Human Nutrition ( 2 )

• Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules
• Role of chemical digestion

  To produce small soluble molecules that can be absorbed
• Enzymes in the digestive system

  • Amylase breaks down starch to simple reducing sugars
  • Protease breaks down protein to amino acids
  • Lipase breaks down fats and oils to fatty acids and glycerol
• Amylase action


  1. Breaks down starch to maltose

  2. Maltose breaks down to glucose
• Amylase


  Acts in the mouth and duodenum
• Protease pepsin


  Breaks down protein to amino acids in the acidic stomach
• Protease trypsin


  Breaks down proteins to amino acids in the alkaline small intestine
• Lipase


  Breaks down fats and oils to fatty acids and glycerol in the duodenum
• Where enzymes are secreted and act

  • Amylase: Salivary glands, mouth and duodenum
  • Pepsin: Stomach
  • Trypsin: Pancreas, duodenum
  • Lipase: Pancreas, duodenum
• Hydrochloric acid in gastric juice


  • Kills harmful microorganisms
  • Provides acidic pH for optimal enzyme activity (pepsin)
• Bile


  Alkaline mixture that neutralizes acidic food and gastric juices entering the duodenum to provide suitable pH for enzymes in small intestine
• Absorption


  • Movement of digested food molecules from digestive system into blood
  • Most water absorbed in small intestine, some also in colon
  • Ileum adapted for absorption with folded surface, villi and microvilli to increase surface area
• Structures in villi


  • Capillaries
  • Thin surface layer
  • Microvilli
  • Blood vessels
  • Lacteals
• Capillaries


  Absorb glucose and amino acids, carry them in blood
• Lacteals


  Absorb fatty acids and glycerol, transport them in lymph