Formation of Urea (Edexcel GCSE Biology)

• Excretion is the removal of the waste substances of metabolic reactions (the chemical reactions that take place inside cells), toxic materials and substances in excess of requirements
• Too much carbon dioxide in the body is toxic and therefore, must be excreted
  • It dissolves in water easily to form an acidic solution which can lower the pH of cells
  • This can reduce the activity of enzymes in the body which are essential for controlling the rate of metabolic reactions

• Urea is also toxic to the body in higher concentrations and so must be excreted via the liver

The main organs involved in excretion. The lungs excrete carbon dioxide, the liver forms urea from excess amino acids and the kidneys excrete water, excess salts and urea.

The role of the liver - forming urea

• 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
• 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