AQA GCSE Biology Biology Paper 1 (Foundation), June 2022: Question 3

✅ Correct Answers (Choose any two)

• Temperature
• Light intensity (or light wavelength / duration)
• Volume of water (or amount of water)
• Size / age of tomato plants (or number/surface area of leaves)
• Amount / type of fertiliser / mineral ions (e.g. nitrate, magnesium) or compost type
• Time before rate readings are recorded

🧠 Exam Technique

Read the question stem carefully: it already states "one type of tomato plant" and that they are in a "greenhouse". Never give a variable that has already been explicitly stated as fixed in the text!

❌ Common Errors

• Writing just "time" unqualified (you must state time before readings are taken or duration of exposure).
• Writing "type of tomato plant" (already controlled in the question stem).
• Writing "greenhouse" (not a measurable variable).
• Stating "carbon dioxide" (this is the independent variable, not a control variable!).

✅ Correct Answer

[✓] From 0.02% to 0.04%

📐 Step-by-Step Data Check

Calculate the difference (increase in rate) across each interval:

• 0.00% → 0.02%: 5 − 0 = +5
• 0.02% → 0.04%: 16 − 5 = +11 (Largest increase!)
• 0.04% → 0.06%: 19 − 16 = +3
• 0.06% → 0.08%: 20 − 19 = +1

🧠 Exam Technique

Never rely on a quick glance. Always do quick mental or written subtractions in the margin for each interval given in the options to avoid picking the initial or steepest apparent step by mistake.

✅ Correct Answer

[✓] Repeat each reading three times and calculate a mean.

💡 Key Knowledge: Why Repeats Matter

Repeating measurements and calculating a mean:

• Allows you to identify and discard anomalies.
• Reduces the effect of random measurement errors.
• Increases the reliability and validity of your conclusion.

❌ Why the other choices are wrong

• "Use concentrations of carbon dioxide above 0.12%": This only extends the range; it does not check whether current values are accurate.
• "Use different tomato plants for each concentration": This introduces plant-to-plant genetic variation as an uncontrolled variable, which actually reduces validity.

✅ Correct Marking Points

• Mark 1 (Observation/Trend): The rate of photosynthesis increases.
• Mark 2 (Biological Reason): (Because) carbon dioxide is a reactant / needed for photosynthesis (or CO₂ was the limiting factor).

💡 Chemical Context

Recall the word and chemical equations for photosynthesis:

carbon dioxide + water → glucose + oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Between 0.00% and 0.08%, carbon dioxide is the limiting factor. Providing more reactant molecules allows photosynthesis to proceed faster.

🧠 "Explain" Command Word Strategy

An explain question always needs two parts:

1. Say what happens: The rate increases.
2. Say why it happens: Because CO₂ is required as a reactant.

Examiner note: Do not just quote numbers from the table—the mark scheme specifically states "ignore values" here.

✅ Correct Answers (Any two from)

• Rate does not increase: It would not increase/change the rate of photosynthesis (the rate stays at 20 arbitrary units).
• Growth/yield does not increase: It would not increase the yield / growth of tomatoes.
• Economic cost: It would cost more money / extra gas is expensive / profit will not increase (waste of money).
• Also allowed: Avoiding unnecessary carbon emissions / global warming.

💡 Limiting Factors in Commercial Farming

Look at Table 2 above 0.08%:

• At 0.08%: Rate = 20
• At 0.10%: Rate = 20
• At 0.12%: Rate = 20

Above 0.08%, carbon dioxide is no longer the limiting factor. Light intensity or temperature is now limiting the rate. Spending money to pump in more CO₂ gives zero extra tomato crop!

❌ Common Errors

• Vague statements like "it damages the plant" (CO₂ at 0.10% is not toxic to tomato plants).
• Saying "the rate decreases" (look closely at the table—it levels off at 20, it does not drop!).
• Failing to mention the cost/money/profit aspect when the question explicitly asks about a commercial farmer.