AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2018: Question 3

✅ Correct answers

Tick:

• They survive in high temperatures
• They survive where it is acidic

💡 Key knowledge

• “Extremophiles” are organisms that live in extreme conditions.
• The question stem says the caves are very hot and acidic, so those are the clues.
• Being dark, humid, or inside rocks does not make an organism an extremophile by itself.

🧠 Exam technique

• Always match your ticks to the environment described in the question.
• Only tick options that are extreme conditions, not just facts about where the organism is found.

❌ Common errors

• Ticking “they survive in the dark” because the caves are dark.
• Ticking “they survive in high humidity” because the cave is humid.
• Thinking “old age” is why they are extremophiles.

✅ Correct answer

C

💡 Key knowledge

• The cave microorganism was reactivated from an environment that is hot and acidic.
• On the graphs, enzyme C has its optimum at about 65 °C and works at low pH around pH 3.
• That makes it the best match for an extremophile living in hot, acidic caves.

🧠 Exam technique

• Use both graphs if needed, but here the key clue is the cave conditions.
• Look for the enzyme with the highest activity at high temperature and low pH.

❌ Common errors

• Choosing A because it has a high peak, but forgetting it is best at a much lower temperature.
• Choosing B because it has a broad peak, but its optimum pH is not acidic.
• Ignoring the cave conditions.

✅ Correct answer

It has high/optimum activity at high temperature or about 65 °C and low pH or about pH 3.

Acceptable alternative: it is the only enzyme active between 55 °C and 75 °C and/or below pH 4.

💡 Key knowledge

• An enzyme works best at its optimum temperature and optimum pH.
• For this microorganism, the right enzyme must suit hot and acidic conditions.
• Phrase your answer using the graph values: 65 °C, pH 3, or the ranges given in the mark scheme.

🧠 Exam technique

• To get the mark, you must state the evidence from the graph.
• Good answers link condition → graph feature → conclusion.
• Example: “C is best because it has its optimum at 65 °C and pH 3, matching the hot acidic cave environment.”

❌ Common errors

• Just writing “because it is an extremophile enzyme” with no graph evidence.
• Giving only the temperature or only the pH.
• Mixing up optimum with minimum.

✅ Correct answer

• The three domains are Archaea, Prokaryota/Bacteria, and Eukaryota.
• They are based on DNA or other chemical evidence.
• Archaea are primitive/simple bacteria.
• Prokaryota/Bacteria are true/modern bacteria.
• Eukaryota include organisms with cells that have a nucleus and include plants, animals, fungi and protists.

💡 Key knowledge

• Carl Woese used molecular evidence, especially DNA, to split life into three domains.
• This is different from older systems that grouped organisms in simpler ways.
• The key idea is that classification is based on relationships shown by DNA, not just appearance.

🧠 Exam technique

• For 3-mark describe questions, give three separate facts.
• Use the names of the domains correctly and add one short description for each.
• If you cannot remember the details, at least naming all three domains can gain some credit.

❌ Common errors

• Writing only kingdoms instead of domains.
• Forgetting that Eukaryota have a nucleus.
• Not mentioning that the system is based on DNA/chemical evidence.

✅ Correct answer

They live in extreme conditions.

Acceptable: most Archaea are extremophiles.

💡 Key knowledge

• Archaea are commonly associated with extreme environments such as very hot, acidic, or salty habitats.
• The cave conditions described are hot and acidic, so this is a good match.

🧠 Exam technique

• Use the wording from the question: extreme conditions.
• Short 1-mark questions usually need one clear point only.

❌ Common errors

• Repeating that they were found in caves without explaining why that matters.
• Saying “they are old” instead of linking to their environment.
• Confusing Archaea with bacteria.

💡 Top-level responses

• Used the graph evidence precisely for Q03.2 and Q03.3.
• Linked hot + acidic cave conditions to extremophiles.
• Named all three domains and included the DNA-based nature of the system.

❌ Where students lost marks

• Ticking environmental facts that were not actually extreme.
• Choosing an enzyme by “looks” instead of by temperature and pH optimum.
• Giving vague classification answers like “there are different groups” without specific domain names.

📐 Quick graph-reading steps

1. Read the question stem: hot, humid, acidic.
2. Identify the enzyme with the highest activity at high temperature.
3. Check which enzyme also works at low pH.
4. Use the exact graph evidence in your explanation.