AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2021: Question 1

13 marks · Standard Demand difficulty · Short Answer

Answer a series of questions about photosynthesis and a required practical investigating how light intensity, changed by distance from a lamp, affects the rate of photosynthesis in pondweed.

Practise this question

Question

The question paper is a multi-part biology item on photosynthesis and a pondweed practical. It begins with a 1-mark multiple-choice question asking for the correct balanced equation for photosynthesis, with four chemical equations and tick boxes. A diagram labelled Figure 1 shows a lamp shining toward a test tube containing pondweed in water, with bubbles of gas rising from the pondweed and a ruler-like horizontal scale marked distance from the lamp in centimetres. The method lists steps: set up the apparatus, place pondweed 10 cm from the lamp, switch on the lamp, record the number of bubbles produced in 5 minutes, and repeat at different distances. Further parts ask for the independent variable, explain how increased water temperature would affect results, suggest improvements to control temperature and improve validity, use Table 1 of results at distances 10, 20, 30, 40 and 50 cm with bubble counts 120, 56, 31, 16 and 10, calculate a rate at 40 cm in bubbles per minute, state a conclusion, plot the data on a graph with axes for distance and number of bubbles in 5 minutes, draw a line of best fit, and predict the value at 60 cm.
Question text

01 Plants absorb light for photosynthesis.

01.1 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 1 shows the apparatus.

Figure 1

This is the method used.

1. Set up the apparatus as shown in Figure 1.

2. Place the pondweed 10 cm away from the lamp.

3. Switch on the lamp.

*02* 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.

01.2 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 4

The lamp gets warm when it is on. This causes the temperature of the water

to increase.

01.3 Explain how an increase in temperature would affect the results of this investigation.

[2 marks]

01.4 Suggest one way the investigation could be improved so the temperature of the water

does not increase.

[1 mark]

01.5 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 1 shows the results.

Table 1

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

01.6 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

01.7 Give one conclusion that can be made from Table 1.

[1 mark]

01.8 Plot the data from Table 1 on Figure 2.

Draw a line of best fit.

[3 marks]

Figure 2

01.9 Predict the number of bubbles that would be produced in 5 minutes if the pondweed

was 60 cm from the lamp.

Use Figure 2.

[1 mark]

Number of bubbles produced in 5 minutes =

Mark scheme

Show the mark scheme The mark scheme is a table with columns for question number, answers, extra information, marks, and AO/specification reference. It gives the correct photosynthesis equation as 6CO2 + 6H2O → C6H12O6 + 6O2; identifies the independent variable as distance of the pondweed from the lamp; explains that higher temperature would make bubbles produced faster because enzymes work faster; and suggests temperature-control improvements such as using an LED lamp, a water bath, water around the tube, or a glass heat shield. For validity, it credits ideas including measuring volume of gas, allowing equilibration time, repeating and calculating a mean or removing anomalies, controlling carbon dioxide concentration, and using the same bulb or same type of pondweed. It gives the calculation answer of 3 bubbles per minute, the conclusion that as light intensity decreases the rate of photosynthesis decreases, graph-marking points for correct plotting and a best-fit line, and a predicted value of 8 bubbles at 60 cm; total marks are 13.

AO /

Question Answers Extra information Mark

Spec. Ref.

01.1 6 CO2 + 6 H2O → C6H12O6 + 6 O2 1 AO1

4.4.1.1

01.2 distance of the pondweed from 1 AO1

the lamp 4.4.1.2

RPA5

01.3 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

01.4 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

01.5 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

• allow the pondweed time to allow a description of this 7

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

01.6 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)

01.7 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

01.8 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

01.9 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

Photosynthesis, Light Intensity and Pondweed Results

What this question tests

Recall the photosynthesis equation, identify variables in an experiment, explain how temperature affects enzyme-controlled reactions, suggest improvements to validity/reliability, and use data to calculate rates, describe trends, plot points, and make a prediction.

Question focus: Part (a) to (i)

Short title: Photosynthesis and light intensity

✅ Correct answer

The correct equation is:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

💡 Key knowledge

  • Photosynthesis uses carbon dioxide and water.
  • It makes glucose and oxygen.
  • The numbers in front of formulas are the balanced coefficients.

🧠 Exam technique

For 1 mark, you only need the fully correct balanced equation. One wrong number or formula loses the mark.

❌ Common errors

  • Writing the equation backwards.
  • Using the wrong formula for glucose.
  • Forgetting the 6 in front of CO₂, H₂O or O₂.

Part (a) 01.1 — Equation for photosynthesis

✅ Mark scheme answer

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

1 mark for the correct balanced symbol equation.

Part (b) 01.2 — Independent variable

✅ Mark scheme answer

Distance of the pondweed from the lamp

💡 Key knowledge

The independent variable is the thing the student changes.

❌ Common errors

Do not choose the number of bubbles, because that is the dependent variable being measured.

Part (c) 01.3 — Effect of temperature increase

✅ Mark scheme answer

Bubbles of gas would be produced faster
because enzymes work faster.

💡 Key knowledge

  • Photosynthesis is controlled by enzymes.
  • Higher temperature speeds up enzyme activity up to the optimum.
  • So the rate of photosynthesis increases, meaning more oxygen bubbles in a given time.

🧠 Exam technique

For 2 marks, give both parts:

  • What happens: faster bubbling / more bubbles in the same time
  • Why: enzymes work faster

❌ Common errors

Students often say “temperature increases so photosynthesis increases” without mentioning enzymes. The mark scheme wants the biological reason.

Part (d) 01.4 — Improve the method so water temperature does not increase

✅ Mark scheme answers — any one

  • Use an LED lamp.
  • Place a tank/beaker of water between the lamp and the pondweed.
  • Put the tube in a beaker of water.
  • Put the tube in a thermostatically controlled water bath.
  • Place a piece of glass / heat shield between the lamp and pondweed.

💡 Key knowledge

The aim is to reduce infrared/thermal radiation from the lamp, so temperature stays constant.

🧠 Exam technique

Pick a method that directly reduces heating. “Move it further away” is not the best answer because it changes light intensity as well.

Part (e) 01.5 — Improve the investigation for validity and reliability

✅ Mark scheme answers — any two

  • Measure the volume of gas produced.
  • Allow the pondweed time to equilibrate.
  • Repeat and calculate a mean, or repeat and remove anomalies.
  • Control the concentration of carbon dioxide in the water.
  • Use the same bulb/lamp.

💡 Key knowledge

Reliability means results are consistent. Validity means the test measures only the effect of the independent variable.

🧠 Exam technique

You need two separate improvements. Choose clear practical steps, not vague comments like “make it better”.

❌ Common errors

  • Writing “control the temperature” when the question says not to refer to that.
  • Only saying “repeat” without explaining what to do with repeats.
  • Confusing bubble count with gas volume.

Part (f) 01.6 — Calculate the rate of photosynthesis at 40 cm

📐 Calculation

At 40 cm, the table shows 16 bubbles in 5 minutes.

  1. Find bubbles per minute: 16 ÷ 5 = 3.2
  2. Use the correct unit: bubbles of gas per minute
  3. Write the final answer: 3 bubbles of gas per minute

Exam note: the mark scheme allows 3.2 , but the expected whole-number answer is 3.

✅ Mark scheme answer

3 bubbles of gas produced per minute

❌ Common calculation traps

  • Using 16 as the final rate without dividing by 5.
  • Writing the unit as “bubbles” with no time unit.
  • Giving 3.0 when the table data support 3.2 or a rounded 3 .

Part (g) 01.7 — Conclusion from Table 1

✅ Mark scheme answer

As the distance from the lamp increases, the rate of photosynthesis decreases.

💡 Key knowledge

Greater distance means lower light intensity, so fewer bubbles are produced.

🧠 Exam technique

Always link the data to the trend. A strong 1-mark answer mentions both distance and rate / bubbles.

Part (h) 01.8 — Plot the data and draw a line of best fit

✅ Points to plot

  • (10, 120)
  • (20, 56)
  • (30, 31)
  • (40, 16)
  • (50, 10)

🧠 Exam technique

  • Use a neat line of best fit through the points.
  • Do not join dot-to-dot.
  • Do not extend the line back to 0,0 unless the data support it.
  • Plot each point accurately, usually within ± ½ small square.

❌ Common errors

  • Swapping axes around.
  • Missing a point because of poor scale reading.
  • Drawing a curve that does not pass near all plotted points.

💡 What the graph should look like

A downward trend: high bubble count near 10 cm, then falling sharply as distance increases, with the line curving or sloping down smoothly through the points.

Part (i) 01.9 — Predict bubbles at 60 cm

✅ Mark scheme answer

8

📐 How to get the prediction

  1. Find 60 cm on the x-axis.
  2. Move up to the line of best fit.
  3. Move across to read the y-value.
  4. Estimate carefully from the graph.

🧠 Exam technique

Predictions from graphs are judged by whether your value matches the line of best fit, not by exact arithmetic.

❌ Common errors

Do not read from the last table value only. The question wants a graph estimate.

Examiner insight: what separated full marks

💡 Strong responses

  • Used the exact photosynthesis equation.
  • Correctly identified the independent variable.
  • Explained temperature using enzymes.
  • Gave realistic method improvements that improve validity or reliability.
  • Calculated rate correctly with units.
  • Described the overall trend in the data.
  • Plotted all points accurately and read the graph sensibly.

❌ Typical mark losses

  • Incorrect formulae or unbalanced equation.
  • Vague explanation with no mention of enzymes.
  • Repeating the same improvement twice in part (e).
  • Forgetting that the rate is “per minute”.
  • Not using the line of best fit for the prediction.

Topics

Biology · Required Practicals · B4: Bioenergetics · Biology Required Practicals

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 1 (Higher), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.