6.1 - Digestion and Absorption

• Digestion breaks down mixture of large carbon molecules into small ions and compounds that can be absorbed into the bloodstream
• Proteins, lipids and polysaccharides digestion involves different stages at different places in the gut
• Digestion requires surfactants to break up lipid droplets and enzymes to catalyse reactions
• Glandular cells in stomach lining and intestines produce some enzymes
• Surfactants and other enzymes are secreted by accessory glands that have ducts leading to the digestive system
• Controlled, selective absorption of nutrients released by digestion occurs in the small intestine and colon
• Some small molecules like alcohol diffuse through the stomach lining before reaching the small intestine
• Mouth

  • Voluntary control of eating and swallowing
  • Mechanical digestion of food by chewing and mixing with saliva that contains lubricants and enzymes that start starch digestion
• Oesophagus

  • Moves food from mouth to stomach through peristalsis
• Stomach

  • Churning and mixing with secreted water and acid kills foreign bacteria and pathogens in food
  • Initial stages of protein digestion occur
• Small intestine

  • Final stages of digestion of lipids, proteins, carbohydrates, nucleic acids, neutralising stomach acid and absorption of nutrients
• Pancreas

  • Secretes protease, lipase and amylase
• Liver

  • Secretion of surfactants in bile to break up lipid droplets
• Gall Bladder

  Storage and regulated release of bile
• Large Intestine

  • Re-absorption of water, further digestion of carbohydrates by symbiotic bacteria, and formation and storage of feces.
• Wall of small intestine is made of layers of living tissue
• 4. Serosa


  • An outer coat
• 3. Muscle layer

  • Longitudinal muscle and circular muscle inside it
• 2. Sub-mucosa

  • A tissue layer containing blood and lymph vessels
• 1. Mucosa

  Lining of small intestine with epithelium that absorbs nutrients on inner surface
• Peristalsis

  • Contraction of circular and longitudinal muscles in layers of small intestines mixes food with enzymes and moves it along the gut
• Gut muscle layer has continuous moderate force with short periods of vigorous contraction instead of being relaxed until stimulated to contract
• Peristalsis


  • Waves of muscle contraction that pass along the intestine
• Contraction of circular muscles behind food constricts gut to prevent it being pushed back to mouth
• Contraction of longitudinal muscles moves on the food along the gut.
• Contraction of muscle layers is controlled unconsciously by the enteric nervous system
• When food returns from stomach during vomiting, abdominal muscles are used rather than the circular and longitudinal muscles
• Main function of peristalsis is to churn semi-digested food to mix it with enzymes and speed up digestion
• The pancreas secretes enzymes into the lumen of the small intestine
• Gland tissue with small groups of gland cells in pancreas secrete hormones insulin and glucagon into blood
• Other gland tissue in pancreas secrete digestive enzymes into gut in response to eating
• Secretion of enzymes and hormones is controlled by hormones synthesised and secreted by stomach and enteric nervous system
• 1. Digestive enzymes are synthesised in pancreatic gland cells on ribosomes of rER
  2. They are processed by golgi apparatus and secreted by exocytosis
  3. Ducts in pancreas merge to form a large pancreatic duct through which a litre of pancreatic juice is secreted per day into the small intestine.
• Amylase digests starch
• Protease digests proteins and peptides
• Lipase digests triglycerides and phospholipids
•  Enzymes digest most macromolecules in food into monomers in the small intestine
• Starch is digested into maltose by amylase
• Triglycerides are digested into fatty acids and glycerols or monoglycerids by lipase
• Phospholipids are digested into fatty acids, glycerol and phosphate by the phospholipase