Digestion and absorption

• Digestion: large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes.
• Amylase

  • hydrolyses starch into maltose, breaking glycosidic bonds
  • produced by salivary glands and pancreas
  • released into the mouth and small intestine
• Membrane-bound disaccharides:

  • found at cell membranes of epithelial cells of the ileum
  • Maltase: maltose -> glucose
  • Sucrase: sucrose -> glucose + fructose
  • Lactase: lactose -> glucose + galactose
• Endopeptidase

  • hydrolyses peptide bonds within a protein
  • produced in stomach with hydrochloric acid, pancreas
  • released in stomach, small intestine
• Exopeptidase

  • hydrolyses peptide bonds at the ends of polypeptide chains
  • produced by pancreas
  • released in small intestine
• Dipeptidase

  • hydrolyses dipeptides into amino acids
  • found at the epithelial cell membrane of the ileum
• Bile salts

  • emulsifies big lipid droplets into smaller ones (emulsification): increases surface area for lipase to work on
  • produced in liver, stored in gall bladder
  • released in small intestine
• Lipase

  • hydrolyses lipids into monoglycerides and fatty acids, breaking ester bonds
  • produced in pancreas
  • released in small intestine
• Absorption of lipids

  1. monoglycerides/fatty acids stick with bile salts to form micelles
  2. micelles make fatty acids soluble in water and bring them to the lining of ileum, they maintain higher concentration gradient of fatty acids to the lining
  3. fatty acids are absorbed into the epithelial cell by diffusion
  4. triglycerides are formed in the endoplasmic reticulum, and packaged into chylomicrons
  5. vesicles move to cell membrane (exocytosis)
• Absorption of monosaccharides

  1. sodium ions are actively transported from epithelial cell into blood via a sodium-potassium pump, which creates a concentration gradient
  2. sodium ions move from the lumen into the epithelial cell bringing glucose with it by co-transport, via a sodium-glucose co-transportor protein
  3. glucose moves from epithelial cell to blood by facilitated diffusion, via a channel protein
• Visking tubing

  1. fill tubing with starch and amylase solution, suspend in water bath
  2. take samples outside of tubing at regular intervals, testing for the presence of starch (using iodine solution) and glucose (using Benedict's reagent)
  3. glucose concentration increases over time, as it is small enough to diffuse across the partially permeable membrane
• Limitations of visking tubing

  • visking tubing <-> membrane of small intestine: both are partially permeable, but small intestine has larger surface area (due to the presence of villi)
  • distilled water <-> blood: both have an initially low solute concentration, but distilled water doesn't flow so no concentration gradient maintained