AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2024: Question 4

8 marks · High Demand difficulty · Extended Answer

Describe hormonal control of increased basal metabolic rate in rats and explain, using table data, how natural selection changes the percentage of warfarin-resistant rats when warfarin is given and after it stops.

Practise this question

Question

The question page shows Question 04 with two parts about rat populations. Part 04.1 states that when rats compete, their basal metabolic rate increases, and says basal metabolic rate is controlled in the same way in humans and rats; it asks the student to describe how basal metabolic rate is increased for 2 marks, with answer lines below. Part 04.2 explains that one female and one male rat can produce 20 offspring every 2 months, that warfarin is a poison used to control rats, and that rats can become resistant by evolution; it says warfarin was given for 4 months and includes a table headed Table 1 with columns for months since warfarin was first given, whether warfarin was being given, and percentage of the rat population resistant to warfarin: 0 months yes 1%, 2 months yes 24%, 4 months yes 81%, 6 months no 99%, 8 months no 92%, 10 months no 85%, 12 months no 70%. Below the table, text states that resistant rats have a smaller mean mass than non-resistant rats and that smaller rats are often at a disadvantage when competing against larger rats, then asks the student to explain the trends in the data when warfarin was being given and when it was no longer given, using Table 1, for 6 marks.
Question text

04 Rat populations can increase rapidly.

When rat populations are large there is competition between rats.

04.1 When rats compete, the basal metabolic rate of the rats increases.

Basal metabolic rate is controlled in the same way in humans and in rats.

Describe how basal metabolic rate is increased.

[2 marks]

04.2 The size of a rat population can increase quickly.

One female rat and one male rat can produce 20 offspring every 2 months.

Warfarin is a poison that has been used to control rat populations.

Rat populations can become resistant to warfarin by the process of evolution.

A population of rats was given warfarin for 4 months.

Table 1 shows information about resistance to warfarin in the rat population for a year

after warfarin was first given.

Table 1

Percentage (%) of rat

Number of months since Was warfarin being

population resistant to

warfarin was first given given?

warfarin

0 Yes 1

2 Yes 24

4 Yes 81

6 No 99

8 No 92

10 No 85

12 No17 70

Rats with resistance to warfarin have a smaller mean mass than rats that

are not resistant to warfarin.

Smaller rats are often at a disadvantage when competing against larger rats.

Explain the trends in the data when warfarin was being given and when

warfarin was no longer given.

Use Table 1.

[6 marks]

Mark scheme

Show the mark scheme Mark scheme for AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2024: Question 4

Question 4

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 (more) thyroxine secreted / allow (more) thyroxine is 1 AO1

released produced / made / created 4.5.3.6

from thyroid (gland) 1

alternative answer

(more) adrenaline secreted /

released / produced / made /

created (1) from adrenal gland

(1)

AO /

Question Answers Mark

Spec. Ref.

04.2 Level 3: Relevant points (reasons / causes) are identified, given in 5–6 AO3

detail and logically linked to form a clear account.

Level 2: Relevant points (reasons / causes) are identified, and 3–4 AO2

there are attempts at logical linking. The resulting account is not

fully clear.

Level 1: Points are identified and stated simply, but their relevance 1–2 AO2

is not clear and there is no attempt at logical linking.

No relevant content 0

4.6.2.2

Indicative content 4.6.3.1

4.6.3.4

trends

• percentage of population with resistance increased for 6 months

or

percentage of population with resistance increased when

warfarin was given

• then decreased after warfarin stopped being given

• the decrease is slower than the increase

• the change from increase to decrease is not instant (when

warfarin stops being given at 6 months)

• manipulated figures

increase explanations

• some rats have allele(s) / gene(s) for resistance to warfarin

• this is due to (random) mutation(s)

• (therefore) more likely to survive if warfarin is being used

• rats with resistance reproduce

• passing on allele / gene (for resistance) to next generation

• repeats for several / many generations until all / most of

population is resistant

• several generations within 6 months

decrease explanations

• when warfarin is not used, rats with resistance are outcompeted

for food / mates / territory / shelter by non-resistant rats

• rats with resistance are killed by larger non-resistant rats

• (therefore) rats without resistance become more likely to

reproduce

For Level 3, answers must explain increase and decrease in

percentage of population resistant to warfarin.

Total Question 4 8

How to answer it

Rat Resistance and Basal Metabolic Rate

AQA GCSE Combined Science: Trilogy · Biology · Question 4
Total marks: 8
What this question tests You need to recall how the body controls basal metabolic rate, then explain natural selection using the rat warfarin data. For the 6-mark part, the examiner wants a clear, linked explanation of the rise in resistance while warfarin is used and the fall after it is stopped, using the table values.

Part (a) — How basal metabolic rate is increased

✅ Correct answer

More thyroxine is secreted from the thyroid gland.

Accept: more thyroxine is produced / made / released / created.

Alternative accepted answer: more adrenaline is secreted from the adrenal gland.

Marking point 1: hormone named correctly. Marking point 2: correct gland named.

💡 Key knowledge

  • Basal metabolic rate is the rate of energy use by the body at rest.
  • Thyroxine is a hormone that increases metabolic rate.
  • The thyroid gland secretes thyroxine.
  • In this question, “controlled in the same way in humans and rats” means the same hormone system is being tested.

🧠 Exam technique

  • Write the hormone and the gland.
  • One mark is for each correct idea.
  • A full 2-mark answer can be very short if it is precise.

❌ Common errors

  • Saying only “metabolism increases” without explaining how.
  • Naming the wrong gland, such as pancreas.
  • Saying “adrenaline from thyroid” — the gland must match the hormone.
  • Using “basal metabolic rate” as if it is a hormone.

Part (b) — Explain the trends in the warfarin resistance data

Use Table 1 and explain the increase when warfarin was given, then the decrease when warfarin stopped.

✅ Correct answer structure

A top answer would say:

  • When warfarin was being given, the percentage of resistant rats increased from 1% at 0 months to 81% at 4 months.
  • This is because some rats had a resistance allele due to random mutation.
  • Resistant rats were more likely to survive the warfarin.
  • They reproduced and passed on the resistance allele to their offspring.
  • After warfarin stopped being given at 6 months, the percentage resistant fell from 99% to 70% by 12 months.
  • Without warfarin, resistant rats were at a disadvantage because smaller rats were outcompeted by larger non-resistant rats for food, mates, territory or shelter.

For full marks, the explanation must cover both the increase and the decrease.

💡 Key knowledge

  • Natural selection happens when organisms with an advantageous allele survive and reproduce more.
  • Mutation creates variation; resistance is not caused by warfarin itself.
  • Warfarin acts as a selection pressure.
  • When the poison is present, resistant rats survive more often.
  • When the poison is absent, resistance may carry a disadvantage, especially if resistant rats are smaller.
  • Several generations can occur in 6 months because rats reproduce quickly.

🧠 Exam technique

  • Use the table data in your answer: examiners reward this.
  • Link the trend to selection pressure, survival, reproduction, and inheritance.
  • To reach Level 3, your answer must explain both the rise and the fall in resistance.
  • Write in a logical chain: mutation → resistance → survival → reproduction → more resistant offspring .

❌ Common errors

  • Saying warfarin “makes rats resistant” — wrong. It selects for already resistant rats.
  • Not mentioning mutation or inherited alleles.
  • Only describing the numbers without explaining why they changed.
  • Explaining only the increase, not the decrease.
  • Claiming the percentage must keep rising forever.

📐 Data interpretation and model answer points

📐 Key data to quote

  • 0 months: 1% resistant
  • 2 months: 24% resistant
  • 4 months: 81% resistant
  • 6 months: 99% resistant
  • 8 months: 92% resistant
  • 10 months: 85% resistant
  • 12 months: 70% resistant

✅ What a strong explanation sounds like

While warfarin was being given, resistant rats survived better than non-resistant rats. These rats reproduced and passed the resistance allele to their offspring, so the percentage resistant rose quickly over several generations. Once warfarin stopped, the resistant rats lost their advantage. Because resistant rats had a smaller mean mass, they were outcompeted by larger non-resistant rats, so the percentage resistant fell.

❌ Common calculation-style traps in data questions

  • Mixing up percentage of resistant rats with the total rat population.
  • Ignoring the fact that the trend changes after 6 months.
  • Using vague words like “changed a lot” instead of specific figures.
  • Forgetting that the data show a decrease after warfarin stops, not a sudden immediate switch.

🧠 How marks are awarded here

  • Level 1 (1–2 marks): simple statements, little or no linking.
  • Level 2 (3–4 marks): relevant biology with some linkage, but not fully clear.
  • Level 3 (5–6 marks): clear, detailed, logically linked explanation of both the increase and the decrease.

Quick revision checklist

💡 For part (a)

  • Thyroxine
  • Thyroid gland
  • Adrenaline from adrenal gland as an alternative

💡 For part (b)

  • Mutation causes variation
  • Selection pressure from warfarin
  • Survival of resistant rats
  • Reproduction and inheritance
  • Outcompeted when warfarin is removed

🧠 Best exam habit

  • Always link cause → effect → next generation.
  • Use the numbers from the table.
  • Answer both halves of the question.

Topics

Biology · B5: Homeostasis and Response · B6: Inheritance, Variation and Evolution · B7: Ecology

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 2 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.