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Digestion is the process by which food is converted into substances that can be absorbed and assimilated by the body. It is accomplished in the gastrointestinal tract by the mechanical and enzymatic breakdown of foods into simpler chemical compounds (1). According to this ultra-shitty lecture notes pack, there are two types of digestion: luminal and membrane. Luminal digestion takes place in the salivary glands, pancreas and stomach. Membrane digestion is the result of enzymes secreted by enterocytes brush border.

Digestion is relatively resiliant, and is safeguarded by a large measure of redundancy. For this reason, 80-90% of the pancreas or 2/3 of the small bowel can be removed before malabsorption develops.

Protein digestion

All we got was this: "Enterokinase activates trypsin which activates all peptidases". Yay.

Lipid digestion

  • Lecithin and bile salts emulsify fat
  • Colipase occupies a space in the emulsion droplet to allow pancreatic lipase to work
  • Pancreatic lipase hydrolyses fat
  • Cholesterol ester hydrolase liberates cholesterol that is bound to fatty acids
  • Phospholipase A digests phospholipids
  • Products of lipid digestion form micelles

Carbohydrate digestion

Carbohydrate digestion is interesting

Cellulose is broken down into organic acids, hydrogen gases, CO2 and methane.

  • α linkages can be cleaved by enzymes in humans.
  • Maltose has to be cleaved into two separate glucoses.
  • Sucrose is readily processed by the sucrase.
  • Lactose is usually broken down more quickly, and gives off the same products as cellulose (organic acids, hydrogen gases, CO2 and methane.)


  • β 1-4 linkages exist in cellulose. Humans cannot break down cellulose because of the β linkages.
  • The body can only take up carbohydrates as monosaccharides
  • Maltose is 2 glucose units.
  • There are lots of oral hypoglycaemic drugs; these are only used in Type 2 diabetes


  • Given the events that occur during lipid absorption, outline possible pathologic mechanisms which could interfere with these events
  • discuss the enterohepatic circulation of bile acids
  • describe the digestion and absorption of a) sucrose and b) lactose
  • discuss the events that result in the activation of pancreatic proteases