3.3.3 Digestion and Absorption

• Digestion

  Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules that are small enough be absorbed across cell membranes into blood
• Digestion of starch in mammals

  Amylase (produced by salivary glands / pancreas) hydrolyses starch to maltose

  Membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose

  Hydrolysis of glycosidic bond
• Digestion of disaccharides in mammals

  Membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides:

  Maltase - maltose → glucose + glucose

  Sucrase - sucrose → fructose + glucose

  Lactase - lactose → galactose + glucose

  Hydrolysis of glycosidic bond
• Digestion of lipids in mammals, including action of bile salts

  Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets, increasing surface area for increased / faster lipase activity

  Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids

  Hydrolysis of ester bond
• Digestion of proteins by a mammal

  Endopeptidases - hydrolyse internal (peptide) bonds within a polypeptide → smaller peptides

  So more ends and surface area for exopeptidases

  Exopeptidases - hydrolyse terminal (peptide) bonds at ends of polypeptide → single amino acids

  Membrane-bound dipeptidases - hydrolyse (peptide) bond between a dipeptide → 2 amino acids

  Hydrolysis of peptide bond
• Membrane-bound enzymes

  • Located on cell membranes of epithelial cells lining ileum
  • Maintain concentration gradients for absorption
• Absorption of products of digestion in mammals

  Lumen (inside) of ileum → cells lining ileum (part of small intestine) → blood
• Absorption of amino acids and monosaccharides in mammals

  1. Na+ is actively transported from epithelial cells lining the ileum to blood (by Na+/K+ pump), this establishes a concentration gradient of Na+

  2. Na+ enters the epithelial cell down its concentration gradient with glucose against its concentration gradient through a co-transporter protein

  3. Glucose moves down a conc. gradient into the blood via facilitated diffusion
• Absorption of lipids by a mammal

  1. Micelles contain bile salts, monoglycerides and fatty acids

  2. Monoglycerides / fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble)

  3. Triglycerides reformed in (epithelial) cells and aggregate into globules

  4. Globules coated with proteins forming chylomicrons which are then packaged into vesicles

  5. Vesicles move to cell membrane and leave via exocytosis

  6. Enter lymphatic vessels and eventually return to blood circulation
• Absorption of lipids by a mammal

  1. Micelles contain bile salts, monoglycerides and fatty acids

  2. Make monoglycerides + fatty acids soluble in water

  3. Carry and release fatty acids and monoglycerides to the cell lining of the ileum

  4. Maintains a high concentration of fatty acids to cell lining

  5. Monoglycerides / fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble)

  6. Triglycerides reformed in (epithelial) cells and aggregate into globules

  7. Globules coated with proteins forming chylomicrons which are then packaged into vesicles

  8. Vesicles move to cell membrane and leave via exocytosis

  9. Enter lymphatic vessels and eventually return to blood circulation