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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

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