Explain why a piece of potato placed in a concentrated salt solution will decrease in mass.

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).

A student cuts identical pieces of potato and places them into different concentrations of salt solution. After 30 minutes, the student removes the potato pieces and measures their mass.

Model answer (5 marks)

Water moves out of the potato cells by osmosis because the salt solution has a lower water concentration than the cytoplasm/vacuole of the cells. Water therefore moves from the higher water concentration inside the cell to the lower concentration in the external solution through the partially permeable cell membrane. As the cells lose water they become flaccid and the tissue shrinks, giving a decrease in mass.

Examiner tips

  • Use the word ‘osmosis’ and explain the direction of water movement (high to low concentration).
  • Mention the partially permeable membrane and the resulting flaccidity/shrinkage of the tissue.

Common mistakes

  • Failing to state that the movement is by osmosis or that the membrane is partially permeable.
  • Confusing the direction of water flow or omitting the link to mass loss.

Mark scheme (5 marks)

  1. Water moves out of the potato cells by osmosis
  2. The salt solution has a lower water concentration than the potato cell cytoplasm / vacuole
  3. Water moves from a higher water concentration (inside the cell) to a lower water concentration (in the salt solution)
  4. Water passes through the partially permeable membrane (of the potato cells)
  5. The potato cells lose water so the cells become flaccid / the tissue shrinks, causing a decrease in mass

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