AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), June 2025: Question 3
9 marks · Standard Demand difficulty · Short Answer
Analyze an investigation into the effect of light intensity and temperature on the rate of photosynthesis in pondweed using the inverse square law and collision theory.
Practise this questionQuestion
Question text
03 Photosynthesis is important for plant growth.
A student used pondweed to investigate the rate of photosynthesis.
Figure 3 shows the apparatus used.
Figure 3
This is the method used.
1. Set up the apparatus as shown in Figure 3.
2. Switch on the lamp.
3. After 2 hours, record the volume of oxygen collected in the measuring cylinder.
4. Repeat steps 1 to 3 a further five times, increasing the distance between the lamp
and the pondweed each time.
5. Calculate the rate of oxygen production for each distance.
Figure 4 shows the results.
Figure 4
03.1 Which measuring cylinder should the student have used to give the most
accurate results?
[1 mark]
Tick ( ) one box.
10 cm3 measuring cylinder with intervals of 0.1 ± 0.01 cm3
10 cm3 measuring cylinder with intervals of 0.2 ± 0.01 cm3
20 cm3 measuring cylinder with intervals of 0.1 ± 0.02 cm3
20 cm3 measuring cylinder with intervals of 0.2 ± 0.02 cm3
03.2 Light intensity can be calculated using the inverse square law.
The equation shows the inverse square law.
light intensity ∝ 2
(distance from light source)
Explain how light intensity changed as the distance of the pondweed from the light
source was doubled.
Include calculations in your answer.
Use data from Figure 4.
[3 marks]
The student did the investigation at 10 °C.
The student repeated the investigation at 20 °C and again at 80 °C.
03.3 Explain how repeating the investigation at 20 °C would affect the results.
[3 marks]
03.4 Explain how repeating the investigation at 80 °C would affect the results.
[2 marks]
Mark scheme
Show the mark scheme
Question 3
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1 10 cm3 measuring cylinder with 1 AO1
intervals of 0.1 ± 0.01 cm3 4.4.1.2
RPA5
AO /
Spec. Ref.
03.2 evidence of squaring allow other appropriate values 1 AO2
for two distances that double: throughout 4.4.1.2
(10 cm and 20 cm) RPA5
100 and 400
or
(20 cm and 40 cm)
400 and 1600
or
(30 cm and 60 cm)
900 and 3600
calculate 1/d2 for two distances 1
that double:
(10 cm and 20 cm)
0.01 / and 0.0025 /
100 400
or
(20 cm and 40 cm)
11 allow 2 marks for these
0.0025 / and 0.000625 / values without working
400 1600
or
(30 cm and 60 cm)
0.001(11…) /
and 0.00027(77…) /
3600
(therefore as distance doubles) 1
light intensity is quartered
AO /
Spec. Ref.
03.3 the rate of oxygen production allow rate of photosynthesis 1 AO3
increases increases
ignore the volume of oxygen
production increases unqualified
(because) reactants / particles / allow reactants / particles / 1 AO2
molecules / enzymes have molecules / enzymes move
greater (kinetic) energy faster 13
(so) collisions are more frequent allow (so) more collisions in a 1 AO2
given time
allow (so) more successful
collisions 4.2.2.1
4.4.1.2
AO /
Spec. Ref.
03.4 no / less oxygen (is produced) allow photosynthesis is stopped 1 AO3
/ reduced
allow the rate of oxygen
production is 0 (cm3/hour)
(because) enzymes are allow the enzymes are 1 AO2
destroyed denatured
allow active site has changed
shape 4.2.2.1
do not accept the cells / plant / 4.4.1.2
pondweed is denatured
do not accept the enzymes are
killed
Total Question 3 9
How to answer it
Photosynthesis Practical: Light Intensity & Temperature
What this question tests
This question evaluates your practical and theoretical understanding of AQA Biology Required Practical 5 (Photosynthesis):
- Selecting measuring apparatus for resolution, precision, and accuracy.
- Applying mathematical relationships: calculating light intensity changes using the Inverse Square Law ( 1/d² ).
- Explaining rate changes with temperature using kinetic theory and enzyme denaturation.
Apparatus Precision & Accuracy
Selecting the most appropriate measuring cylinder
✅ Correct Answer
10 cm³ measuring cylinder with intervals of 0.1 ± 0.01 cm³
💡 Key Knowledge
The total volume of gas collected in 2 hours is around 7.2 cm³ (since the peak rate is 3.6 cm³/hour). A 10 cm³ cylinder has sufficient capacity without being oversized.
Intervals of 0.1 cm³ offer higher resolution (smaller scale divisions) and an uncertainty of only ± 0.01 cm³, minimising measurement errors.
❌ Common Errors
Choosing the 20 cm³ cylinder because it is "bigger". In science, choosing a container far larger than needed reduces resolution and makes readings less accurate.
🧠 Exam Technique
Always pick measuring equipment that:
- Accommodates the maximum expected volume.
- Has the smallest scale divisions (finest resolution).
- Shows the smallest margin of uncertainty (± value).
The Inverse Square Law
Explaining changes in light intensity as distance doubles
📐 Step-by-Step Calculation
For example: d₁ = 10 cm and d₂ = 20 cm (or 20 cm and 40 cm, or 30 cm and 60 cm).
10² = 100 and 20² = 400
At 10 cm: 1 / 100 = 0.01 (or 1/100)
At 20 cm: 1 / 400 = 0.0025 (or 1/400)
Compare values: 0.0025 ÷ 0.01 = 1/4
Therefore, as distance doubles, light intensity quarters (or decreases by a factor of 4).
🧠 Top Tip for Full Marks
The prompt explicitly asks to "include calculations". Stating only that "light intensity quarters" without showing the squared distances or the 1/d² values will cost you 2 out of the 3 marks!
❌ Common Errors
- Saying light intensity "halves" because distance doubles (ignoring the square relationship).
- Using rate values directly from the y-axis instead of using the formula 1/d² as instructed.
Effect of Increasing Temperature to 20 °C
Collision theory and enzyme activity
✅ Model Answer
- The rate of oxygen production increases (or rate of photosynthesis increases).
- Reactants, particles, and enzymes gain greater kinetic energy (move faster).
- Resulting in more frequent collisions (or more successful collisions per second).
💡 Why 20 °C Increases Rate
Photosynthesis is controlled by enzymes. At 10 °C, the enzymes have low kinetic energy. Raising temperature to 20 °C brings it closer to the optimum temperature, speeding up enzyme-substrate collisions without causing denaturation.
❌ Common Misconceptions
- Writing "more oxygen is produced" unqualified without mentioning rate or per hour.
- Saying "more collisions occur" without specifying frequency ("more collisions per second" or "more frequent collisions").
🧠 Exam Technique: Rate vs Total
Always refer to the rate of oxygen production. A plant at 10 °C could eventually make the same total volume of oxygen if left long enough; what changes is how fast it makes it.
Effect of Extreme Temperature (80 °C)
Enzyme denaturation
✅ Model Answer
- No oxygen is produced (rate falls to 0 / photosynthesis stops).
- Because the photosynthetic enzymes are denatured (their active site has changed shape).
❌ Forbidden Phrasing (Zero Marks)
- "Enzymes are killed" → NEVER say this! Enzymes are biological catalysts (protein molecules), not living organisms.
- "The plant / pondweed / cells are denatured" → Only proteins / enzymes denature, not whole organisms or cells.
Topics
Biology · B4: Bioenergetics · B2: Organisation
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 1 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.