AQA GCSE Biology Biology Paper 1 (Higher), June 2025: Question 3

💡 Key Knowledge (Evaluation Points)

An evaluation requires both strengths and improvements with scientific reasons:

• Strengths in method: Mass measured before and after; change in mass calculated; potato cut into consistent shape (cubes); different concentrations tested.
• Improvements needed:
  • Dry / blot cubes before reweighing to remove surface water.
  • Calculate percentage change in mass because starting masses will vary.
  • Control time: Leave cubes for a set, identical time (e.g. 30 mins) rather than vague "some time".
  • Control surface area / dimensions: Cut cubes with identical dimensions or use a cork borer.
  • Peel potatoes: Remove skin as it forms an impermeable barrier to osmosis.
  • Control temperature: Use a thermostatic water bath.
  • Repeat: At least 3 times at each concentration to calculate a mean and identify anomalies.
  • Test more concentrations: Expand beyond 3 concentrations to plot a reliable graph.

✅ Model Level 3 Answer (5–6 Marks)

Strengths:

"The student correctly measured the mass before and after immersion, calculated the change in mass, and tested different concentrations of salt solution."

Improvements & Reasons:

• "Blot the potato cubes with a paper towel before reweighing, because excess surface solution artificially increases final mass."
• "Calculate the percentage change in mass, because the initial mass of each cube will vary, making absolute mass change non-comparable."
• "Specify an exact, identical time (e.g., 60 minutes) for each cube rather than 'some time', so water movement duration is controlled."
• "Peel the skin off the potato before cutting, because skin acts as an impermeable barrier preventing osmosis."
• "Repeat at each concentration (at least 3 times) to spot anomalies and calculate a concordant mean."

🧠 Exam Technique: Securing Level 3 (5–6 Marks)

• The "Improvement + Because" Rule: Level 3 requires a linked judgement supported by reasons. Never just write "repeat the test"; write "repeat at each concentration so that anomalies can be identified and a mean calculated."
• Balance is mandatory: The mark scheme states that for Level 3, both a strength and multiple improvements with reasons must be included.
• Avoid vague statements: Writing "keep everything fair" scores zero. Explicitly name the variable (e.g., volume of solution, temperature, immersion time).

❌ Common Errors

• Only listing weaknesses: Forgetting to acknowledge that the student did some things correctly (cutting shapes, measuring initial and final mass).
• "Measure mass accurately": Too vague. State the tool or action: "use an electronic digital balance to 2 decimal places" or "blot excess water".
• Confusing precision with validity: Improvements must directly ensure that only the independent variable (salt concentration) affects the change in mass.

✅ Correct Answer

☑ The solution in the potato cells was more concentrated than the distilled water.

Award 1 mark for selecting option 2.

💡 Scientific Explanation

• Distilled water is pure water (0.0 mol dm⁻³ solute). It has a very high water concentration (dilute solute).
• The cytoplasm and vacuole inside potato cells contain dissolved salts and sugars (a more concentrated solute solution / lower water concentration).
• Water molecules move by osmosis from a dilute solution (high water concentration) into a more concentrated solution (low water concentration) across the partially permeable cell membrane.
• This net influx of water causes the cell to swell and gain mass.

❌ Common Error: "Volume" vs "Concentration"

Students frequently confuse volume with concentration. Osmosis is entirely driven by concentration gradients across membranes, not the total volume of liquid in the beaker!

✅ Acceptable Answers (Give ANY ONE)

• Energy requirement: Active transport requires energy (from respiration), whereas osmosis is a passive process (does not require energy).
• Direction of gradient: Active transport moves substances against / up a concentration gradient (from low to high concentration), whereas osmosis involves net movement down a water concentration gradient.
• Substance transported: Active transport moves mineral ions or solutes (e.g. nitrates, glucose), whereas osmosis refers specifically to the transport of water.

🧠 Examiner Tips on Keywords

• "Against" vs "Along": You MUST say against or up the concentration gradient. Saying "transports along" or "across" the gradient is ignored and will not gain credit.
• Definite "No": Do NOT write that active transport moves substances "down" a concentration gradient. That is diffusion!
• Specificity: Simply saying "it is active" is ignored; state clearly that it requires energy (from cellular respiration).