Digestion and Absorption

• Define digestion.

  The hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes.
• Which enzymes are involved in carbohydrate digestion and where are they found?

  • Amylase- salivary glands
  • Maltase, sucrase and lactase in the membrane of the small intestine. Collectively, these are known as membrane-bound disaccharidases.
• Where are lipids digested?

  The small intestine.
• What are the substrates and products of each carbohydrate enzyme?

  • Amylase- starch into smaller polysaccharides
  • Maltase- maltose into two glucose molecules
  • Sucrase- sucrose into glucose and fructose
  • Lactase- lactose into glucose and galactose
• What needs to happen to lipids before they are digested?

  They must be emulsified by bile salts produced by the liver. This breaks down large fat molecules into smaller, soluble molecules called micelles which increases surface area.
• Name the parts of lipid digestion.

  A) Emulsification

  B) Lipase

  C) Large fat globule

  
• How are lipids digested?

  Lipase hydrolyses the ester bond between monoglycerides and fatty acids.
• Which enzymes are involved in the digestion of protein and what are their roles?

  • Endopeptidases- break peptide bonds between specific amino acids in the middle of the polypeptide chain.
  • Exopeptidases- break peptide bonds between specific amino acids at the end of the polypeptide chain.
  • Dipeptidases- break the peptide bond in dipeptides to form two amino acids.
• How are certain molecules absorbed in the ileum against their concentration gradient?

  Co-transport
• Which molecules require co-transport?

  Amino acids and monosaccharides.
• Explain the role of sodium ions in co-transport in the ileum.

  1. Sodium ions are actively transported out of cells into the lumen of the ileum.
  2. This creates a concentration gradient.
  3. Sodium ions bind to co-transport proteins, which are also bound to amino acids/monosacchardies.
  4. The sodium ions move down their concentration gradient, bringing the co-transport protein and nutrient with them.
• Why do fatty acids and monoglycerides not require co-transport?

  They are non-polar meaning they can easily diffuse over the cell-surface membrane of epithelial cells.