3.3 Digestion and absorption

• Explain what happens in digestion

  • ● Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules
  • ● That are small enough be absorbed across cell membranes into blood
• Describe the 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

  
• Describe the 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
• Describe the digestion of lipids in mammals, including action of bile salts

  • ● Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets
  • ● This increases surface area of lipids for increased / faster lipase activity
  • ● Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids
  • ● Hydrolysis of ester bond

  
• Describe the digestion of proteins by a mammal

  • ● Endopeptidases - hydrolyse internal (peptide) bonds within a polypeptide → smaller peptides
  • ○ So more ends / 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
• Suggest why membrane-bound enzymes are important in digestion

  • ● Membrane-bound enzymes are located on cell membranes of epithelial cells lining ileum
  • ● (By hydrolysing molecules at the site of absorption they) maintain concentration gradients for absorption
• Describe the pathway for absorption of products of digestion in mammals

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

  Co-transport:

  1. Na+ actively transported from epithelial cells lining ileum to blood (by Na+/K+ pump)
  2. ● Establishing a conc. gradient of Na+ (higher in lumen than epithelial cell)
  3. Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient
  4. ● Via a co-transporter protein
  5. Glucose moves down a conc. gradient into blood via facilitated diffusion

  
• Describe the absorption of lipids by a mammal, including the role of micelles - part 1: up to absorption into epithelial cells

  • ● Micelles contain bile salts, monoglycerides and fatty acids
  • ○ Make monoglycerides and fatty acids (more) soluble in water
  • ○ Carry / release fatty acids and monoglycerides to cell / lining of ileum
  • ○ Maintain high concentration of fatty acids to cell / lining
  • ● Monoglycerides / fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble)
• Describe the absorption of lipids by a mammal, including the role of micelles - part 2: from absorption into epithelial cells

  • ● Triglycerides reformed in (epithelial) cells and aggregate into globules
  • ● Globules coated with proteins forming chylomicrons which are then packaged into vesicles
  • ● Vesicles move to cell membrane and leave via exocytosis
  • ○ Enter lymphatic vessels and eventually return to blood circulation