CIE IGCSE Biology: 复习笔记：13.1.3 The Role of the Liver in Excretion

• Many digested food molecules absorbed into the blood in the small intestine are carried to the liver for assimilation (when food molecules are converted to other molecules that the body needs)
• These include amino acids, which are used to build proteins such as fibrinogen, a protein found in blood plasma that is important in blood clotting
• Excess amino acids absorbed in the blood that are not needed to make proteins cannot be stored, so they are broken down in a process called deamination
  • The amino group of all  amino acids - NH₂ (which contains the nitrogen atoms) is removed, hence the term de-amin(o)-ation
• Enzymes in the liver split up the amino acid molecules
• The part of the molecule which contains carbon is turned into glycogen and stored
• The other part, which contains nitrogen, is turned into ammonia, which is highly toxic, and so is immediately converted into urea, which is less toxic
• The urea dissolves in the blood and is taken to the kidney to be excreted
• A small amount is also excreted in sweat

Amino acid groups

• In deamination, the nitrogen-containing amino group is removed and converted into ammonia and then urea to be excreted
• The toxic consequences of high urea levels, if it is not excreted effectively, are very serious:
  • Cell death
  • Reduced response to insulin, leading to diabetes
  • Deposits inside blood vessels