Digestion of carbohydrates

• The digestion of carbohydrates takes place in the mouth and the small intestine
• Carbohydrates require more than one enzyme to hydrolyse them into monosaccharides:

  • Amylases
  • Membrane-bound disaccharidases
• Digestion of carbohydrates:

  1. Begins in the mouth
  2. Continues in the duodenum
  3. Completed in the ileum
• Amylase:

  • Produced by the pancreas and salivary glands
• Amylase:

  • Hydrolyses polysaccharides (e.g starch) into the disaccharide maltose by breaking the glycosidic bonds
• There are Three membrane-bound disaccharides:

  • Maltase
  • Sucrase
  • Lactase
• Membrane-bound disaccharidases - Enzymes in the cell membranes of the epithelial cells in the ileum (the small intestine)
• Membrane-bound disaccharidases:

  • They catalyse the hydrolysation of disaccharides (e.g maltose) into monosaccharides (e.g glucose)
  • Occurs in duodenum, and then into the ileum as well
• Example of membrane-bound disaccharidase: SUCRASE

  • SUCRASE can hydrolyse sucrose into fructose and glucose
• Example of membrane-bound disaccharidase: LACTASE

  • LACTASE can hydrolyse lactose into galactose and glucose
• Once the disaccharides have been hydrolysed into monosaccharides:

  • The monosaccharide products are then small enough to be transported across the epithelial cell membrane through specific transporter proteins
• Once the disaccharides have been hydrolysed into monosaccharides:

  • Glucose and galactose are actively transported using sodium ions through co-transporter proteins
• Once the disaccharides have been hydrolysed into monosaccharides:

  • Fructose is absorbed by facilitated diffusion through a different transporter protein
• Polysaccharides and disaccharides are digested into monosaccharides by hydrolysis of the glycosidic bonds

  • Starch is digested to glucose in two stages:
  • Starch --> (amylase) Maltose --> (maltase) a-glucose