Digestion & absorption

• Lipase


  The enzyme that digests lipids, produced in the pancreas and secreted into the duodenum
• Bile salts


  Produced in the liver and released into the duodenum, help emulsify lipids to increase surface area for lipase
• Micelle


  Vesicles containing fatty acids, monoglycerides, and bile salts formed after lipase digestion of triglycerides
• Role of micelles


  • Make fatty acids more water-soluble
  • Carry fatty acids and monoglycerides to epithelial cells of the ileum
  • Maintain high concentration of fatty acids outside epithelial cells to drive absorption by diffusion
• Absorption of lipids


  1. Fatty acids and monoglycerides diffuse into epithelial cells of ileum down concentration gradient

  2. In cells, fatty acids and monoglycerides are re-esterified into triglycerides in smooth ER or Golgi

  3. Triglycerides or chylomicrons (with proteins) are packaged into vesicles

  4. Vesicles fuse with cell membrane and contents are released by exocytosis

  5. Lipids enter lymphatic system via lacteals and are transported to bloodstream
• Digestion & absorption


  Carbohydrates, proteins, lipids (micelles)
• Digestion


  Large insoluble molecules hydrolysed into smaller soluble molecules that can be absorbed across cell membranes
• Carbohydrates


  • Require more than one enzyme to hydrolyse them into monosaccharides
  • Amylases
  • Membrane-bound disaccharidases
• Amylase


  Produced by pancreas and salivary glands, hydrolyses polysaccharides into disaccharides called maltose
• Sucrase and lactase


  Membrane-bound enzymes that hydrolyse sucrose and lactose into monosaccharides
• Proteins


  • Large polymer molecules that can be hydrolysed by 3 enzymes: endo peptidases, exopeptidases, membrane-bound dipeptidases
• Protein digestion


  Starts in the stomach, continues in duodenum, fully digested in the ileum
• Lipids


  • Digested by lipase and the action of bile salts
  • Lipase hydrolyses the ester bond in triglycerides to form monoglycerides and fatty acids
  • Bile salts emulsify lipids to form tiny droplets, micelles, increasing surface area for lipase
• Lipid digestion


  1. Physical: emulsification & micelle formation

  2. Chemical: lipase hydrolysis
• Micelles


  Water soluble vesicles formed of the fatty acids, glycerol, monoglycerides and bile salts, deliver the products of lipid digestion to the epithelial cells of the ileum for absorption
• Absorption


  • Villi in the ileum wall have thin walls surrounded by a network of capillaries and epithelial cells have even smaller microvilli, maximising absorption by increasing surface area, decreasing diffusion distance and maintaining concentration gradient
• Monosaccharide & amino acid absorption

  Require higher concentration in the lumen compared to the epithelial cell for facilitated diffusion, active transport and co-transport are required
• Lipid absorption


  Fatty acids in micelles can diffuse across the cell surface membrane of the ileum epithelial cells, then modified back into triglycerides inside the endoplasmic reticulum and Golgi body