Digestion

• Digestive system


  A tube that extends from the mouth, through the body, to the anus. Each organ has a specialised role in the breakdown and absorption of food molecules and absorption of water.

  
• Digestive juices


  Produced by the gland cells of the digestive system, they release hydrolytic enzymes and other molecules that facilitate digestion.
• Hydrolysis of large insoluble food molecules (polymers) into smaller soluble molecules (monomers)

  1. Enzymes hydrolyse the molecules

  2. Monomers can then be absorbed through the lining of the intestine

  
• Hydrolysis of food molecules

  • Polypeptides & proteins into amino acids by proteases
  • Carbohydrates into simple sugars by carbohydrases
  • Fats into glycerol, fatty acids and monoglycerides by lipases

  
• Digestive glands and enzymes

  • Salivary glands - Salivary Amylase
  • Stomach - Endopeptidases (e.g. Pepsin) & Exopeptidases
  • Pancreas - Pancreatic Amylase, Lipases & Exopeptidases
  • Ileum - Membrane bound Dipeptidases & Dissacharidases

  
• Carbohydrate digestion


  1. Food enters mouth, mixed with saliva, salivary amylase hydrolyses starch to maltose

  2. In stomach, salivary amylase denatured due to acidic pH

  3. In small intestine, pancreatic amylase hydrolyses starch to maltose

  4. Maltose hydrolysed to glucose by maltase enzymes in epithelial cell membrane

  
• Mammals cannot hydrolyse cellulose as they do not produce the enzyme cellulase
• Summary of starch digestion

  • Amylase (salivary and pancreatic) hydrolyses starch to maltose
  • Maltase hydrolyses maltose to glucose

  
• Monosaccharides


  Only monosaccharides (glucose, fructose, galactose) can be transported across the epithelial cell membrane as they are small enough and complementary to specific carrier/channel proteins
• Absorption of monosaccharides


  Monosaccharides absorbed by facilitated diffusion and co-transport using specific carrier proteins
• Glucose absorption


  1. Na+ actively transported out of cell, creating Na+ gradient

  2. Na+ and glucose enter cell by facilitated diffusion using cotransporter proteins

  3. Glucose moves into cell against concentration gradient

  4. Glucose moves into blood by facilitated diffusion

  
• Epithelial cells contain many mitochondria to produce ATP for active transport
• Humans cannot hydrolyse cellulose as they do not produce cellulase
• Some people do not produce lactase and are unable to hydrolyse lactose
• Lactose and all disaccharides are soluble so they lower water potential
• Bacteria in large intestine can produce enzymes to hydrolyse cellulose and lactose
• Protein digestion


  1. Proteases hydrolyse peptide bonds in polypeptides

  2. Endopeptidases hydrolyse internal peptide bonds, producing shorter polypeptides

  3. Exopeptidases hydrolyse terminal peptide bonds, producing amino acids and dipeptides

  4. Dipeptidases in epithelial cell membrane hydrolyse dipeptides into amino acids for absorption

  
• Amino acid absorption


  Amino acids absorbed by facilitated diffusion and cotransport using specific carrier proteins
• Lipid digestion and absorption

  1. Lipid droplets emulsified by bile salts, increasing surface area for lipase

  2. Lipases hydrolyse triglycerides into glycerol, fatty acids and monoglycerides

  3. Bile salts, glycerol and fatty acids form micelles

  4. Fatty acids enter epithelial cells by simple diffusion

  5. In ER, fatty acids and glycerol recombined into triglycerides

  6. In Golgi, triglycerides modified into lipoproteins (chylomicrons) and packaged into vesicles

  7. Chylomicrons transported into lymph vessel by exocytosis, then enter blood