Digestion

• Digestion

  Large insoluble molecules hydrolyzed into smaller soluble molecules which can then be absorbed across cell membranes and into the bloodstream
• Types of molecules digested

  • Carbohydrates
  • Lipids
  • Proteins
• Enzymes involved in digestion

  • Amylases
  • Membrane-bound disaccharidases
  • Endo peptidases
  • Exo peptidases
  • Membrane-bound dipeptidases
  • Lipase
• Enzymes in biology ending in ASE indicate they are enzymes
• Amylase is produced in the pancreas and saliva glands, starting digestion of carbohydrates in the mouth
• Membrane-bound disaccharidases in the duodenum and ileum hydrolyze disaccharides into monosaccharides
• Examples of membrane-bound disaccharidases: sucrase (hydrolyzes sucrose into fructose and glucose) and lactase (hydrolyzes lactose into galactose and glucose)
• Protein digestion involves endo peptidases, exo peptidases, and membrane-bound dipeptidases
• Protein digestion starts in the stomach and continues in the duodenum and ileum
• Lipase, produced in the pancreas, is involved in the digestion of lipids
• Digestion

  1. Enzymes for chemical digestion and physical breakdown using bile salts

  2. Lipase is produced in the pancreas and secreted into the duodenum and ileum

  3. Lipase hydrolyzes the ester bond in triglycerides into fatty acids and glycerol

  4. Bile salts emulsify lipids, splitting large droplets into tiny droplets
• Physical digestion

  • Emulsification and micelle formation
  • Bile salts coat lipids causing them to split into tiny droplets creating a large surface area for faster hydrolysis by lipase
• Micelles

  • Spheres or vesicles made up of fatty acids, monoglycerides, and bile salts
  • Deliver fatty acids and monoglycerides to epithelial cells in the ileum for absorption
• Absorption

  1. Occurs within the cells lining the ileum

  2. Villi increase surface area for absorption

  3. Microvilli cells on villi have further foldings to increase surface area for rapid absorption
• Villi structure

  Folding of the ileum surface, covered in several microvilli cells with further foldings to increase surface area for rapid absorption
• Capillary network in villi

  Enables the concentration gradient to be constantly maintained with a very short diffusion distance
• Absorption of monosaccharides and amino acids

  Both absorbed by Co-transport, a type of active transport
• Co-transport

  • Type of active transport used for absorption of glucose and amino acids
• Lipid absorption

  Fatty acids and monoglycerides carried by muscles to epithelial cells, where they diffuse through the plasma membrane due to their lipid-soluble nature
• Formation of triglycerides

  Inside the cell, fatty acids and monoglycerides are formed back into triglycerides in the Golgi apparatus or endoplasmic reticulum
• Formation of chylomicron

  Combination of triglyceride with a protein, released inside a Golgi vesicle and then exocytosed from the cell
• Transport of chylomicron

  Absorbed into the lacteal or lymph vessel, transported around in the lymph, and eventually drained into the capillary system