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AQA GCSEAQA Combined Science Walkthrough: Osmosis Required Practical — Interpreting Results and 6-Mark Questions
Required practical questions appear on every AQA Combined Science Biology paper and are consistently poorly answered. This walkthrough covers the osmosis practical and the 6-mark question format that examiners say students most commonly drop marks on.
The question
A student investigated the effect of sucrose concentration on potato cylinders. She cut five potato cylinders of equal mass (2.50 g each) and placed each into a different concentration of sucrose solution (0%, 0.2 mol/l, 0.4 mol/l, 0.6 mol/l, 0.8 mol/l) for 30 minutes. She then removed, dried, and reweighed each cylinder. Her results are shown in the table below. [Table: sucrose concentration vs final mass, showing the cylinders gaining mass at low concentrations and losing mass at high concentrations] (a) Describe and explain the results for the cylinder placed in 0% sucrose solution. [3 marks] (b) The student concludes that the solute concentration inside the potato cells is approximately 0.4 mol/l. Evaluate this conclusion using the data. [3 marks]
[6 marks]
Part (a): Describe and explain — 3 marks
Describe the result (1 mark)
State what happened using the data: "The potato cylinder placed in 0% sucrose solution gained mass — its final mass was greater than its initial mass of 2.50 g." Always use specific data from the table or graph. "It gained mass" alone is not enough without reference to the numbers.
Explain using osmosis (2 marks)
Apply the definition of osmosis and the water potential gradient: "The 0% solution is pure water, which has a higher water potential than the solution inside the potato cells. Water therefore moves by osmosis from the solution (high water potential) into the potato cells (lower water potential) through the partially permeable cell membrane. This causes the cells to become turgid, increasing the mass of the cylinder." Two marks require two ideas: the direction of the water potential gradient AND the resulting movement of water by osmosis. State both explicitly.
Part (b): Evaluate the conclusion — 3 marks
Evaluate questions are worth 3 marks and require a structured response: state whether the conclusion is supported, use specific data to justify, and identify a limitation. Here is a full Band 3 response:
"The conclusion is partially supported by the data. If the solute concentration inside the potato is 0.4 mol/l, we would expect the cylinders placed in 0.4 mol/l sucrose to show no change in mass — the water potential inside and outside the cells would be equal, so no net movement of water would occur by osmosis. The data supports this because the cylinder in 0.4 mol/l shows the smallest change in mass compared to other concentrations. However, the conclusion would be strengthened by using more concentrations around 0.4 mol/l (e.g., 0.3, 0.35, 0.4, 0.45, 0.5 mol/l) to identify the isotonic point more precisely. The current data does not confirm that 0.4 mol/l is exactly the isotonic point — only that it is closest to it in this experiment."
What makes an evaluate answer score full marks
- State whether you agree or disagree with the conclusion first — do not hedge
- Use specific data from the experiment (actual masses or concentrations, not just "the results show")
- Reference the scientific principle (osmosis, water potential) that supports or challenges the conclusion
- Identify a limitation or improvement — the most common mark that is missed
Required practical: what to revise
The osmosis practical is one of the 21 AQA required practicals across Biology, Chemistry, and Physics. For every required practical, revise: the independent variable (what you change), the dependent variable (what you measure), control variables (what you keep the same and why), the expected result, and how you would improve the reliability of the investigation. AQA questions on required practicals follow predictable structures — describe the method, explain a result, evaluate the conclusion, or suggest an improvement. Practising one past question on each required practical before the exam is the highest-value use of Combined Science revision time.
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