Digestion and absorption

• What are the two stages of digestion?

  Physical breakdown

  Chemical digestion
• What happens during digestion?

  Large biological molecules are hydrolysed into smaller molecules and then can be absorbed across cell membranes
• What are the three types of digestive enzymes?

  Carbohydrases - hydrolyse carbohydrates into monosaccharides

  Lipases - hydrolyse lipids into glycerol and fatty acids

  Proteases - hydrolyse proteins into amino acids
• Name types of carbohydrases

  Amylase - starch -> maltose

  Maltase - maltose -> glucose

  Sucrase - sucrose -> glucose + fructose

  Lactase - lactose -> glucose + galactose
• Where is amylase produced?

  In the mouth and pancreas
• Where is maltase produced?

  In the lining of the ileum
• Describe the process of carbohydrate digestion

  1. Salivary amylase hydrolyses starch into maltose
  2. Food enters the stomach and the acid denatures the amylase to stop further hydrolysis
  3. Food is passed into the small intestine and mixes with pancreatic juice
  4. Pancreatic juice contains pancreatic amylase
  5. Pancreatic amylase hydrolyses any remaining starch to maltose
  6. Food is pushed along the ileum
  7. Maltase is produced - it is bound to the cell-surface membrane - membrane-bound disaccharidase
  8. Maltase hydrolyses maltose to glucose
• Explain how glucose/amino acids are absorbed in the ileum

  Glucose/amino acids and sodium move in by facilitated diffusion (symport) down a concentration gradient

  Sodium ions are actively transported from the ileum to the blood

  This maintains a concentration gradient

  Potassium is pumped into the ileum at the same time (antiport)

  Glucose/amino acids leave the ileum by facilitated diffusion
• Where are lipases produced?

  In the pancreas
• What is a monoglyceride?

  A glycerol molecule with one fatty acid molecule attached
• Describe the role of micelles in the process of lipid absorption

  Bile salts are produced by the liver

  They emulsify lipids into tiny droplets called micelles

  Micelles make fatty acids/monoglycerides more soluble in water

  Micelles increase the surface area for lipases to work on

  They bring the fatty acids/monoglycerides to the lining of the ileum

  Fatty acids/monoglycerides are absorbed by diffusion

  Triglycerides are formed in the cell

  Triglycerides are packaged into vesicles

  Vesicles move to cell membrane via exocytosis

  They enter lymphatic capillaries called lacteals
• What do micelles contain?

  Bile salts and fatty acids/monoglycerides
• Name three types of peptidases (proteases)

  Endopeptidases

  Exopeptidases

  Dipeptidases
• What is the role of endopeptidases?

  Hydrolyse the peptide bonds in the middle of a polypeptide to produce shorter polypeptides
• What is the role of exopeptidases?

  Hydrolyse at the end of polypeptides to produce dipeptides/amino acids
• What is the role of dipeptidases?

  Hydrolyse dipeptides into single amino acids
• How do villi increase the efficiency of absorption?

  • Increase surface area for diffusion
  • Thin walls - so reduces diffusion distance
  • Well supplied with blood vessels - so blood can carry away absorbed molecules which maintains a diffusion gradient
  • Epithelial cells lining villi have microvilli - further increase surface area for absorption