The Role of the Liver in Excretion (Cambridge International IGCSE Biology)

Excretion by deamination of amino acids: Extended

Spec point — 13.1.3 · Describe the role of the liver in the assimilation of amino acids by converting them to proteins State that urea is formed in the liver from excess amino acids Describe deamination as the removal of the nitrogen-containing part of amino acids to form urea Explain the importance of excretion, limited to toxicity of urea

Excretion by deamination of amino acids: Extended

• 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 in the blood that are not needed by the body 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 for storage
  • the nitrogen-containing amino group is removed and 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 of urea is also excreted in sweat

Amino acid groups

• The toxic consequences of high urea levels, if it is not excreted effectively, are very serious:

  • cell death
  • reduced response to insulin, potentially leading to diabetes
  • deposits inside blood vessels

Exam Hint Take care that you understand the difference between excretion and egestion:

• excretion is the removal from the body of waste products of metabolic reactions, toxic substances and substances in excess of requirements.
• egestion is the expulsion of undigested food waste from the anus.