Digestion and Absorbtion

• Explain what happens in digestion:

  • Large (insoluble) biological molecules are hydrolysed to smaller (soluble) molecules
  • That are small enough to be absorbed across cell membrane into blood
• Where is amylase produced?

  Salivary glands and pancreas
• Describe the digestion of starch in mammals:

  • Amylase 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 into 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 the surface area of lipids for increased/faster lipase activity
  • Lipase (made in pancreas) hydrolysis lipids into monoglycerides an fatty acids
  • Hydrolysis of ester bond
• Describe the digestion of proteins by a mammal:

  • Endopeptidases hydrolyses internal peptide bonds within a polypeptide = smaller peptides

  -So more ends/surface area for exopeptidases

  • Exopeptidases hydrolyse terminal peptide bonds at ends of polypeptides - single amino acids
  • Membrane bound dipeptidases hydrolyse peptide bonds between a dipeptide - 2 amino acids
  • Hydrolysis of peptide bond
• Suggest why membrane bound enzymes are important in digestion:

  • They are located on cell membranes of epithelial cells lining the ileum
  • By hydrolysing molecules at site of absorption they maintain concentration gradient 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 lipids by a mammal including the role of micelles:

  • Micelles contain bile salts, monoglycerides, and fatty acids

  -Make monoglycerides and fatty acids more soluble in water

  -Carry/release fatty acids and monoglycerides to lining of ileum

  -Maintain high concentration gradient

  • Monoglycerides/fatty acids absorbed into epithelial cell by diffusion
  • 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