AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2020: Question 5

✅ Correct answer

Measure the volume of oxygen produced in a given time.

Alternative: If more oxygen is produced in a given time, the rate of photosynthesis is faster.

💡 Key knowledge

• Photosynthesis produces oxygen.
• Rate means amount per unit time.
• So oxygen output can be used as a measure of photosynthesis rate.

🧠 Exam technique

To get the mark, make sure you mention time or rate. Just saying “oxygen is produced” is not enough.

❌ Common errors

• Only saying “oxygen is made” without linking it to rate.
• Not mentioning a time period.
• Writing the reverse idea vaguely, without a clear link to photosynthesis rate.

✅ Correct answer

Light intensity (or light).

💡 Key knowledge

• A limiting factor is the factor stopping the rate increasing further.
• At point X, increasing the light intensity would increase the rate, so light is the limiting factor.

🧠 Exam technique

Read the graph carefully. This point is on the rising part of the curve, so the rate is still increasing with light intensity.

❌ Common errors

• Saying temperature because it is shown on the graph, even though the question asks about point X on the curve.
• Writing “carbon dioxide” without checking the curve shape.

✅ Correct answer

20 000 lux

Acceptable range: 19 500 to 20 500 lux

💡 Key knowledge

• The maximum rate is where the curve becomes flat.
• The minimum usable light intensity is the point where the curve first levels off.

🧠 Exam technique

Use the graph scale carefully. The answer is the start of the plateau, not the highest point on the axis.

❌ Common errors

• Reading the wrong point on the curve.
• Giving the maximum rate value instead of the light intensity.
• Forgetting to include lux.

✅ Correct answer

• There is a cost for heating the greenhouse.
• There is a cost for increasing the carbon dioxide concentration in the greenhouse.
• Therefore the extra costs might be greater than the extra profit from selling more tomatoes.

💡 Key knowledge

• Commercial growers must balance output against running costs.
• More photosynthesis can increase yield, but that does not guarantee more profit.
• In winter, there may also be a cost for lighting.

🧠 Exam technique

For 3 marks, give at least two costs and then a clear conclusion about profit. You need a chain of reasoning: costs go up → profit may go down.

❌ Common errors

• Only saying “it costs money” without naming the costs.
• Talking about plant growth but not linking to profit.
• Repeating the same cost in different words.

✅ Correct answer

• When there is no light, there is no photosynthesis.
• So no oxygen is produced.
• Respiration still happens all the time, so oxygen is used up.
• Therefore the net/overall oxygen production is negative, about −2 arbitrary units.

💡 Key knowledge

• Photosynthesis needs light energy.
• Respiration is a continuous process in plant cells.
• At 0 lux, the graph shows oxygen production below zero because oxygen use is greater than oxygen release.

🧠 Exam technique

To score all 4 marks, use the graph and explain the biology. A strong answer usually includes:

1. No light → no photosynthesis.
2. No oxygen made by photosynthesis.
3. Respiration continues and uses oxygen.
4. Net oxygen is negative, matching the graph at about −2.

❌ Common errors

• Saying “no oxygen is produced” and stopping there.
• Forgetting that respiration still uses oxygen.
• Writing the graph value as just “−2” without the words net/overall and arbitrary units.
• Claiming photosynthesis continues in the dark.

💡 Remember

• Rate = amount per unit time.
• Oxygen production can be used to measure photosynthesis rate.
• The limiting factor is the factor stopping the rate increasing.
• At 0 lux, respiration continues but photosynthesis stops.

🧠 How to pick up marks fast

• Use the exact graph value when asked.
• Use the correct unit: lux.
• When explaining, always link the science to the result.
• For longer answers, write in a clear chain: because / therefore / so.

📐 Calculation-style reading tip

There are no numerical calculations in this question, but graph questions work like calculations: choose the right axis, read the scale carefully, and give the value with the correct unit.