Osmosis in potato — required practical

AQA GCSE Biology

Investigate the effect of sugar (or salt) concentration on the mass of potato cylinders — the effect of osmosis.

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

Method

  1. Use a cork borer to cut potato cylinders, then cut them to the same length; measure and record the starting mass of each.
  2. Place each cylinder in a different concentration of sugar solution (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³).
  3. Leave for the same length of time (e.g. 30 minutes).
  4. Remove, gently pat dry to remove surface liquid, and measure the final mass.
  5. Calculate the percentage change in mass for each concentration.

Variables

  • Independent variable: Concentration of sugar/salt solution
  • Dependent variable: Percentage change in mass of the potato
  • Control variables: Potato variety, temperature, time in solution, volume of solution, surface area of cylinders

Results & analysis

Percentage change = (final mass − initial mass) ÷ initial mass × 100. Plot % change against concentration; where the line crosses the x-axis (0% change) the solution is the same concentration as the potato cell contents (isotonic).

Risk assessment

  • Care with the cork borer and scalpel — cut on a tile, away from fingers.

Improving accuracy & reliability

  • Pat each cylinder dry the same way before weighing.
  • Repeat and take a mean.
  • Cut cylinders to the same size and surface area.
Exam tip: Use percentage change in mass, not just change in mass — it corrects for cylinders starting at slightly different sizes.

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