AQA GCSE Combined Science: Trilogy Biology Paper 1 (Foundation), June 2025: Question 3

✅ Mark Scheme Answers (Any Two)

• Repeat the experiment at each concentration (and calculate a mean) 1 mark
• Dry / blot the potato cubes with a paper towel before re-weighing 1 mark
• Use the same size / mass of potato cubes 1 mark
• Use the same volume of salt solution (or ensure fully submerged) 1 mark
• Calculate percentage change in mass 1 mark
• Use a more precise balance (e.g. to 2 decimal places) 1 mark
• Control the temperature of the solutions 1 mark

❌ Common Errors & Examiner Traps

• Vague statements: Writing just "measure better" or "use more potato" gains zero marks.
• Missing the "Why": Students often forget to blot potato cubes. Surface liquid adds variable extra mass that is not part of the tissue.
• Initial mass variation: Because potato pieces are never identical in mass, calculating percentage change in mass is essential for fair comparison.

✅ Correct Answer

0.5 mol/dm³ 1 mark

💡 Key Knowledge

Look at Table 3 for the point where Change in mass = 0.0 g:

• At 0.5 mol/dm³, start mass is 30.8 g and final mass is 30.8 g.
• When change in mass is zero, there is no net movement of water into or out of the cells.
• This means the external solution is isotonic to the potato cell cytoplasm.

✅ Correct Answer

Tick: Osmosis 1 mark

🧠 Exam Technique: Key Definitions

• Osmosis: Diffusion of water from a dilute solution to a concentrated solution across a partially permeable membrane.
• Active Transport: Movement of mineral ions/substances against a concentration gradient using energy.
• Respiration: Chemical reaction that releases energy (ATP) inside mitochondria.

📐 Step-by-Step Calculation

1. Step 1: Calculate area of one face:
   3 cm × 3 cm = 9 cm² 1 mark
2. Step 2: Multiply by total number of faces on a cube (6 faces):
   9 × 6 = 54 1 mark
3. Step 3: State correct unit for surface area:
   cm² 1 mark

Final Answer: 54 cm²

❌ Common Errors

• Calculating volume instead: Doing 3 × 3 × 3 = 27 cm³ is volume, not surface area!
• Wrong number of faces: Multiplying by 4 (sides only) instead of 6 faces.
• Incorrect unit: Giving cm or cm³ instead of area ( cm² ). The unit mark is independent, so always write it down!

✅ Correct Answer

Cut the potato into smaller pieces (or thin slices / chips / quarters) 1 mark

💡 Key Principle

Subdividing any solid object into smaller fragments exposes fresh internal surfaces to the solution without changing the total volume, thus significantly increasing its total surface area (and surface area to volume ratio).

✅ Correct Matching

• Starch ➔ Iodine solution 1 mark
• Sugar ➔ Benedict's reagent 1 mark

🧠 Recall Guide for Food Reagents

• Benedict's : Reducing sugars (needs heating in a water bath).
• Iodine : Starch.
• Biuret : Proteins (peptide bonds).
• Ethanol : Lipids (fats/oils - emulsion test).

Warning: The mark scheme explicitly states: "do not accept more than one line from a box on the left". Only draw one line per carbohydrate!

✅ Correct Answer

Tick: Sugar and starch 1 mark

💡 Decoding the Colour Changes

• Protein (Biuret): Result is Blue. The original colour of Biuret is blue, so blue means negative (no protein). A positive result would be purple/lilac.
• Sugar (Benedict's): Result is Red (brick-red precipitate). This is a positive result for sugar.
• Starch (Iodine): Result is Blue-black. This is a positive result for starch (iodine starts orange-brown).