Explain why urea moves from liver cells into the blood.

Eduqas GCSE Biology — 1.1.2 Movement of substances · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Urea is a waste product made in liver cells. It is transported in the blood to the kidneys, where it is removed from the body.

Model answer (4 marks)

Urea moves into the blood by diffusion.
Urea moves from an area of high concentration to an area of low concentration.
The concentration of urea is higher inside the liver cell than in the blood.
A concentration gradient exists between the liver cell and the blood, allowing net movement of urea into the blood.

Examiner tips

Use the word ‘diffusion’ and explain the direction of movement. Mention the concentration gradient and that it drives the net movement. Keep the answer short and to the point, matching the four points in the mark scheme.

Common mistakes

Confusing diffusion with active transport. Forgetting to state that the concentration is higher inside the cell. Using vague terms like ‘movement’ without specifying direction or mechanism.

Mark scheme (4 marks)

Urea moves by diffusion
Urea moves from an area of high concentration to an area of low concentration
The concentration of urea is higher inside the liver cell than in the blood
A concentration gradient exists (between the liver cell and the blood), allowing net movement of urea into the blood

Key terms in this question

urea