Digestive system

• Digestion


  The process of breakdown of food into smaller parts which are absorbed and taken up into the body
• Components of the digestive system

  • Mouth
  • Oesophagus
  • Stomach
  • Pancreas
  • Liver
  • Small intestine - Duodenum, ileum
  • Large intestine (colon)
• Chemical digestion


  1. Digestive enzymes catalyse hydrolysis reactions that break down complex/large molecules in food (proteins, lipids and polysaccharides such as starch) into smaller, simpler molecules

  2. Starts from the mouth with action of salivary amylase and lingual lipase

  3. Hydrochloric acid secreted by stomach wall glands helps to sterilise food and provides acidic conditions required by stomach proteases such as pepsin

  4. Bile produced in the liver and stored in the gallbladder, then released into the duodenum via the bile duct, acts as a surfactant to break up large fat globules into smaller droplets so lipases can work more effectively
• Mechanical digestion


  1. Breaking down of food into smaller pieces, enhancing efficiency of chemical digestion

  2. Starts with chewing food (mastication)

  3. Mixing food with saliva

  4. Peristalsis - rhythmic movement of the gut wall which moves food along the length of the GI tract, involving circular and longitudinal muscles controlled by the autonomic nervous system
• Absorption


  • Digestion of the main food types: Proteins into amino acids, Polysaccharides into monosaccharides, Lipids into fatty acids and glycerol
  • Absorption occurs mainly in the ileum, in the epithelial cells lining the wall of the gut
  • The ileum have villi which are finger-like projections on the wall of the gut, with microvilli on their surface to increase surface area for absorption
• Cell transport


  • Polar molecules cannot cross the phospholipid bilayer in the plasma membrane of the epithelial cells lining the ileum
  • Polar substances require specialised transport systems involving proteins and glycoproteins embedded in the phospholipid bilayer
  • Fatty acids can pass into epithelial cells via simple diffusion, all other products of digestion are absorbed via active transport and facilitated diffusion
• Gut bacteria in humans aid digestion particularly with breakdown of complex carbohydrates (particularly plant cellulose) that are not broken down by enzymes such as amylase
• Gastrointestinal tract (GIT)


  • Four layers of the wall: mucosa, submucosa, muscularis and serosa
  • Mucosa is the innermost layer, serosa is the outermost layer