• 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