AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), June 2025: Question 4

✅ Model Answer (4-Step Chain)

1. Herbicide is sprayed onto the crop field.
2. Weeds are killed while the genetically modified maize survives (is unaffected).
3. There is less competition for light, water, minerals/ions, or space.
4. The crop can carry out more photosynthesis to produce more glucose for growth (e.g. into proteins, starch, or cellulose).

🧠 Exam Technique: Chain of Cause & Effect

Four-mark "explain" questions need a logical narrative:

Action (spray herbicide) → Immediate outcome (weeds die, GM maize lives) → Biological advantage (less competition for light/water/minerals) → Growth mechanism (more photosynthesis → glucose → yield).

❌ Common Errors to Avoid

• Saying "nutrients": The mark scheme explicitly states: "ignore reference to nutrients". You must specify mineral ions, minerals, nitrates, water, or light.
• Vague competition: Simply writing "it has less competition" without naming the limiting factors scores zero.
• Stopping at photosynthesis: State what photosynthesis makes (glucose) and what it is used for (growth/biomass).

💡 Key Knowledge: Plant Competition

Plants compete for four key resources:

• Light (energy for photosynthesis)
• Space (room for roots and canopy to spread)
• Water (reactant for photosynthesis and cell turgor)
• Mineral ions (e.g. nitrates to make proteins)

✅ Model Answer

• The use of herbicide kills wild plants / weeds.
• This means there is less food (or less variety of food / fewer habitats) for animals or insects, reducing animal numbers and species.

❌ Examiner Warning: Chemical Confusion

Many students confuse herbicides with insecticides/pesticides.

• Herbicides kill plants (weeds), not animals directly.
• Never say: "The herbicide poisons the insects and animals." Examiners will reject this.
• The decrease in animals is an indirect ecological effect via the food chain.

✅ Acceptable Answers (Any Two)

1. Insects (such as bees) pollinate the crops/flowers (allowing fertilization and fruit/seed development).
2. Some insects act as predators of crop pests (eating pests that would otherwise damage crops).

💡 Biological Control & Pollination

In modern sustainable farming:

• Pollinators transfer pollen grains between flowers, which is essential for fruit and seed production.
• Beneficial predators (like ladybirds eating aphids) reduce the need for chemical insecticides.

✅ Acceptable Answers (Any One)

• Provides a source of food (nectar, berries, leaves).
• Provides shelter / nesting sites.
• Provides protection / cover from predators.

🧠 Exam Tip: Be Direct

For 1-mark "Give one reason" questions, keep it simple and precise. Stating simply "Provides shelter" or "Provides a source of food" gets straight to the mark with zero risk.

✅ Model Answer

• Higher oxygen levels increase the rate of aerobic respiration by microorganisms (bacteria / fungi / decomposers) during decay.
• This process releases carbon dioxide (CO₂), which is a greenhouse gas that causes global warming.

❌ Common Error: Repeating the Question

Do NOT write: "...which releases carbon dioxide causing climate change."

The stem of the question already told you it contributes to climate change! The mark scheme explicitly notes: "ignore (which) releases carbon dioxide causing climate change".

You must state that CO₂ is a greenhouse gas or causes global warming / enhanced greenhouse effect.

💡 Why Peat Bogs Matter

Undisturbed peat bogs are waterlogged and acidic, meaning there is very little oxygen. Dead plant matter cannot fully decay and carbon remains trapped for thousands of years (acting as a massive carbon store).

When peat is drained or dug up for compost, oxygen rushes in → decomposers respire aerobically:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O → CO₂ enters the atmosphere.