Explain why red blood cells would be damaged if placed in a solution with a lower water concentration than the cytoplasm of the cells.

Edexcel GCSE Biology (1BI0) — 1.5 Transport in cells · Explain · 4 marks · View as Markdown

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

Red blood cells are specialised cells that lack a cell wall. They are normally found in blood plasma, which has a water concentration very similar to that of the cytoplasm inside the cells.

Model answer (4 marks)

The external solution has a lower water concentration than the cytoplasm, creating a water concentration gradient.
Water therefore moves out of the red blood cell by osmosis.
The movement occurs through the partially permeable cell membrane.
As water leaves, the cell shrinks (crenates) and loses its normal shape because it has no cell wall to resist the loss.

Examiner tips

Use the word ‘osmosis’ and mention the membrane is partially permeable.
Show the direction of water movement (outside to inside or vice versa).
Explain the consequence (crenation) and link it to lack of a cell wall.

Common mistakes

Confusing water concentration with solute concentration.
Forgetting to mention the membrane’s permeability.
Saying the cell ‘expands’ instead of ‘shrinks’ in a hypotonic solution.

Mark scheme (4 marks)

The solution outside has a lower water concentration than inside the red blood cell / water concentration gradient exists
Water moves out of the red blood cell by osmosis
Water moves through the partially permeable / cell surface membrane
The cell shrinks / crenates / loses its normal shape because there is no cell wall to prevent water loss

Key terms in this question

water concentration