AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2022: Question 4

✅ Correct answers

• Light intensity
• Water
• Temperature
• Mass or volume of soil
• Soil type or soil pH
• Other mineral content of the soil
• Number of plants in each box
• Starting mass, height or age of the corn
• Type or variety of corn
• Harvested at the same time

💡 Key knowledge

• A control variable is kept the same to make the test fair.
• The only variable that should change is soil nitrate concentration.
• Good examples are factors that affect plant growth, such as light, water, and temperature.

🧠 Exam technique

• Write two different control variables.
• Make sure they are genuine controls, not the variable being changed.
• Simple answers are fine: “water” and “temperature” would both score.

❌ Common errors

• Writing “nitrate concentration” — this is the independent variable, so it does not count.
• Saying just “fertiliser” when the question is about nitrate concentration.
• Giving vague answers like “conditions” without naming a specific variable.

✅ Correct answers

• X-axis: Soil nitrate ion concentration in ppm
• Y-axis: Mean mass of corn per plant in grams or g
• Plot these points correctly:
  • (0, 122)
  • (10, 190)
  • (20, 256)
  • (30, 268)
  • (40, 240)
  • (50, 184)
• Draw a smooth line of best fit through the pattern.

💡 Key knowledge

• Use the independent variable on the x-axis.
• Use the dependent variable on the y-axis.
• A line of best fit should show the overall trend, not join point to point.

🧠 Exam technique

• Choose a scale that uses most of the graph paper.
• Label axes with both the quantity and the unit.
• For AQA, graph marks are often split:
  • 1 mark for suitable axis labels and scale
  • 2 marks for plotting points accurately
  • 1 mark for a suitable line of best fit

❌ Common errors

• Missing units, e.g. writing only “mass”.
• Using an awkward scale that wastes most of the grid.
• Joining the points with straight lines instead of drawing a smooth trend.
• Plotting points in the wrong order or swapping x and y values.

✅ Correct answer

The mean mass of corn per plant increases as nitrate concentration increases up to 30 ppm, where it reaches a maximum of 268 g. Above 30 ppm, the mean mass decreases.

💡 Key knowledge

• Always describe the trend and support it with data.
• The data are not just “higher nitrate = higher yield” forever.
• There is a clear peak at 30 ppm.

🧠 Exam technique

• For 2 marks, aim to mention:
  • the rise and then fall
  • a correct value from the table
• Use phrases like “maximum at 30 ppm” for full credit.

❌ Common errors

• Saying only “it increases” — this misses the decrease.
• Giving no data, or using the wrong values.
• Describing the graph as a “straight line trend” when it clearly is not.

✅ Correct points to include

• Increasing nitrate concentration up to 30 ppm increases yield, so it can increase income / sales / profit.
• Above 30 ppm, yield decreases, so adding more fertiliser would waste money.
• Any extra yield must be balanced against the cost of the fertiliser.
• The amount added should depend on the original nitrate level in the soil.
• Fertiliser can run off into rivers, lakes, seas or oceans and cause pollution.
• Run-off can cause eutrophication.
• There may also be environmental costs from the energy used to make and transport the fertiliser.

💡 Key knowledge

• Economic = cost, profit, income, sales, waste of money.
• Environmental = pollution, run-off, eutrophication, effects on rivers and lakes.
• The best answers use both sides: benefits and drawbacks.

🧠 Exam technique

• For a 4-mark evaluation, give at least four relevant points.
• Link your points to the data:
  • 0 to 30 ppm: yield rises
  • above 30 ppm: yield falls
• Use balanced language such as “however”, “but”, and “therefore”.

❌ Common errors

• Only talking about profit and ignoring the environment.
• Only talking about pollution and ignoring the economic side.
• Saying “more fertiliser is always better” — this is contradicted by the data.
• Not using the result that the maximum is at 30 ppm.

04.1

Any two sensible control variables = 2 marks

04.2

Axes/scale, points, and line of best fit = 4 marks

04.3

Describe increase then decrease, with maximum at 30 ppm = 2 marks

04.4

Economic and environmental evaluation with data reference = 4 marks

✅ Do

• Use the data given.
• Give specific values when possible.
• Use clear scientific vocabulary.
• Balance economic and environmental ideas.

❌ Don’t

• Repeat the same point in different words.
• Ignore the graph shape.
• Say nitrate concentration is a control variable.
• Forget to mention the maximum at 30 ppm.