AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), June 2025: Question 3

✅ Correct Answer

Distance from the tree (or simply distance).

💡 Key Knowledge

• Independent Variable: The factor deliberately changed by the experimenter (here: 1 m vs 10 m).
• Dependent Variable: What is measured (buttercup distribution / % cover).
• Control Variables: Factors kept constant to ensure valid results.

📐 Step-by-Step Calculation

1. Count qualifying squares: Inspect the 10 m quadrat (5 × 5 grid = 25 total squares). Count squares that are at least half (≥ 50%) covered by buttercups.
   Count = 7 squares
2. Calculate fraction of total area:
   (7 ÷ 25) × 100 or each square is worth 100% ÷ 25 = 4% , so 7 × 4% .
3. Calculate final percentage:
   Percentage cover = 28%

❌ Common Errors & Traps

• Counting squares with tiny patches: The rule states at least half covered. Counting squares with only tiny tips (e.g. counting 9 or 10) loses mark 1, though error carried forward (ECF) is allowed for math working if in range 6–10.
• Dividing by 100 instead of 25: The grid is 5 × 5 (25 squares total), not 100 squares!

✅ Any TWO from the Mark Scheme

• Whether the sample is representative (large enough sample size).
• The size of the quadrat used.
• The number of students (or number of quadrats available).
• How many quadrats physically fit around the tree at 1 m (circumference / area limit).
• Obstacles / environmental factors such as buildings, paths, or other trees.

🧠 Exam Technique: Read the Restrictions!

The question explicitly says: "Do not refer to time available."

Any mention of "not having enough time" or "how long the lesson lasts" scores 0 marks. Focus on biological and physical constraints (representativeness, spatial limits around the tree, or equipment).

📐 Calculations Required

Advantage 1: Quadrat B covers a larger total area

• Area of Quadrat A = 0.25 m × 0.25 m = 0.0625 m² (or 625 cm²)
• Area of Quadrat B = 0.5 m × 0.5 m = 0.25 m² (or 2500 cm²)
• Quadrat B samples 4 times larger area!

Advantage 2: Quadrat B gives higher precision / resolution

• Quadrat A: 5 × 5 = 25 squares → each square = 4%
• Quadrat B: 10 × 10 = 100 squares → each square = 1%
• Quadrat B estimates cover to the nearest 1% instead of 4%.

✅ Mark Breakdown (4 Marks)

• Mark 1: Quadrat B samples a larger area (or is more representative).
• Mark 2: Calculation showing 0.25 m² vs 0.0625 m² (or 4 times larger).
• Mark 3: Quadrat B gives more accurate / precise results (or higher resolution).
• Mark 4: Calculation showing percentage resolution: to the nearest 1% rather than nearest 4% (or 100 squares vs 25 squares).

✅ Two Correct Statements (Tick Boxes)

• ☑️ With a point quadrat, a plant that is smaller than half a square is more likely to be counted.
• ☑️ With a point quadrat, no judgement of cover is needed.

💡 Why Are These True?

• No subjective judgement: A pin either physically touches a leaf or it doesn't (objective binary data). With square quadrats, students must estimate whether a plant covers ≥ 50% of a square.
• Detecting small plants: Tiny leaves that never fill half a square are ignored in standard square quadrats, but can easily touch a needle-thin pin.