the liver

• Liver


  Located in the upper abdominal cavity, a very large organ with a host of different functions, one of which is the preparation of materials for excretion
• Proteins


  • Primarily 'body builders'; make up the structural materials of cells
  • If the body has a sufficient supply of carbohydrates and fats, or has a supply of stored fat, then little protein is used in energy-releasing reactions
  • Excess protein from the diet cannot be stored in the cells, and so processes are required to remove it from the body
• Protein breakdown


  1. Worn-out cells, such as red blood cells, are a source of protein, and are broken down into the constituent amino acids

  2. Most of these amino acids are then used to make new proteins

  3. A very small amount is lost from the body via the urine, skin, hair and fingernails
• Proteins


  • Become the primary constituents of cell structures, enzymes, antibodies and many glandular secretions
  • If other energy sources have been used up, the body is able to metabolise large amounts of proteins, breaking them down to produce energy
• Deamination


  1. The amino group (NH2) must first be removed from the amino acids

  2. This process occurs in the liver with the aid of enzymes

  3. Once the amino group has been removed, it is converted by the liver cells to ammonia (NH3) and then finally to urea
• Ammonia


  • Extremely soluble in water and is highly toxic to cells
  • One-thousandth of a milligram of ammonia in a litre of blood is sufficient to kill a person
  • The cells of the liver rapidly convert ammonia to the less toxic molecule urea
• Urea


  • Moderate amounts are harmless to the body
  • It is easily excreted by the kidneys and is eliminated from the body in the urine
  • Small amounts of urea are also lost in sweat from the sweat glands