The Role of the Liver in Excretion (Cambridge O Level Biology)

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Excretion by Deamination of Amino Acids

  • 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 allĀ  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

An Amino Acid's Chemical Structure Diagram

an amino acid's structure before deamination

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

Examiner Tip

Excretion and egestion are two terms that often get confused:

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.

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Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.