AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2022: Question 4

💡 Key knowledge

• Light provides the energy needed for photosynthesis.
• Photosynthesis is an endothermic reaction.
• The word and symbol equation must be memorised.
• Blue and red light usually give a higher rate than green light.

🧠 Exam technique

• Use short, exact science phrases for 1-mark answers.
• For practical methods, give a sequence: set up, measure, time, repeat, mean.
• When reading graphs, use the labelled scale carefully and give the full range asked for.

✅ Correct answer

Light transfers energy to the plant for photosynthesis.

Also accepted: photosynthesis is an endothermic reaction.

💡 Key knowledge

• Light is not a raw material in the equation — it is the energy source.
• Endothermic means energy is taken in.

❌ Common errors

• “Light makes glucose” — too vague and not the marking point.
• “Light is used up” — not accepted unless linked to energy transfer.
• “Light creates energy” — incorrect wording.

✅ Correct answer

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

💡 Key knowledge

• Reactants: carbon dioxide and water.
• Products: glucose and oxygen.
• This is the balanced symbol equation.

❌ Common errors

• Using the wrong equation with oxygen on the left.
• Missing the coefficients 6.
• Writing CH₂O instead of glucose.
• Forgetting subscripts: use CO₂, H₂O, C₆H₁₂O₆, O₂.

✅ Correct answer points

• Allow the pondweed to equilibrate in the light before taking measurements.
• Use a gas syringe or a measuring cylinder to measure or collect the oxygen produced.
• Measure the time for which oxygen is collected using a timer/stopwatch.
• Repeat the measurements and calculate a mean.

🧠 Exam technique

• To get full marks, include the idea of measurement of oxygen, time, and repeats.
• Use clear practical language: “collect oxygen”, “measure time”, “calculate a mean”.
• If you mention a control variable, it must be linked to the method to count.

💡 Key knowledge

• Equilibration means letting the plant settle in the new light conditions.
• More oxygen produced in a fixed time = higher rate of photosynthesis.

✅ Correct answer

• In the student’s investigation, the independent variable is categoric / discrete because the light colour is in separate groups.
• In the scientist’s investigation, the independent variable is continuous because wavelength can take many values.

💡 Key knowledge

• Colour is a category: red, blue, green.
• Wavelength is measured in nm and changes smoothly across a range.

❌ Common errors

• Talking about the y-axis scale — ignored by the mark scheme.
• Saying “discontinuous” instead of discrete; the mark scheme allowed categorical/discrete.
• Calling wavelength a category — incorrect.

✅ Correct answer

492 nm to 577 nm

Also accepted as ranges: 475 to 525 nm and 575 to 650 nm when using the graph evidence correctly.

🧠 Exam technique

• Use both Figure 8 and Figure 9 together.
• Read off the wavelength values where green light fits the low-rate region / green band.
• Give units: nm.

❌ Common errors

• Giving only one number instead of a range.
• Forgetting units.
• Using the y-axis values instead of wavelength on the x-axis.
• Mixing up colour names and wavelength values.

💡 Key knowledge

• Light provides energy for photosynthesis.
• The balanced equation is essential recall.
• Practical measurements should be repeatable and fair.
• Colour is categorical; wavelength is continuous.

🧠 Exam technique

• Be precise: use “oxygen produced”, not just “gas”.
• Always include units when reading graphs.
• For practicals, write enough steps to show how valid data is obtained.

❌ Final traps to avoid

• Wrong equation order.
• No repeat or mean in the method.
• Mixing up colour and wavelength.
• Not matching the graph to the question asked.