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AQA GCSE Biology (9-1): Getting the most out of Required Practicals - Osmosis

Posted by James Simms on November 22, 2019

Specification location: AQA GCSE Biology Paper 1, Section 4.1 - Cell biology


Summary of method: Small pieces of potato (or other vegetables) are cut and their masses are recorded. They are then immersed in a variety of strengths of sugar or salt solutions and left for osmosis to occur (an absolute minimum of 15 minutes). They are then removed and their exteriors are dried. Their masses are then measured again so that the change in their mass and their percentage change in mass can be calculated. From this, the approximate concentration inside the potato cells can be determined by plotting the results on a graph and looking for the point at which the line of best fit crosses the x-axis (no change in mass means that the solution is isotonic).


For more detail on the set-up of this experiment, watch our high-quality required practical tutorial on YouTube.


Osmosis required practical variables:

Independent variable: Concentration of sugar or salt solution.

Dependent variable: Percentage mass change of potato pieces (or length change of potato pieces)

Control variables: Length, surface area, temperature, potato age and type, time


Common issues

Potential solutions


With a sixty-minute lesson, it can be challenging to outline the practical, prepare it, leave the potato pieces for enough time for sufficient osmosis to occur and analyse the results. 

If necessary, the potato pieces could be prepared to minimise practical prep within the lesson. However, experience has taught me that it still can prove to be insufficient, especially if the osmosis subject content needs to be recapped. Pupils can also set up the experiment first: blindly without understanding what the investigation is. However, I feel this removes the educational value of a significant chunk of the lesson.

By far, the best solution I have found is for pupils to preload the investigation’s methodology through a flipped learning technique (e.g. viewing and note-taking tutorials and undertaking quizzes on This minimises lag time at the start through misunderstanding and also the time needed to be spent on covering osmosis content again.

Control of potato piece surface area

Obviously, if the solution concentration is the independent variable and the mass change of the potato pieces through osmosis is the dependent variable, all other variables must be controlled.

By far, the easiest way to control the surface area is to use a cork borer to produce potato cylinders of uniform diameter. These can then be measured and trimmed to a predetermined length using a ruler and scalpel.

Factors affecting the rate of osmosis

When cutting the potato, it is important to trim any skin from the pieces to be used. The potato skin has a different permeability to water to the internal plant tissues, so a varying presence of skin will cause osmosis to occur at different rates and the results to be less precise. This will need to be trimmed off neatly using a scalpel. It is important to note that this must not affect the total length of the potato piece as this must be controlled between different solution concentrations.

Student mathematical skills

In my experience, students often struggle to calculate percentage change. This can be combated by sufficient teacher modelling and through flipped learning (which also addresses the earlier time issue).

Student graph skills

Again, in my experience, students often struggle to draw graphs which have both positive and negative x-axis values (in this case, percentage mass change). This is worth modelling before and during the required practical lesson itself, and any flipped learning that can be done beforehand (e.g. drawing example graphs or completing worksheets) is also helpful.


Sample osmosis required practical results:


1.0mol/dm3 sugar solution

0.75mol/dm3 sugar solution

0.5mol/dm3 sugar solution

0.25mol/dm3 sugar solution

Distilled water

Initial mass (g)






Final mass (g)






Change in mass (g)






Percentage change in mass






Source: AQA GCSE Biology (9-1) required practical handbook

AQA GCSE Science subject-specific terminology:

  • As students will be measuring small changes in mass, probably with a two-decimal-place balance, this is an ideal opportunity to discuss the idea of resolution.
  • As the concentrations of the solutions will vary, this is also an ideal opportunity to discuss both the ideas of both intervals and range.


Other blog posts in our AQA GCSE Biology (9-1) Required Practical series:

Getting the most out of GCSE Biology Required Practicals: Microscopy

Getting the most out of GCSE Biology Required Practicals: Microbiology

Getting the most out of GCSE Biology Required Practicals: Food tests

Getting the most out of GCSE Biology Required Practicals: Enzyme activity

Getting the most out of GCSE Biology Required Practicals: Photosynthesis

Getting the most out of GCSE Biology Required Practicals: Reaction times

Getting the most out of GCSE Biology Required Practicals: Plant responses

Getting the most out of GCSE Biology Required Practicals: Field Investigations

Getting the most out of GCSE Biology Required Practicals: Decay