AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2018: Question 3

✅ Correct answer

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

This is the balanced equation for photosynthesis.

💡 Key knowledge

• Photosynthesis uses carbon dioxide and water.
• It produces glucose and oxygen.
• It must be balanced, so the number of each type of atom is the same on both sides.

🧠 Exam technique

• Look for the equation with the correct products and the correct balancing.
• One mark means you only need the exact correct option.

❌ Common errors

• Choosing the equation with the wrong arrow direction.
• Forgetting the coefficient 6 in front of CO₂, H₂O, and O₂.
• Writing glucose as just CH₂O or similar.

✅ Correct answer

Endothermic

Photosynthesis absorbs energy from light.

💡 Key knowledge

• Endothermic = energy taken in from the surroundings.
• In photosynthesis, light energy is absorbed by chlorophyll.

🧠 Exam technique

Use the meaning of the word: endo = in, so energy goes in.

❌ Common errors

• Choosing exothermic because “energy is involved”.
• Choosing aerobic or oxidation because they sound science-like.

✅ Correct answers

• Measure the volume of gas released instead of counting bubbles.
• Increase the length of time the bubbles are counted/measured.
• Repeat the measurement at each distance several times and calculate a mean.
• Use a measuring cylinder, gas syringe or capillary tube.
• Video the investigation so bubbles can be recounted later.

💡 Key knowledge

• Bubble size can vary, so counting bubbles alone is less reliable.
• Repeated measurements improve reliability.
• Measuring gas volume is usually more accurate than counting bubbles.

🧠 Exam technique

For 2 marks, give two separate improvements. Make sure each one clearly improves the measurement.

❌ Common errors

• Saying “use different distances” — this changes the variable, not the accuracy.
• Saying “count faster” or “watch more carefully” without explaining how that improves accuracy.

✅ Correct answer

Temperature affects the rate of photosynthesis / the rate of bubble production.

This is because photosynthesis is controlled by enzymes, and high temperatures can denature them.

💡 Key knowledge

• Enzymes control reactions in photosynthesis.
• Temperature changes the enzyme activity and therefore the rate.
• If temperature changes, it becomes a fair test problem.

🧠 Exam technique

You need a cause and effect answer: temperature changes enzyme action, so the rate changes.

❌ Common errors

• Just saying “so the plant is healthy” — too vague.
• Only saying “temperature affects photosynthesis” without explaining why.
• Mentioning denaturation but not linking it to high temperatures.

📐 Calculations: step-by-step

1. Use the formula pattern: I ∝ 1 / d²
2. If the distance doubles, square the new distance.
3. Example distances: 5 cm and 10 cm
4. Calculate 1 / d²:
   • At 5 cm: 1 / 5² = 1 / 25 = 0.04
   • At 10 cm: 1 / 10² = 1 / 100 = 0.01
5. Compare the values: 0.01 is one quarter of 0.04
6. So when distance doubles, light intensity is quartered.

✅ Correct answer

When the distance from the light source is doubled, the light intensity becomes one quarter of its original value.

Using 5 cm and 10 cm: intensity changes from 0.04 to 0.01.

💡 Key knowledge

• Inverse square law means intensity decreases with the square of distance.
• If distance doubles, intensity does not halve — it becomes 1/4.
• Using two doubling distances is enough to show the pattern.

❌ Common errors

• Saying intensity halves when distance doubles.
• Forgetting to square the distance.
• Not showing any working.
• Mixing up distance and intensity units — light intensity here is a relative value, not in cm.

✅ Correct answer

2 bubbles would be produced in 1 minute.

Reason: as the distance doubles, the light intensity is quartered, so the number of bubbles should also fall to about a quarter.

📐 Calculations: step-by-step

1. From 10 cm to 20 cm, the distance doubles.
2. 31 bubbles at 10 cm becomes about a quarter at 20 cm: 31 ÷ 4 ≈ 7.75, which matches the result of 8.
3. From 20 cm to 40 cm, double the distance again.
4. 8 ÷ 4 = 2

🧠 Exam technique

• Use the pattern in the table to predict the next value.
• Give a short reason linked to the inverse square law or the idea of less light for photosynthesis.
• Round to a whole bubble, since you cannot produce a fraction of a bubble.

❌ Common errors

• Predicting 0 bubbles — the mark scheme does not accept “no light”.
• Saying 4 bubbles instead of 2.
• Giving a prediction with no reason.
• Ignoring the pattern in the data table.

✅ Correct answer

Independent variable: use different concentrations of sodium hydrogencarbonate solution.

Control variables: any two from:

• distance from light source
• temperature of solution
• same plant
• time for plant to equilibrate

💡 Key knowledge

• Sodium hydrogencarbonate releases carbon dioxide.
• Changing its concentration changes the carbon dioxide available for photosynthesis.
• A fair test keeps other variables the same.

🧠 Exam technique

• State exactly how to change the independent variable.
• Then give two control variables only.
• Use scientific wording such as “same plant” or “same distance from the lamp”.

❌ Common errors

• Saying “use more sodium hydrogencarbonate” without mentioning different concentrations.
• Writing “keep light the same” without stating distance from the light source.
• Choosing the wrong control variables or repeating the same idea twice.

💡 What distinguished top responses

• Precise scientific vocabulary: endothermic, enzymes, denature, inverse square.
• Short, direct explanations linked to the question.
• Clear calculations with the correct pattern: doubling distance → quarter intensity.
• For practical questions, students named a better measurement method and a sensible way to improve reliability.

❌ Where students lost marks

• Confusing endothermic with exothermic.
• Not giving a reason for why temperature must be controlled.
• Failing to explain the inverse square relationship.
• Giving vague practical changes instead of specific methods.