A red blood cell is placed into a solution that has a lower water concentration than the cytoplasm of the cell. Explain what happens to the red blood cell.

AQA A-Level Biology (7402) — 3.2.3 Transport Across Membranes · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Water moves out of the red blood cell by osmosis. The water moves from an area of higher water concentration (inside the cell) to an area of lower water concentration (in the surrounding solution) through the partially permeable membrane. As a result, the red blood cell shrinks (crenates) and decreases in size.

Examiner tips

Use the word ‘osmosis’ to show understanding of the mechanism.
Explain the direction of water movement in terms of concentration gradients.
Mention the membrane’s permeability to water.
Describe the physical change in the cell (shrinkage/crenation).

Common mistakes

Failing to mention osmosis or the direction of water movement.
Describing the cell as swelling instead of shrinking.
Using vague terms like ‘changes’ without specifying crenation.

Mark scheme (5 marks)

Water moves out of the red blood cell
By osmosis
Water moves from an area of higher water concentration (inside the cell) to an area of lower water concentration (in the surrounding solution)
Water moves through the partially permeable membrane
The red blood cell shrinks / crenates / decreases in size

Key terms in this question

water concentration