Digestion and absorption

• Digestion

  Large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes
• Carbohydrates are digested by

  • Amylases
  • Membrane bound disaccharidases
• Amylase


  • Produced in pancreas - secreted into ileum and duodenum
  • Produced in salivary glands
• Amylase function

  1. Hydrolyses polysaccharides into the disaccharide

  2. By hydrolysing the glycosidic bond

  3. This begins in mouth
• Enzyme which hydrolyse disaccharides

  Membrane bound disaccharidases
• Membrane-bound disaccharidases

  • Maltase - hydrolyses maltose into two glucose molecules
  • Lactase - hydrolyses lactose into galactose and glucose
  • Sucrase - hydrolyses sucrose into fructose and glucose
• Stages/places of carbohydrate digestion

  1. Begins in mouth with amylase in saliva

  2. Continues into the duodenum

  3. Completed in the ileum where membrane bound disaccharidases are found
• Enzymes that digest protein

  • Endopeptidases
  • Exopeptidases
  • Membrane bound dipeptidases
• Endopeptidases


  Hydrolyses peptide bonds between the amino acid in the middle of a polypeptide chain
• Exopeptidases

  Hydrolyses peptide bonds between amino acids at the end of a polypeptide chain
• Membrane bound dipeptidases

  Hydrolyses peptide bonds between two amino acids/ a dipeptide
• Stages/places of protein digestion

  1. Begins in the stomach

  2. Partially digested and churned up proteins move down duodenum

  3. Fully digested in ileum
• Lipids are digested

  • Physically - emulsification and micelle formation
  • Chemically - action of lipase
• Lipase

  • Produced by pancreas
  • Secreted into duodenum and ileum
• Lipase function

  Hydrolyses ester bonds in triglycerides to form monoglycerides and fatty acids
• Action of bile salts

  Occurs before lipase action
• Bile salts

  • Produced in liver
  • Stored in gallbladder
  • Travel to the small intestine/duodenum through the bile duct
• Bile salts function

  Emulsify lipids to form tiny droplets called micelles
• Emulsification of lipids by bile salts

  1. Lipids are coated in bile salts to create an emulsion

  2. Bile salts causes them to split up into tiny droplets
• Advantage of emulsification

  Many tiny droplets of lipids create larger surface area to enable faster hydrolysis of lipids into monoglycerides and fatty acids by lipase
• Micelles

  Water soluble vesicles made up of the fatty acids, glycerol, monoglycerides and bile salts
• Micelles deliver

  The fatty acids, glycerol and monoglyceride the epithelial cells of the ileum
• All products of digestion are absorbed across the lining of the ileum
• Ileum wall adaptations for efficient absorption

  • Covered in villi (folding of ileum)
  • Villi are covered in smaller microvilli
  • Villi surrounded by network of capillaries
  • Villi and microvilli have thin walls
• Villi surrounded by network of capillaries

  Maintains concentration gradient
• Villi and microvilli have thin walls


  Short diffusion distance
• Villi are covered in smaller microvilli

  Increasing the surface area
• Absorption of glucose and amino acids

  Occurs via co-transport - see in topic 2
• Lipid absorption

  1. Micelles deliver fatty acid to epithelial cells

  2. Due to non-polar nature, fatty acids and monoglycerides can simply diffuse across the cell surface membrane and enter the epithelial cells

  3. Once in cells - fatty acids and monoglycerides are modified back into triglycerides inside the endoplasmic reticulum and Golgi body
• Chylomicron

  • Fatty acid is combined with protein
  • Produced when micelles are being processed by the Golgi body
• Lipids entering capillaries

  1. Chylomicron released in Golgi vesicle

  2. Vesicle released by exocytosis

  3. Lipids/chylomicron are absorbed by lymph vessel (lacteal)

  4. Transported in lymph which drains into capillary network/system of the body