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Question
A UK pond has: water-fleas (eat algae), small fish (eat water-fleas), heron (eats small fish), pond algae. Draw the food chain with correct arrows. (3 marks)
Solution
Producer first: algae.
Then the primary consumer that eats the producer: water-flea.
Then secondary consumer (small fish), then tertiary consumer / apex (heron).
Answer
Pond algae → water-flea → small fish → heron. Arrows show energy flow from food to feeder (1 for each correct arrow, max 3).
Question
A graph shows tawny owl and field vole numbers over 10 years. Vole peaks appear at years 2, 5 and 8; owl peaks appear at years 3, 6 and 9. Explain this pattern. (4 marks)
Solution
Voles peak first because they have plenty of food and few predators.
Owls respond to abundant voles — they have more food, survive better and breed more — so owl numbers rise (peak about a year later).
More owls eat more voles → vole numbers fall.
With fewer voles, owls have less food → owl numbers fall (after another delay) → voles can recover and the cycle repeats.
Answer
Voles peak first because food is abundant and predators few (1). Plentiful voles let owls breed more, so owl numbers peak about a year later (1). Increased predation reduces vole numbers (1). Fewer voles → less food for owls → owl numbers fall, vole numbers recover, cycle repeats (1).
Question
Describe what happens to a carbon atom from when it is in atmospheric CO₂ to when it is back in the atmosphere again, having passed through a cow. (5 marks)
Solution
CO₂ is absorbed from the air by a grass plant during PHOTOSYNTHESIS.
The carbon is incorporated into glucose, then into starch / proteins / fats / cellulose in the plant.
The cow eats the grass — carbon moves to the cow's body (digestion + assimilation).
The cow RESPIRES the glucose / fats, releasing CO₂ back to the atmosphere.
(Alternatively: cow dies, decomposers break down the body and respire, releasing CO₂.)
Answer
CO₂ is absorbed by grass during photosynthesis (1) and built into glucose / starch / proteins (1). Cow eats and digests the grass (1). Cow respires glucose, releasing CO₂ back to the air (1). Alternatively, cow dies and decomposers respire the body, returning CO₂ (1).
Question
Name the four key processes in the water cycle and describe one in detail. (5 marks)
Solution
Evaporation, transpiration, condensation, precipitation (1 mark each).
Choose one (e.g. evaporation) and describe: Sun's heat converts liquid water in oceans/rivers to water vapour, which rises into the atmosphere.
Answer
The four key processes are evaporation, transpiration, condensation and precipitation (4 × 1 mark). Detail (e.g.): evaporation is when liquid water at the sea surface absorbs heat from the Sun and changes into water vapour, which then rises into the atmosphere (1).
Question
Explain why food in a compost heap decays much faster than food in a fridge. (4 marks)
Solution
Compost is WARM (microbial respiration generates heat, ~40–60 °C) — enzymes work faster.
Compost is MOIST — microbes need water for enzyme action and cell processes.
Compost is TURNED, providing OXYGEN for aerobic respiration of decomposers.
A fridge is cold, slowing enzyme action and microbe growth → much slower decay.
Answer
Compost is warm, moist and oxygenated — perfect for decomposers (1+1+1). A fridge is cold, so enzymes and microbes work slowly (1). All four factors together make compost decay much faster.
Question
Milk pH falls from 6.5 to 5.0 in 10 minutes at 30 °C, and from 6.5 to 5.0 in 5 minutes at 40 °C. Calculate the rate of pH change at each temperature and explain the difference. (4 marks)
Solution
Change in pH = 1.5 units in both cases.
Rate at 30 °C = 1.5 ÷ 10 = 0.15 pH units/min.
Rate at 40 °C = 1.5 ÷ 5 = 0.30 pH units/min — twice as fast.
At 40 °C, enzymes from decomposers have more kinetic energy, so more successful collisions per second → faster fatty-acid release → faster pH fall.
Answer
Rate at 30 °C = 0.15 pH units/min (1). Rate at 40 °C = 0.30 pH units/min (1) — double the rate. The higher temperature increases kinetic energy of enzyme and substrate molecules (1), giving more successful collisions per second and faster fatty-acid production (1).
Question
Records show the speckled wood butterfly has spread from southern England as far as the Scottish borders over the last 50 years. Suggest how climate change has driven this shift. (4 marks)
Solution
Average UK temperatures have risen ~1 °C over this period.
Northern UK now has summers warm enough for speckled wood caterpillars to develop.
Milder winters allow more individuals (or eggs/pupae) to survive in northern areas.
Population expands northward where conditions are newly suitable; food plants (grasses) are widespread, so no food barrier.
Answer
Rising average UK temperatures (1) mean northern areas are now warm enough for caterpillars to develop (1) and for more adults to survive winter (1). With food-plants (grasses) widespread, populations can expand northwards into newly suitable habitat (1).
Question
Atlantic cod numbers in the North Sea have dropped. Suggest TWO ways human activity contributes to this decline. (2 marks)
Solution
Climate change has warmed the North Sea, pushing cold-adapted cod northwards (changing distribution).
Overfishing has removed adult cod faster than populations can replace them.
Answer
Climate change has warmed the sea, shifting cod northwards out of the area (1). Overfishing has removed adults faster than they can be replaced (1).
A green plant or alga that traps light energy by photosynthesis and forms the start of a food chain.
An organism that obtains energy by eating other organisms (primary = eats producer; secondary = eats primary; tertiary = eats secondary).
A predator at the top of its food chain with no natural predators.
A diagram showing how many food chains in a community link together through shared species.
The reaction in chloroplasts that uses light energy to build glucose from CO₂ and water — removes CO₂ from the air.
The reaction in all living cells that breaks down glucose to release energy — releases CO₂ into the air.
Burning a fuel (wood, coal, oil, gas) — releases CO₂ into the air.
The breakdown of dead organisms and waste by decomposers (bacteria and fungi), releasing CO₂ and mineral ions.
Change of liquid water into water vapour, driven by heat from the Sun.
Loss of water vapour from the leaves of plants, mainly through stomata.
Change of water vapour back into liquid droplets — forms clouds.
Water falling to the ground as rain, snow, hail or sleet.
A microorganism (bacterium or fungus) that breaks down dead organic matter and releases nutrients back to the environment.
Breakdown of dead matter by decomposers that respire using oxygen — releases CO₂ and water.
Breakdown of dead matter without oxygen — produces methane and organic acids; slower than aerobic.
A fuel mainly of methane and CO₂ produced by anaerobic decomposition in digesters.
How quickly dead matter is broken down — measured as mass loss or chemical change per unit time.
A change in temperature, water availability or atmospheric gas composition in a habitat.
Where a species lives — the geographic range and the abundance within that range.
Long-term shift in global or regional climate driven mainly by extra atmospheric greenhouse gases from human activity.
The study of the timing of natural events such as flowering, migration and hibernation.
Mistake
Drawing arrows from consumer to food.
Why it happens
Thinking 'eats' means the arrow points where the eater goes.
How to avoid it
Arrows show ENERGY FLOW — energy goes FROM the food INTO the eater. Always 'food → feeder'.
Mistake
Drawing predator and prey curves in phase (peaks at the same time).
Why it happens
Assuming the relationship is immediate.
How to avoid it
There's a TIME LAG — predator numbers respond after a delay because they need to breed up from extra food.
Mistake
Drawing a predator curve TALLER than the prey curve.
Why it happens
Forgetting energy loss.
How to avoid it
Predators are fewer than prey — you need many prey to support a few predators (pyramid of biomass).
Mistake
Saying 'plants only photosynthesise; animals only respire'.
Why it happens
Oversimplified models.
How to avoid it
Plants RESPIRE as well, 24/7. They photosynthesise only when there's light. At night, plants release CO₂ from respiration.
Mistake
Treating combustion and decomposition as the same process.
Why it happens
Both release CO₂.
How to avoid it
Combustion is BURNING (rapid, with flames). Decomposition is microbial respiration on dead matter (slow, no flame). Both add CO₂ but they are different processes.
Mistake
Forgetting transpiration from the water cycle.
Why it happens
It's the only 'biological' step.
How to avoid it
AQA explicitly includes transpiration. Plants pump huge volumes of water from soil to atmosphere — name it.
Mistake
Drawing decomposition rate rising indefinitely with temperature.
Why it happens
Forgetting enzyme denaturation.
How to avoid it
Rate rises to an OPTIMUM (~40 °C) then FALLS sharply as enzymes denature.
Mistake
Saying 'water makes the food softer'.
Why it happens
Surface explanation.
How to avoid it
Microbes NEED water to live (cell processes, enzyme action). It's not about softness.
Mistake
Thinking anaerobic decomposition is faster.
Why it happens
Confusing 'no oxygen' with 'no competition'.
How to avoid it
Anaerobic respiration releases far less energy per glucose, so microbes grow slowly — anaerobic decay is SLOWER but produces methane.
Mistake
Blaming ONLY climate change for a species decline.
Why it happens
Climate change is the headline issue.
How to avoid it
Examiners want you to mention multiple human pressures: climate, habitat loss, pollution, introduced species, disease.
Mistake
Saying species 'just evolve' to keep up with rapid change.
Why it happens
Misunderstanding evolution timescales.
How to avoid it
Natural selection works over MANY generations. Human-caused change is often too fast — species must move or face extinction.
Mistake
Assuming species can always move.
Why it happens
Forgetting geography.
How to avoid it
Mountain-top plants run out of mountain. Islands have no escape. Fragmented habitats block movement. Some species are physically stuck.