AQA GCSE Combined Science: Trilogy Biology Paper 2 (Foundation), 2024: Question 5

Key skills

• Food chains: producer → primary consumer → secondary consumer
• Area calculations using cm and m
• Percentages and scaling up a sample
• Sampling methods and fairness/randomness
• Hazards and water pollution examples

How to score well

• Use the correct arrow direction in food chains
• Show units in calculations
• Use the answer from the previous part if asked
• Give one clear reason for 1-mark questions

✅ Correct answer

algae → limpets → crabs

💡 Key knowledge

• Algae are the producer.
• Limpets eat algae, so they are the primary consumer.
• Crabs eat limpets, so they are the secondary consumer.

🧠 Exam technique

• Always use arrows to show the direction of energy transfer.
• The arrow goes from the food to the animal that eats it.

❌ Common errors

• Reversing the arrows
• Writing crabs → limpets → algae
• Leaving out one organism

📐 Calculations step-by-step

1. Side length = 50 cm
2. Area of a square = side × side
3. 50 × 50 = 2500 cm²
4. Convert cm² to m²: divide by 10,000
5. 2500 ÷ 10,000 = 0.25 m²

Answer: 0.25 m²

✅ Correct answer

0.25 m²

🧠 Exam technique

• Write the units at each stage.
• You can also use 0.5 m × 0.5 m = 0.25 m² .
• Either method earns full marks if the working is clear.

❌ Common errors

• Giving 2500 m² instead of converting units
• Using 50 × 50 = 100 or another incorrect square area
• Forgetting that cm² must be converted using squared units

📐 Calculations step-by-step

1. Find 2% of 1800 m²: 1800 × 2/100 = 36 m²
2. Each quadrat covers 0.25 m² from part (b)
3. Number of quadrats = 36 ÷ 0.25 = 144

Answer: 144 quadrats

✅ Correct answer

144 times

💡 Key knowledge

• 2% means 2/100 or 0.02 .
• Using the answer from part (b) correctly is part of the method.

❌ Common errors

• Using 1800 × 0.2 instead of 0.02
• Forgetting to divide by the quadrat area
• Mixing up area sampled with number of samples

✅ Correct answer

Throwing the quadrat is not random because the students may choose where to throw it, or the throw may be limited by how far they can throw. This means the sample location is biased and not representative.

💡 Key knowledge

• A random sample should give every area an equal chance of being chosen.
• Non-random sampling can overestimate or underestimate the population.

🧠 Exam technique

• Use science words like biased and representative.
• Explain the consequence, not just the method.

❌ Common errors

• Only saying “it is not random” without explaining why
• Talking about safety, which the question says not to do
• Describing the throw but not linking it to bias

✅ Correct answers

• Measure two perpendicular sides of the sea shore with a tape measure and use coordinates
• Divide the area into a grid and use a random number generator

💡 Key knowledge

• Random sampling means each grid square or coordinate has an equal chance.
• Coordinates or grid references help locate the quadrat fairly.

🧠 Exam technique

• Give a full method, not just “use randomness”.
• Examples: random numbers from a generator, numbers drawn from a container.

❌ Common errors

• “Throw the quadrat” — this does not count here
• Only saying “use random numbers” without saying what they are used for
• No clear link to coordinates/grid

✅ Correct answers

• Water
• Tide
• Rocks
• Quicksand
• Sewage
• Animals, e.g. crabs or jellyfish
• Slippery or uneven surfaces

💡 Key knowledge

In GCSE fieldwork, shore habitats can be dangerous because conditions change quickly, surfaces are uneven, and natural hazards may be present.

❌ Common errors

• Giving an answer too vague to be a hazard
• Writing “the quadrat” or “throwing” when the question excludes it
• Not naming a real hazard

✅ Correct answers

• Fertiliser
• Sewage
• Toxic chemicals
• Herbicides / pesticides / insecticides
• Crude oil
• Microplastics
• Heavy metals
• Thermal pollution

💡 Key knowledge

• Pollution can reduce survival or reproduction of shore animals.
• Some pollutants poison organisms directly; others change the water conditions.

🧠 Exam technique

• Name a specific pollutant or pollution type.
• One clear example is enough for the mark.

❌ Common errors

• Saying just “pollution” without a type
• Giving land pollution instead of water pollution
• Writing vague answers like “dirty water”

High-scoring features

• Used the correct biological order in the food chain
• Showed the area calculation clearly and converted units correctly
• Used the previous answer from part (b) to scale up the sample
• Explained why a method was biased and not representative
• Gave a realistic random sampling method using coordinates or a grid

Where marks were lost

• Incorrect arrow direction in the food chain
• Forgetting that 50 cm = 0.5 m
• Using 2% as 0.2 instead of 0.02
• Describing a method without saying why it is non-random
• Giving an example too vague to count as a hazard or pollutant

Remember

Food chain: algae → limpets → crabs

Quadrat area: 0.25 m²

2% of 1800 m²: 36 m²

Number of quadrats: 144

Best words to use

biased, representative, random, coordinates, grid, pollution