AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), June 2025: Question 4

✅ Correct Answer

To take account of the different starting masses of the potato groups (allows a valid comparison).

❌ Common Errors

• Vague answers like "to make it a fair test" (scores 0).
• Writing "they were not all the same size" without explicitly mentioning mass.
• Claiming mass is a "control variable".

✅ Correct Answer

• Mark 1: As the total surface area increases, the percentage increase in mass increases (positive correlation).
• Mark 2: It increases by a decreasing amount (the rate of increase slows down / begins to level off).

📐 Data Evidence from Table 1

• 7.4 to 8.1 cm² → increase of +0.5% (4.4 → 4.9)
• 8.1 to 8.9 cm² → increase of +0.4% (4.9 → 5.3)
• 8.9 to 9.7 cm² → increase of +0.2% (5.3 → 5.5)
• 9.7 to 10.5 cm² → increase of +0.1% (5.5 → 5.6)

🧠 Exam Technique

For 2-mark trend questions, look past the general rule ("as X goes up, Y goes up"). Always look at the differences between values to see if the change is constant, accelerating, or decreasing.

❌ Common Errors

Do not say they are "directly proportional". Directly proportional means doubling surface area exactly doubles the mass increase and the line passes through (0,0), which is not true here.

✅ Correct Answer

• Mark 1: Water moved in by osmosis.
• Mark 2: Water moved from a dilute solution (distilled water outside) to a more concentrated solution (inside the potato cells)
  OR from a higher concentration of water to a lower concentration of water.

💡 Key Knowledge

Osmosis Definition: The net movement of water molecules from a dilute solution (higher water potential) to a more concentrated solution (lower water potential) across a partially permeable membrane.

🧠 Exam Technique

Always state the direction of movement and specify where the dilute and concentrated solutions are located (e.g., "distilled water is dilute, potato cytoplasm is concentrated").

❌ Common Errors

• Saying water moves by "active transport" (scores 0).
• Saying water moves "along" or "across" a concentration gradient instead of down its concentration gradient.
• Confusing solute concentration with water concentration.

✅ Correct Answer

• Mark 1: All the potato groups would have the same percentage change in mass.
• Mark 2: Because the plant cells become fully turgid (cell walls are completely stretched).
• Mark 3: So the cells cannot absorb any more water.

💡 Key Knowledge

Surface area alters the rate of osmosis, not the final equilibrium position. Over 30 minutes, higher surface area means faster uptake. Over 24 hours, all pieces have reached their absolute capacity and become fully turgid.

🧠 Exam Technique: Rate vs. Final Endpoint

When an experiment runs for a very long time (like 24 hours), think about the biological limit: the rigid cellulose cell wall prevents plant cells from taking in more water once maximum turgor pressure is reached.

❌ Common Errors

• Predicting that Beaker E would continue to absorb more water indefinitely.
• Saying the cells "burst" (plant cells have cell walls and do not burst in water; animal cells lyse).

✅ Correct Answer

• Mark 1 (Independent Variable): Use different concentrations of salt solution (e.g. 0.2 mol dm⁻³, 0.4 mol dm⁻³, etc.) instead of distilled water.
• Mark 2 (Control Variable): Keep the surface area constant (e.g., do not cut cylinders into different numbers of pieces / use 1 whole cylinder in each beaker).

💡 Experimental Design Rule

When you swap the independent variable to salt concentration, your previous independent variable (surface area) must now become a controlled variable to ensure validity.

❌ Common Errors

• Only stating what to change, forgetting that a valid method must control surface area.
• Writing "use the same shape and size" (too vague; the mark scheme explicitly ignores "same shape/size" and expects mention of controlling surface area or cutting into the same number of pieces).