Mechanisms of Digestion and Absorption in Vertebrates

Carbohydrates

• In typical omnivore, most are polysaccharides, starch and cellulose with some monosaccharides and disaccharides
• Mouth: starch digestion by salivary amylase
• Additional starch digestion by pancreatic amylase
• Monosaccharides transported across intestinal epithelium into the blood

Proteins

• Pepsin in the stomach
• Pancreas secretes trypsin and chymotrypsin
  • Pancreas secretes them as inactive precursors
  • Activated in small intestine
• Small intestine enzymes complete digestion of proteins into amino acids
• Transported into intestinal cells and into bloodstream

Lipids

• Most in the form of triglycerides
• Digestion entirely in small intestine
• Pancreatic lipase
• Phospholipids and bile salts emulsify fat into small droplets with increased surface area for digestion
• Bile salts form micelles allowing lipids to diffuse into intestinal cells
• Triglycerides reform inside intestinal cells
• Aggregate into chylomicrons released by exocytosis to pass into lacteals
• Will enter into general blood circulation

Vitamins, Minerals, and Water

• Do not require digestion
• Absorbed in complete form
• Fat-soluble vitamins follow pathway for fat absorption
• Small amounts of water absorbed in stomach but most in small intestine