Deamination

In the syllabus and the exam

Deamination is introduced in the topic of homeostasis and the human urinary system, where candidates learn how the liver regulates the amount of amino acids circulating in the blood. Because the body cannot store surplus amino acids the way it stores fat or glycogen, any protein eaten in excess of the body's needs is broken down in the liver: the amino group is removed from each amino acid, converted first into toxic ammonia and then, in a linked reaction, into the less toxic urea. This urea is released into the bloodstream and later filtered out by the kidneys during ultrafiltration, so deamination is frequently tested alongside excretion rather than as a stand-alone topic. In Paper 2 structured questions, candidates are commonly asked to state the organ where deamination occurs (the liver, not the kidney) and to name both products (ammonia converted to urea), and marks are lost when a candidate confuses deamination with excretion itself or names the kidney as the site of the reaction. Another frequent error is describing deamination as breaking down glucose or lipids; the process acts specifically on the amino group of amino acids, not on carbohydrates or fats. Some candidates also forget that ammonia must still be converted to urea before it is safe to transport in the blood, since ammonia itself is too toxic to be excreted directly by the human body. It is also worth noting that deamination happens continuously in a healthy liver whenever dietary protein intake exceeds immediate need, not only after an unusually large meal, so the amount of urea produced tends to track a person's overall protein intake over time.

Example

In an SPM structured question describing a high-protein diet, a candidate explains: 'Excess amino acids cannot be stored, so the liver breaks them down by deamination, removing the amino group to form ammonia, which is converted to urea before it enters the blood for excretion by the kidneys.'

Related terms

Amino acidAssimilationMetabolism

Source:SRC-DSKP-EN