finger-like projections increasing intestinal surface area.
- Chylomicron — Lipoprotein particle that transports dietary lipids via lymph.
- Micelle — Aggregate of bile salts and lipid digestion products aiding absorption.
- SGLT1/GLUT5/GLUT2 — Transport proteins for carbohydrate absorption.
- Endopeptidase/Exopeptidase — Enzymes digesting internal/external peptide bonds in proteins.
Action Items / Next Steps
- Review summary tables for digestion and absorption of carbohydrates, proteins, and fats.
- Prepare for next lecture on fermentation in ruminants and other animals.