Core Practical: Investigate osmosis in potatoes

The idea

This core practical measures the effect of different concentrations of sugar or salt solution on potato tissue. Cut potato cylinders of equal length and diameter, blot them dry and record their initial mass. Place each cylinder in a labelled solution of known concentration (including pure water as a control) for a set time, then remove, blot dry and reweigh. Calculate the change in mass for each. In pure water the potato gains mass because water enters by osmosis; in concentrated solutions it loses mass because water leaves. Plot change in mass against concentration and read off the concentration at which there is no change, where the water potential of the solution equals that of the potato. Control variables include cylinder size, temperature, time and volume of solution.

What earns the marks

• Identifies the independent variable as the concentration of the sugar or salt solution and the dependent variable as the change in mass (or length) of the potato cylinder.
• States that potato cylinders must be equal in size and blotted dry, and that temperature, time and solution volume are controlled, to make the test fair.
• Explains that in pure water the potato gains mass because water enters by osmosis, and in concentrated solution it loses mass because water leaves by osmosis.
• Describes using the results to find the concentration at which there is no change in mass, where the solution is isotonic to the potato tissue.
• Recognises that change in mass is calculated by subtracting initial mass from final mass and that results should be plotted on a graph.

Common mistakes and how to put them right

• Using potato cylinders of different sizes.

  Put it right by stating all cylinders must be the same length and diameter so mass change is comparable.

• Forgetting to blot the cylinders dry before weighing.

  Put it right by stating excess surface water must be removed so the mass recorded reflects the tissue.

• Assuming the potato always gains mass.

  Put it right by stating it loses mass in concentrated solutions because water leaves by osmosis.

Exam tips

• Name the independent and dependent variables explicitly before describing the method.
• Explain the direction of water movement for each solution concentration, not just the mass change.
• State how the graph is used to identify the concentration with no net change in mass.