CIE A Level Biology复习笔记14.1.2 Production of Urea

Production of Urea

  • Many metabolic reactions within the body produce waste products
  • The removal of these waste products is known as excretion
  • Many excretory products are formed in humans, with two in particular (carbon dioxide and urea) being formed in much greater quantities than others


  • Urea is produced in the liver
  • It is produced from excess amino acids
  • If more protein is eaten than is required, the excess cannot be stored in the body
  • However, the amino acids within the protein can still provide useful energy
  • To make this energy accessible, the amino group is removed from each amino acid
  • This process is known as deamination:
    • The amino group (-NH2) of an amino acid is removed, together with an extra hydrogen atom
    • These combine to form ammonia (NH3)
    • The remaining keto acid may enter the Krebs cycle to be respired, be converted to glucose, or converted to glycogen / fat for storage


Deamination of an amino acid

  • Ammonia is a very soluble and highly toxic compound that is produced during deamination; it can be very damaging if allowed to build up in the blood
    • It dissolves in the blood to form alkaline ammonium hydroxide, disrupting blood pH
    • It can impact the reactions of cell metabolism such as respiration
    • It interferes with cell signalling processes
  • This is avoided by converting ammonia to urea
    • Urea is less soluble and less toxic than ammonia
  • Ammonia is combined with carbon dioxide to form urea


Formation of urea