IGCSE · Practice questions · Excretion and the liver

From amino acid to urine.

Six original Cambridge-style questions on what the lungs and kidneys excrete, the route urine takes out of the body, and (Extended) how the liver deals with amino acids, with data on protein intake and urea output.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Lungs, kidneys, liver.

Lungs: excrete carbon dioxide from respiration.
Kidneys: excrete urea, excess water and excess ions as urine.
Route: kidney, ureter (two), bladder, urethra (one).
Extended: liver assimilates amino acids into proteins; excess are deaminated and the nitrogen-containing part forms urea, which is toxic.

Recall

[3 marks]

State the waste product excreted through the lungs, and the substances excreted by the kidneys.

The lungs excrete carbon dioxide (produced by respiration). ✓
The kidneys excrete urea. ✓
The kidneys also excrete excess water and excess ions (salts). ✓

Recall

[4 marks]

A diagram of the human excretory system shows structures P, Q, R and S. P is a bean-shaped organ that forms urine; Q is a tube leaving P; R is a muscular bag that stores urine; S carries urine out of the body. Name P, Q, R and S.

P: kidney. ✓
Q: ureter. ✓
R: bladder. ✓
S: urethra. ✓

Explain

[4 marks]

Extended. Describe what happens in the liver to amino acids absorbed from the small intestine.

Some amino acids are assimilated: the liver uses them to make proteins (for example plasma proteins such as fibrinogen). ✓
Excess amino acids cannot be stored. ✓
They are deaminated: the nitrogen-containing part of each amino acid is removed. ✓
The nitrogen-containing part is used to form urea, which passes into the blood and is carried to the kidneys; the rest of the molecule can be respired for energy. ✓

Data

[4 marks]

The table shows the mass of urea excreted per day by an adult on diets containing different masses of protein.

Protein eaten / g per day	Urea excreted / g per day
40	12
80	25
120	37

(a) Describe the relationship shown. (b) Extended. Explain the relationship.

(a) As the mass of protein eaten increases, the mass of urea excreted increases; it roughly triples when protein triples (approximately proportional). ✓
(b) Protein is digested to amino acids, which are absorbed into the blood. ✓
The more amino acids absorbed beyond what the body needs to make proteins, the more are deaminated in the liver. ✓
So more urea is formed, and the kidneys excrete more urea in the urine. ✓

Explain

[3 marks]

Extended. A person's kidneys stop working. Explain why this is life-threatening, with reference to urea.

The liver continues to deaminate excess amino acids and make urea. ✓
Without working kidneys, urea is not removed from the blood, so its concentration rises. ✓
Urea is toxic, so it damages / poisons cells; without treatment such as dialysis or a transplant the person would die. ✓

Analysis

[3 marks]

A student writes: "Excretion is the removal of faeces, and urea is made in the kidneys." Identify the two errors and correct them.

Removal of faeces is egestion, not excretion: faeces are mainly undigested food that was never part of the body's metabolism. ✓
Excretion is the removal of the waste products of metabolism (such as carbon dioxide and urea) and substances in excess (such as water and ions). ✓
Urea is made in the liver (from excess amino acids by deamination), not in the kidneys; the kidneys excrete it. ✓

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