Digestive System

• Epiglottis


  When we swallow the bolus, the epiglottis closes off the opening to the trachea, preventing food from entering the trachea
• Esophagus


  The tube that carries the bolus from the throat to the stomach
• Bolus entering the stomach

  Bolus enters the stomach through the cardiac sphincter (valve)
• Stomach


  • J-shaped organ made up of smooth muscle
  • Thick-walled
  • Contains mucus-secreting goblet cells and gastric glands
• Gastric juice


  Secreted by the stomach, contains hydrochloric acid (HCl) with a pH of 1.0
• Functions of HCl in gastric juice

  • Kills bacteria, a line of defense against illness
  • Activates the protein-digesting enzyme pepsin
  • Denatures (unfolds) proteins into polypeptides, an example of mechanical digestion
• Contents of gastric juice

  • HCl
  • Pepsin
  • Water
• Mucus-secreting goblet cells


  Secrete mucus to protect the stomach lining from the gastric juice
• Stomach function


  1. Stores food

  2. Mechanically and chemically continues to break down the bolus
• Enzyme


  • Has a specific 3D shape that makes it substrate-specific
  • Active site is where the chemical reaction happens
• Lock and key theory

  One enzyme, one substrate
• Factors affecting enzyme activity

  • Temperature (optimal range 36-37°C, damage above 39-40°C)
  • pH
  • Substrate concentration
  • Enzyme concentration
  • Presence of inhibitors