Explain why a plant cell that has been placed in a concentrated salt solution will eventually reach a state of plasmolysis.

IB DP Biology Standard Level (2023 syllabus) — D2.3 Water potential · Explain · 4 marks · View as Markdown

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

A plant cell is transferred from distilled water into a concentrated sodium chloride solution.

Model answer (4 marks)

The concentrated NaCl solution has a lower (more negative) water potential than the cell’s cytoplasm/vacuole. Consequently water moves out of the cell by osmosis down the water‑potential gradient. As water leaves the vacuole the vacuole and cytoplasm shrink, losing volume. The plasma membrane (and tonoplast) then pulls away from the rigid cell wall, producing plasmolysis.

Examiner tips

Use the term ‘water potential’ and ‘osmosis’ explicitly.
Show the direction of water movement (out of the cell).
Explain that the membrane pulls away from the wall to give plasmolysis.

Common mistakes

Confusing the direction of water movement (e.g., saying water enters the cell).
Using vague terms like ‘shrinks’ without linking to plasmolysis.
Omitting the role of the cell wall in the final step.

Mark scheme (4 marks)

The concentrated salt solution has a lower (more negative) water potential than the cell's cytoplasm/vacuole.
Water moves out of the cell by osmosis down the water potential gradient.
The vacuole (and cytoplasm) shrinks/loses volume as water is lost.
The plasma membrane/tonoplast pulls away from the cell wall, resulting in plasmolysis.

Key terms in this question

plasmolysis