AQA GCSE Combined Science: Trilogy Biology Paper 1 (Foundation), 2021: Question 5
13 marks · Standard Demand difficulty · Short Answer
Analyze a photosynthesis experiment involving pondweed, including identifying the chemical equation, experimental variables, data plotting, and rate calculations.
Practise this questionQuestion
Question text
05 Plants absorb light for photosynthesis.
05.1 Which is the equation for photosynthesis?
[1 mark]
Tick ( ) one box.
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
6 CO2 + 6 H2O → C6H12O6 + 6 O2
6 H2O + 6 O2 → C6H12O6 + 6 CO2
6 O2 + 6 CO2 → C6H12O6 + 6 H2O
A student investigated the effect of light intensity on the rate of photosynthesis.
Figure 6 shows the apparatus.
Figure 6
This is the method used.
1. Set up the apparatus as shown in Figure 6.
2. Place the pondweed 10 cm away from the lamp.
3. Switch on the lamp.
4. Record the number of bubbles of gas produced in 5 minutes.
5. Repeat steps 2 to 4 with the pondweed at different distances from the lamp.
05.2 What was the independent variable in this investigation?
[1 mark]
Tick ( ) one box.
Distance of the pondweed from the lamp
Length of the piece of pondweed
Number of bubbles of gas produced
Time taken to collect the gas 26
The lamp gets warm when it is on. This causes the temperature of the water
to increase.
05.3 Explain how an increase in temperature would affect the results of this investigation.
[2 marks]
05.4 Suggest one way the investigation could be improved so the temperature of the water
does not increase.
[1 mark]
05.5 Suggest two improvements to the investigation so the results would be more valid.
Do not refer to controlling the temperature of the water.
[2 marks]
Table 2 shows the results.
Table 2
Distance of pondweed from Number of bubbles of gas
the lamp in cm produced in 5 minutes
10 120
20 56
30 31
40 16
50 10
05.6 Calculate the rate of photosynthesis when the pondweed was 40 cm from the lamp.
Give the rate of photosynthesis as the number of bubbles of gas produced per minute.
[1 mark]
Rate = bubbles of gas produced per minute
05.7 Give one conclusion that can be made from Table 2.
[1 mark]
05.8 Plot the data from Table 2 on Figure 7.
Draw a line of best fit.
[3 marks]
Figure 7
05.9 Predict the number of bubbles that would be produced in 5 minutes if the pondweed
was 60 cm from the lamp.
Use Figure 7.
[1 mark]
Number of bubbles produced in 5 minutes =
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark
Spec. Ref.
05.1 6 CO2 + 6 H2O → C6H12O6 + 6 O2 1 AO1
4.4.1.1
05.2 distance of the pondweed from 1 AO1
the lamp 4.4.1.2
RPA5
05.3 bubbles (of gas) would be allow more / bigger bubbles of 1 AO3
produced faster gas would be produced (in a 4.4.1.2
given time) RPA5
(because) enzymes work faster allow (because) photosynthesis 1 AO2
is controlled by enzymes 4.2.2.1
allow (because) photosynthesis 4.4.1.2
would be faster
05.4 any one from: 1 AO3
• use an LED (lamp) allow use a light that does not 4.4.1.2
emit (a lot of) infrared / thermal RPA5
radiation
• place a tank / beaker of water
between the lamp and tube /
pondweed
• put the tube in a beaker of
water
• put the tube in a
(thermostatically controlled)
water bath
• place a piece of glass allow place a heat shield
between the lamp and tube / between the lamp and tube /
pondweed pondweed
05.5 any two from: 2 AO3
• measure the volume of gas allow amount for volume 4.4.1.2
produced allow use a cylinder / gas RPA5
syringe to collect the gas
16 • allow the pondweed time to allow a description of this
equilibrate
• repeat and calculate a mean ignore repeat unqualified
or
repeat and remove anomalies
• control the concentration of allow put the pondweed in
carbon dioxide (in the water) sodium hydrogen carbonate
(solution) or sodium bicarbonate
(solution)
• use the same bulb / lamp
allow use the same type / size /
age / piece of pondweed
allow record the number of
bubbles of gas produced in a
longer period of time
05.6 3 (bubbles of gas produced per allow 3.2 (bubbles of gas 1 AO2
minute) produced per minute) 4.4.1.2
do not accept 3.0 (bubbles of RPA5
gas produced per minute)
05.7 as light intensity decreases the allow as distance from lamp 1 AO3
rate of photosynthesis increases rate of photosynthesis 4.4.1.2
decreases decreases RPA5
allow as distance from lamp
increases number of bubbles
produced decreases
05.8 all points plotted correctly allow tolerance of ± ½ a small 2 AO2
square 4.4.1.2
allow 1 mark for four points RPA5
plotted correctly
line of best fit through their do not accept line extended to 1
points 0, 0
ignore extrapolations of line
05.9 8 allow correct value from their 1 AO3
line ± ½ a small square 4.4.1.2
RPA5
allow value in range 6 to 9 if a
curved line of best fit is not
drawn
Total 13
How to answer it
Investigating Photosynthesis and Light Intensity
What this question tests
This question assesses your knowledge of the Photosynthesis Required Practical (RPA 5). You need to recall the chemical equation for photosynthesis, identify experimental variables, suggest improvements for validity, and demonstrate data skills including rate calculations and graphing.
The Photosynthesis Equation
Correct Answer
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Key Knowledge
- Reactants: Carbon dioxide and Water.
- Products: Glucose and Oxygen.
- Light energy is required (absorbed by chlorophyll).
Identifying Variables
Correct Answer
Distance of the pondweed from the lamp.
Exam Technique
The Independent Variable is the thing you change. In this method, step 5 says "repeat... at different distances." This confirms distance is the variable being tested.
Temperature Control
05.3: The Effect
An increase in temperature would make the bubbles be produced faster because enzymes controlling photosynthesis work faster at higher temperatures.
05.4: The Fix
- Use an LED lamp (they release very little heat).
- Place a glass tank of water between the lamp and the pondweed to act as a heat shield.
Common Error
Students often forget why temperature matters. Photosynthesis is an enzyme-controlled reaction. If the lamp heats the water, you are no longer just testing light—you are testing temperature too, which ruins the experiment's validity.
Improving Validity
Suggest two improvements (not temperature related)
Accepted Improvements
- Measure volume: Use a gas syringe or measuring cylinder instead of counting bubbles (bubbles vary in size).
- Equilibration: Leave the pondweed for a few minutes at each new distance before counting.
- Control CO₂: Add sodium hydrogen carbonate to the water to ensure CO₂ is not a limiting factor.
- Repeats: Repeat the test at each distance and calculate a mean.
Examiner Tip
When suggesting "repeats," you must mention calculating a mean or identifying anomalies to get the mark. Simply saying "do it again" is usually not enough at GCSE.
Rate Calculation
Step-by-Step Calculation
The question asks for the rate at 40 cm distance.
- Find the data in Table 2: At 40 cm, 16 bubbles were produced.
- The method states these were produced in 5 minutes.
- Calculation: 16 bubbles ÷ 5 minutes = 3.2
Answer: 3.2 (The mark scheme also accepts 3, but 3.2 is more precise).
Calculation Trap
Always check the time period! If you just wrote "16," you missed that the rate is "per minute," not "per 5 minutes."
Drawing Conclusions
Conclusion
As the distance from the lamp increases, the rate of photosynthesis decreases.
Linking Variables
A good conclusion always links the Independent Variable (distance) to the Dependent Variable (rate/bubbles).
Graphing and Prediction
05.8: Plotting Figure 7
- Points: Use a sharp pencil to plot (10, 120), (20, 56), (30, 31), (40, 16), and (50, 10).
- Line of Best Fit: This should be a smooth curve passing through or near all points.
- Warning: Do NOT extend the line to (0,0). The data shows that at 50cm there are still bubbles, so it wouldn't hit zero at the lamp!
05.9: Prediction
To predict the value at 60 cm, extend your curve naturally. Based on the pattern, the answer is 8 (allowance for 6 to 9 depending on your curve).
Topics
Biology · B4: Bioenergetics · B2: Organisation
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 1 (Foundation), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.