Chapter 23 digestion and absorption

Overview of the Digestive System

• Focus on final digestion and absorption steps in the small intestine, primarily in the jejunum and ileum.

• The majority of nutrient absorption occurs here.

Anatomy of the Small Intestine

• Surface Area Enhancements:

  • Circular Folds: Large folds in the intestinal wall.

  • Villi: Finger-like projections that further increase surface area.

  • Microvilli: Cellular extensions on enterocytes, known as brush border.

    • Location: At the apical surface (lumen side) of enterocytes.

    • Brush Border Enzymes: Enzymes residing on microvilli; crucial for final digestion before absorption.

Membrane Structures

• Basolateral Membrane: Interfaces with connective tissue and interstitial fluids.

• Tight Junctions: Connects enterocyte cells, preventing substances from passing freely between them; forces absorption through membrane transport.

Digestive Process

• Digestion: Chemical breakdown of food occurs before absorption.

  • Enzymes are secreted via:

    • Saliva (from the mouth)

    • Gastric secretions (in the stomach)

    • Pancreatic secretions (in the small intestine)

    • Brush border enzymes (on microvilli).

  • Last Step of Digestion: Brush border enzymes convert macromolecules into monomers or dimers, which can be easily absorbed.

Carbohydrate Digestion

• Beginning: Starts in the buccal cavity; limited action in saliva.

• Pancreatic Amylase: Secreted in the duodenum, breaks down starch into oligosaccharides and disaccharides.

• Brush Border Enzymes: Finalize breakdown to monosaccharides (e.g., glucose, galactose, fructose).

  • Transport Mechanisms:

    • Glucose and Galactose: Secondary active transport via sodium co-transport.

    • Fructose: Facilitated diffusion (independent of sodium).

• Nutrient movement from lumen through enterocytes into interstitial fluids.

Protein Digestion

• Initiation in the Stomach:

  • Pepsin (active form of pepsinogen) begins protein digestion but does not complete it.

• Pancreatic Enzymes: Continue to digest proteins into smaller peptides in the small intestine.

• Brush Border Enzymes: Breakdown peptides into amino acids and small peptides for absorption.

  • Transport: Utilizes secondary active transport dependent on sodium for moving amino acids across membranes.

• Both dipeptides and tripeptides can also be absorbed under these mechanisms.

Fat Digestion

• Initiation: Begins with lingual lipase in the mouth; minimal impact.

• Gastric Lipase: Continues in the stomach, but its effect is limited.

• Emulsification: Bile salts from the gallbladder break lipids into smaller droplets to increase surface area, allowing enzymes to act effectively.

• Pancreatic Lipase: Main enzyme responsible for fat breakdown into fatty acids and monoglycerides.

  • Diffusion of these molecules occurs across the apical surface into the enterocytes.

• Repackaging: Fatty acids and monoglycerides are reassembled back into triglycerides in enterocytes and packed into chylomicrons.

  • Chylomicrons exit the enterocyte into interstitial fluid via exocytosis, entering the lymphatic system for transport.

Recap of Digestion and Absorption

• Digestive processes take place in the buccal cavity, stomach, and small intestine.

• Ion transport mechanisms are crucial for absorption.

• Main goal: Break down food into smallest units for efficient absorption.

• Absorption predominantly occurs in the jejunum and ileum, ensuring that nutrients enter the body effectively.