Ch 23 Digestion and Absorption

Digestive System Overview

• The small intestine is the primary site for absorption in the digestive system, particularly in the jejunum and, to a lesser extent, the ileum.
• Key structures in the small intestine enhancing absorption:
  • Circular folds: Increase surface area for absorption.
  • Villi: Finger-like projections that further increase surface area.
  • Microvilli: Cellular extensions on enterocytes (intestinal cells) that create a brush border, the site of absorption.

Microvilli and Brush Border Enzymes

• Microvilli are found on the apical surface of enterocytes, facing the intestinal lumen, crucial for nutrient absorption.
• Brush border enzymes: Located on microvilli, they catalyze the final steps of digestion before absorption. Example: enzymes breaking down disaccharides into monosaccharides.
• Basolateral membrane: Or basal membrane, interfaces with connective tissue and interstitial fluid, facilitating nutrient transport after absorption.

Transportation Mechanisms

• Tight junctions: Connect enterocytes, preventing substances from passing between them; nutrients must be transported across membranes and through enterocytes to be absorbed.
• Absorption involves:
  • Transporting nutrients from the intestinal lumen across the apical membrane into the cytosol of enterocytes.
  • Moving nutrients across the basolateral membrane into interstitial fluids.
• Lumen: Represents the external environment; anything here is technically outside the body until absorbed.

Carbohydrate Digestion and Absorption

• Digestion begins in the buccal cavity (mouth) with salivary amylase, though its impact is minimal.
• Major carbohydrate digestion occurs:
  • In the small intestine via pancreatic amylase that breaks down starch.
  • By brush border enzymes into monosaccharides (e.g., glucose, galactose, fructose).
• Transport mechanisms for monosaccharides:
  • Glucose and Galactose: Transported via secondary active transport (co-transport mechanism with sodium).
  • Fructose: Utilizes facilitated diffusion, a sodium-independent process.

Protein Digestion and Absorption

• Protein digestion begins in the stomach with pepsin (activated from pepsinogen) and continues in the small intestine.
• Proteins are broken down into peptides by pancreatic enzymes; brush border enzymes further digest them into amino acids.
• Transport mechanisms:
  • Amino acids and small peptides use secondary active transport dependent on the sodium gradient for absorption into the enterocytes.
  • Dipeptides and tripeptides can also be absorbed by this mechanism.

Fat Digestion and Absorption

• Fat digestion differs as fats are lipid-soluble and do not require carriers like water-soluble molecules.
• Begins in the buccal cavity with lingual lipase, but this is minimal.
• Digestion process:
  • Gastric lipase continues fat digestion in the stomach.
  • In the small intestine, bile (from gallbladder) emulsifies fats to increase surface area for pancreatic lipases to act on them.
• Fat digestion produces fatty acids and monoglycerides, which can easily diffuse across the apical membrane into enterocytes.
• Inside enterocytes, fats are reassembled into triglycerides and packaged into chylomicrons.
• Chylomicrons are then secreted via exocytosis into interstitial fluid and transported through lymphatic and circulatory systems.

Key Takeaways

• Digestion: A series of catabolic reactions that breakdown food into simpler units for absorption (e.g., macromolecules into monomers).
• Absorption: The process of transporting nutrients from the intestine into the body, predominantly occurring in the small intestine (jejunum and ileum).
• Goal: To absorb the simplest forms of nutrients, allowing for efficient transport into the cells of the body.