Metabolism: breakdown of excess proteins to form urea for excretion.

The idea

Amino acids absorbed from digested protein cannot be stored by the body, so any surplus is broken down in the liver. The nitrogen-containing amino group is removed (deamination) and converted into urea, while the remaining carbon skeleton is respired to release energy. Urea is toxic, so it passes into the blood and is filtered out by the kidneys, then excreted in urine. For example, a person eating more protein than their cells need will produce extra urea, which is why urine tests can reveal how well the kidneys are working. This links metabolism, the liver, the kidneys and excretion together.

What earns the marks

• Excess amino acids from protein digestion are transported to the liver in the blood.
• The liver removes the amino group from excess amino acids, a process called deamination.
• The nitrogen from the removed amino group is converted into urea.
• Urea is toxic and must be excreted rather than stored in the body.
• Urea travels in the blood plasma to the kidneys, where it is filtered out of the blood.
• Urea leaves the body dissolved in urine, which is excreted via the ureters, bladder and urethra.
• The carbon skeleton left after deamination can be respired to release energy or converted to carbohydrate or fat.

Common mistakes and how to put them right

• Thinking that excess protein is stored unchanged in the body.

  Put it right: Surplus amino acids are broken down in the liver, and the nitrogen is converted to urea.

• Confusing deamination with digestion.

  Put it right: Digestion breaks protein into amino acids in the gut, whereas deamination breaks down amino acids in the liver.

• Saying that urea is made in the kidneys.

  Put it right: Urea is made in the liver; the kidneys filter urea out of the blood and excrete it in urine.

Exam tips

• Use the term deamination accurately and link it to the liver, not the kidney.
• When asked about excretion, name urea as the nitrogenous waste and state that it leaves in urine.
• Sequence the route clearly: excess amino acids → liver → urea → blood → kidneys → urine.
• If asked why urea must be removed, state that it is toxic and would build up if not excreted.