AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2018: Question 4

✅ Correct answer

To control for the starting mass of the carrot pieces.

💡 Key knowledge

• The carrot pieces were not all the same mass at the start.
• Using percentage change makes the results fairer to compare.

🧠 Exam technique

• Say what is being controlled, not just “it is a control variable”.
• For 1 mark, a short clear sentence is enough.

❌ Common errors

• “To make it fair” alone is too vague.
• “Because they were not the same size” may be accepted in context, but mass is the best answer.
• Do not say “it is a control variable” — that was specifically not accepted.

💡 Key knowledge

• y-axis label: Percentage change in mass (%)
• x-axis is already given: Concentration of sugar solution (mol/dm³)
• Plot these points exactly:
• (0.0, +24)
• (0.2, +12)
• (0.4, +1)
• (0.6, -8)
• (0.8, -15)

🧠 Exam technique

• Choose a scale that uses most of the graph and is easy to read.
• The mark scheme allows 5 or 6 squares per 2 cm for the y-axis.
• You need a sensible scale and a label on the y-axis for 1 mark.
• For the plotting marks, each point should be correct to within about half a square.
• Then draw a smooth line of best fit, not dot-to-dot.

❌ Common errors

• Missing the minus signs for negative values.
• Using a scale that squashes the graph into a corner.
• Forgetting to label the y-axis with both quantity and unit.
• Joining points with straight line segments instead of a best-fit line.
• Plotting 0.4 as 0.04 or using the wrong axis scale.

✅ Correct answer

About 0.4 mol/dm³ is the best estimate.

💡 Key knowledge

• The estimate is where the line of best fit crosses 0% change in mass.
• This point is the isotonic concentration: no net movement of water.

🧠 Exam technique

• Use your own plotted graph, not the table alone.
• Read from the x-axis where the line crosses y = 0.
• An answer close to 0.4 mol/dm³ would be credited.

❌ Common errors

• Giving 0.0 or 0.2 because the mass change is still positive there.
• Forgetting to include the unit mol/dm³ .
• Using the raw table values instead of the graph intercept.

✅ Correct answer

The mass decreased because water moved out of the carrot cells by osmosis through a partially permeable membrane. The sugar solution outside the carrot was more concentrated than inside the cells, so the water concentration was higher inside the carrot than outside. Water moved from a higher water concentration to a lower water concentration.

💡 Key knowledge

• Osmosis = net movement of water molecules through a partially permeable membrane.
• Water moves from a region of higher water concentration to lower water concentration.
• In a more concentrated sugar solution, the water concentration outside the cells is lower.

🧠 Exam technique

• To get full marks, include all four ideas:
  • mass decreased
  • water lost by osmosis
  • through a partially permeable membrane
  • because the outside solution was more concentrated / had lower water concentration
• Use “water moved out” or “lost water” rather than just “changed”.

❌ Common errors

• Saying “diffusion” instead of osmosis.
• Leaving out the membrane.
• Saying sugar moved out of the carrot — the mark scheme is about water movement.
• Confusing concentration of sugar with concentration of water.

✅ Correct answer

The partially permeable cell membrane was damaged, so osmosis could not happen properly.

💡 Key knowledge

• Boiling damages the cell membrane.
• A damaged membrane may be more permeable or leaky.
• The membrane may be denatured and no longer intact.

🧠 Exam technique

• One clear biological reason is enough for the mark.
• Link the boiling to membrane damage.

❌ Common errors

• “The cells are dead” alone is not enough here.
• “The water cannot move because the carrot is cooked” is too vague.
• Do not say the membrane is “selectively permeable” after boiling — it has been damaged.