CIE A Level Biology复习笔记14.1.2 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

• 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