AQA GCSE Biology Biology Paper 1 (Foundation), June 2025: Question 8
8 marks · Standard Demand difficulty · Extended Answer
Evaluate a method investigating osmosis in potato cubes to suggest improvements for validity, explain why a potato cube increases in mass in distilled water, and compare active transport to osmosis.
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Question text
08 A student investigated osmosis in pieces of potato.
This is the method used.
1. Cut a piece of potato into a cube.
2. Record the mass of the cube.
3. Place the cube into a beaker containing 0.4 mol/dm3 salt solution.
4. After some time, remove the cube and record the mass.
5. Calculate the change in the mass of the cube.
6. Repeat steps 1 to 5 using two more different concentrations of salt solution.
08.1 Evaluate the student’s method.
You should include how the student could improve the validity of their results.
[6 marks]
08.2 Why would a potato cube placed into distilled water increase in mass?
[1 mark]
*36* Tick ( ) one box.
The solution in the potato cells was less concentrated
than the distilled water.
The solution in the potato cells was more concentrated
than the distilled water.
The volume of water in the potato cells was greater
than the volume of distilled water.
The volume of water in the potato cells was less
than the volume of distilled water.
08.3 Active transport is another process used by potato cells.
Give one way active transport is different from osmosis.
[1 mark]
Mark scheme
Show the mark scheme
Question 8
AO /
Question Answers Mark
Spec Ref.
08.1 Level 3: A judgement, strongly linked and logically supported by a
5–6 AO3
sufficient range of correct reasons, is given.
Level 2: Some logically linked reasons are given. There may also
3–4 AO3
be a simple judgement.
Level 1: Relevant points are made. They are not logically linked. 1–2 AO3
4.1.3.2
No relevant content. 0 RPA3
Indicative content:
(strengths)
• same / a / one plant / potato used
• all potato pieces used were cubes
• mass was recorded before and after
• change in mass was calculated
• different concentrations of salt solution were used
(improvement and reason)
• use the same / a / one potato for all pieces
o (because the student) may have used different potatoes
• remove skin or peel the potato
o (because) skin (may still be) present
• blot / dry cube before measuring mass
o (because) cube will have excess water on surface
• use same / set size / surface area of potato piece each time
o (because) size / surface area of cubes not controlled /
specified
• repeat (whole investigation or at each concentration)
o (because there were) no repeats
o to spot anomalies
• calculate mean (from repeats)
o (because there was) no mean calculated
• same time for each piece to be in solution
o (because) length of time in solution not specified
• control temperature of solution using water bath
o (because) temperature (of solution) not controlled /
specified
• use same volume of solution for each piece of potato
or ensure all pieces are completely submerged
o (because) volume of solution not specified / controlled
• calculate percentage change in mass
o (because) percentage change in mass not calculated
or
(because) each potato piece had a different starting mass
24 • include control experiment with boiled cube
o (because) no control experiment included
• increase the number of concentrations (of salt solution) used
o (because) only 3 different concentrations (of salt solution)
used
For Level 3, a strength and improvements / reasons must be
included.
AO /
Question Answers Extra information Mark
Spec Ref.
08.2 the solution in the potato cells 1 AO2
was more concentrated than the 4.1.3.2
distilled water
AO / 25
Spec Ref.
08.3 AO2
any one from: 1
4.1.3.2
• requires energy ignore it is active 4.1.3.3
allow it is not passive
• (transports substances) allow (transports substances)
against / up a concentration from low concentration to high
gradient concentration
ignore (transports substances)
along / across the concentration
gradient
do not accept (transports
substances) down a
concentration gradient
• transports minerals / ions allow transports other named
(and not water) substances such as glucose
allow does not transport water
Total Question 8 8
How to answer it
Investigating Osmosis in Potato Tissue (Required Practical 3)
This question evaluates practical competency in AQA Biology RPA 3 (Osmosis), understanding water potential gradients, and comparing cellular transport mechanisms (osmosis vs active transport). It tests experimental critique, variable control, validity, and precise biological terminology.
Evaluating the Osmosis Method & Improving Validity
Extended response (AO3 Practical Skills & Evaluation)
💡 What Makes an Evaluation "Level 3"?
To score 5–6 marks, you must include:
- At least one strength of the current method.
- Multiple clear improvements paired with their scientific reason (why it improves validity).
- A coherent, logically structured conclusion or overall judgement.
✅ Model Evaluation Points
- Strength: Mass was measured before and after, allowing mass change to be calculated; potato was cut into cubes; different concentrations were tested.
- Improvement 1: Blot cylinders/cubes dry with a paper towel before weighing → removes excess surface liquid that adds anomalous mass.
- Improvement 2: Calculate percentage change in mass → controls for differences in starting masses.
- Improvement 3: Keep immersion time identical (e.g. 30 mins) using a timer → "after some time" is unstandardised.
- Improvement 4: Repeat at each concentration → enables spotting anomalies and calculating a mean.
- Improvement 5: Control temperature using a water bath → temperature affects the kinetic energy and rate of water movement.
🧠 Exam Technique: The "Point + Because" Rule
Examiners frequently report students writing "do repeats" without explaining why. Always link the modification to validity:
- "Use a cork borer instead of cutting by hand because it keeps surface area consistent."
- "Peel potato skin because potato skin is impermeable to water."
- "Test more concentrations (e.g. 5 intervals) to plot a calibration curve and find the exact isotonic point."
❌ Common Errors & Misconceptions
- Vague timings: Simply stating "leave it longer" gets 0 marks. You must specify controlling the time (e.g. identical duration for all cubes).
- Confusing accuracy with reliability: Writing "repeats make it accurate" loses marks. Repeats allow you to identify anomalies and calculate a mean.
- Forgetting percentage change: Forgetting that different starting masses make raw change in mass an invalid comparison.
• Level 3 (5–6 marks): Logically supported judgement with a range of valid points; must include both a strength and improvements with explanations.
• Level 2 (3–4 marks): Relevant improvements given with some linked reasoning; simple judgement.
• Level 1 (1–2 marks): Isolated points/improvements listed without reasons.
Potato Mass Increase in Distilled Water
Multiple Choice (AO2 Application)
✅ Correct Answer
[ × ] The solution in the potato cells was more concentrated than the distilled water.
💡 Osmosis Principle
Osmosis is the diffusion of water from a dilute solution (high water concentration/potential) to a more concentrated solution (low water concentration/potential) across a partially permeable membrane.
- Distilled water = pure water (0 mol dm⁻³ solute).
- Potato cytoplasm contains dissolved solutes (sugars, salts) → it is more concentrated than distilled water.
- Water moves into the cells by osmosis, causing an increase in mass.
❌ Trap: Volume vs Concentration
Two distractor options refer to the volume of water. Osmosis is driven by differences in solute concentration / water potential, NOT by the total volume of liquid present in the beaker versus the cell.
Active Transport vs Osmosis
Recall & Comparison (AO2 / AO1)
✅ Acceptable Answers (Give ANY ONE)
- Energy requirement: Active transport requires energy / ATP (it is an active process, whereas osmosis is passive).
- Direction of gradient: Active transport moves substances against / up a concentration gradient (from low to high concentration), whereas osmosis moves water down a concentration gradient.
- Substance moved: Active transport moves mineral ions / solutes (e.g. nitrates, glucose), whereas osmosis moves water molecules only.
🧠 Comparison Reference Table
| Feature | Osmosis | Active Transport |
|---|---|---|
| Substance | Water only | Mineral ions, glucose |
| Gradient | High → Low water potential | Against concentration gradient |
| Energy (ATP) | Passive (no energy) | Requires cellular energy |
❌ Examiner Watch-outs
- DO NOT write: "Transports down a concentration gradient" for active transport (automatic 0).
- Vague terms: Writing "across" or "along" the gradient is ignored. Must state against or up the gradient.
- Just saying "it is active": The mark scheme specifically states: ignore "it is active". You must mention energy / ATP.
Topics
Biology · Required Practicals · B1: Cell Biology · Required Practicals
Question and mark scheme from the AQA GCSE Biology examination, Biology Paper 1 (Foundation), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.