AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2019: Question 3

✅ Correct answer

Open and close the tap so that water from the reservoir moves the air bubble back to 0 mm.

💡 Key knowledge

• The reservoir contains water that can be used to push the bubble back.
• This lets each investigation start from the same position.

🧠 Exam technique

For 1 mark, give the action clearly. Words like open the tap and water enters from the reservoir match the mark scheme.

❌ Common errors

• Only saying “reset it” without explaining how.
• Talking about changing the bubble with air instead of using the reservoir.

✅ Correct answer

The air bubble moves further in a given time, so the resolution is improved and small changes are easier to measure.

💡 Key knowledge

• Narrow tubing makes movement of the bubble easier to see.
• A small change in water uptake gives a bigger movement on the scale.
• This makes measurements more accurate.

🧠 Exam technique

The mark scheme rewards a cause and effect answer: narrow tube → bubble moves more → easier to measure / better resolution.

❌ Common errors

• Saying “it is more precise” without explaining why.
• Using vague phrases like “better” with no science point.
• Talking about validity, which the mark scheme does not credit here.

📐 Calculations: step-by-step

1. Read the graph carefully: at 5 minutes, the air bubble has moved to about 66 mm.
2. Find the rate in mm/min:
   rate = distance ÷ time = 66 ÷ 5
3. 66 ÷ 5 = 13.2 mm/min
4. Convert to mm³/min using the cross-sectional area of the tube:
   13.2 × 0.8 = 10.56 mm³/min

Final answer: 10.56 mm³/min
Accept 10.6 or 11.

✅ Correct answer

Rate = 10.56 mm³/min

💡 Key knowledge

• Rate = distance ÷ time.
• Use the graph reading correctly before calculating.
• Because the answer is in volume per minute, multiply by the tube area.

🧠 Exam technique

• Show working clearly to get method marks.
• Include units at each stage: mm/min, then mm³/min.
• If your graph reading is slightly off, you can still gain marks for the correct method.

❌ Common calculation traps

• Using 63 ÷ 5 instead of 66 ÷ 5.
• Forgetting to multiply by 0.8.
• Leaving the answer in mm/min instead of mm³/min.
• Writing the wrong units or no units at all.

✅ Correct answer

Plot these points accurately and draw a suitable line of best fit:

• (0, 0)
• (1, 6)
• (2, 16)
• (3, 22)
• (4, 30)
• (5, 42)

🧠 Exam technique

• Points must be plotted correctly, usually within about half a square.
• Then draw a sensible line of best fit — not join-the-dots.
• Label the line B.

💡 Key knowledge

A line of best fit should show the overall trend and pass close to the points. It should be smooth and suitable for the data.

❌ Common errors

• Joining each point with straight line segments.
• Missing the origin point (0,0).
• Labeling the wrong line or forgetting the label.

✅ Correct answer

Draw a straight line starting at 0,0 with a steeper gradient than A, and label it C.

💡 Key knowledge

Dry air at 25 °C causes faster transpiration than moist air, so the air bubble should move more quickly.

🧠 Exam technique

You do not need exact points here — just the correct shape, start, and relative steepness.

❌ Common errors

• Starting the line above 0,0.
• Drawing a curve instead of a straight line.
• Making line C less steep than A.

✅ Correct answer

• No photosynthesis happens in the dark.
• So the stomata close because carbon dioxide is not needed.
• So there is no transpiration, or very little water loss.

💡 Key knowledge

In daylight, stomata are open for gas exchange. In the dark, photosynthesis stops, so the stomata close and water vapour loss from the leaves is reduced.

🧠 Exam technique

This is a 3-mark chain of reasoning. Try to link the ideas in order: dark → no photosynthesis → stomata close → little/no transpiration → bubble does not move.

❌ Common errors

• Saying only “plants need light” without linking it to transpiration.
• Confusing transpiration with photosynthesis.
• Forgetting the stomata connection.

💡 What examiners wanted

• Accurate practical knowledge of a potometer.
• Clear use of units and correct graph reading.
• Reasoned explanations, especially for light/dark and transpiration.

🧠 How to get full marks

• Answer exactly what is asked: action, reason, calculation, or graph feature.
• Use scientific vocabulary: transpiration, stomata, resolution.
• Show working in calculations and always include units.

❌ Biggest traps

• Not reading the graph carefully.
• Giving vague statements with no biology link.
• Forgetting that the potometer measures water uptake indirectly, not transpiration directly.