AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2024: Question 1

✅ Correct answer

algae → limpet → crab

1 mark for the correct organisms in the correct order. The arrows must point the right way to show energy flow.

💡 Key knowledge

• Food chains show who eats whom.
• Arrows mean “is eaten by” or show energy transfer.
• Algae are the producer; limpets eat algae; crabs eat limpets.

🧠 Exam technique

• Put the living things in a line from food source to predator.
• Check the direction of each arrow carefully.

❌ Common errors

• Reversing the arrows.
• Writing the organisms in the wrong order.
• Missing out one organism.

📐 Calculations: step by step

1. The quadrat side length is 50 cm.
2. Area = length × width
   50 cm × 50 cm = 2500 cm²
3. Convert cm² to m²:
   2500 ÷ 10000 = 0.25 m²

Area of quadrat = 0.25 m²

✅ Correct answer

Both stages are needed for full marks:

• 2500 cm²
• 0.25 m²

The mark scheme allows alternative methods, such as converting 50 cm to 0.5 m first, then doing 0.5 × 0.5 = 0.25 m².

💡 Key knowledge

• 1 m = 100 cm
• 1 m² = 10,000 cm²
• Area of a square = side × side

❌ Common errors

• Using 100 instead of 10,000 when converting cm² to m².
• Giving the answer in cm² only.
• Doing 50 ÷ 100 = 0.5 but forgetting to square it.

📐 Calculations: step by step

1. Find 2% of the total sea shore area:
   1800 × 2 ÷ 100 = 36 m²
2. Use the area of one quadrat from 01.2:
   36 ÷ 0.25 = 144
3. So the number of quadrats is 144.

Number of times = 144

✅ Correct answer

Marks were awarded for:

• working out the sample area = 36 m²
• dividing by the area of one quadrat = 0.25 m²

🧠 Exam technique

• Always use your answer from the previous part if the question says so.
• Show both the percentage step and the division step.
• Include units: m² for area and no unit for the number of quadrats.

❌ Common errors

• Finding 2% incorrectly.
• Dividing by 1800 instead of by the quadrat area.
• Forgetting that 2% means 2 ÷ 100.

The mark scheme allows follow-through marks, so a method can still gain credit if part 01.2 was wrong, as long as the rest is consistent.

✅ Correct answer

• The sample location may be biased or chosen.
• Throwing is limited by how far students can throw, so the results may not be representative.

💡 Key knowledge

• A random sample should give each area an equal chance of being chosen.
• If students throw, they may hit areas they prefer or avoid areas with limpets.

🧠 Exam technique

• Use the word biased and explain why that makes the estimate unreliable.
• For 2 marks, give one point about the method and one point about the result.

❌ Common errors

• Only saying “it is not random” without explaining why.
• Talking about safety, which was not allowed here.
• Not linking the method to the sample being unrepresentative.

✅ Correct answer

Any one valid method:

• Use a tape measure along two perpendicular sides of the sea shore and choose coordinates using random numbers.
• Allocate co-ordinates to the sea shore and use a random number generator.
• Divide the area into a grid and choose grid squares at random.

💡 Key knowledge

• Random sampling means every location should have an equal chance of being chosen.
• A random number generator can be a computer, calculator, or numbers drawn from a container.

🧠 Exam technique

• Describe the full method, not just “make it random”.
• Names of tools matter: tape measure, grid, coordinates, random numbers.

❌ Common errors

• Saying “throw the quadrat” — this was specifically not accepted.
• Giving only one vague word like “randomly” with no method.
• Forgetting to explain how the random location is chosen.

✅ Correct answer

Any one hazard, for example:

• water
• tide
• rocks
• quicksand
• slippery or uneven surfaces
• hazardous material such as needles or glass
• sewage
• animals such as crabs or jellyfish

💡 Key knowledge

• Sea shores are naturally risky because of moving water, unstable ground, and sharp objects.
• The mark scheme accepts a named example of a hazard.

🧠 Exam technique

• Give one clear hazard only.
• If you name an animal, make it obvious why it is a hazard, e.g. jellyfish.

❌ Common errors

• Repeating the instruction about throwing quadrats.
• Giving something too vague, such as “danger”.

✅ Correct answer

Any one from:

• fertiliser
• sewage
• toxic chemicals

Other accepted examples include pesticide, herbicide, insecticide, DDT, crude oil, microplastics, dissolved carbon dioxide, heavy metals, or thermal pollution.

💡 Key knowledge

• Pollution can reduce food supply, poison organisms, or damage habitats.
• Any valid named pollutant that could affect limpets gets the mark.

🧠 Exam technique

• Use a named pollutant, not a description like “dirty water”.
• One correct example is enough for 1 mark.

❌ Common errors

• Giving a type of land pollution instead of water pollution.
• Writing something too general, such as “waste”.

💡 What top answers did well

• Gave the exact food chain order.
• Showed area conversion carefully.
• Used the 2% sample correctly to scale up.
• Explained bias clearly for the non-random method.
• Used precise fieldwork language for random sampling and hazards.

🧠 Final exam tips

• Check units in every calculation.
• For ecology questions, think: sample, random, representative, bias.
• If the question says “use your answer”, always carry it forward.
• For 1-mark questions, keep the answer short and exact.