AQA GCSE Biology · Topic 19
Food chains, quadrats and transects, the carbon and water cycles, and decay.
172 questions 4 subtopics Spec 4.7
Every subtopic below has its own questions, mark schemes and notes once you start.
Every food chain starts with a producer that builds biomass using energy from light. This sub-point also carries the fieldwork: quadrats, transects and the maths for turning a handful of counts into an estimate for a whole habitat.
There is no new supply of materials on Earth, so everything living things are built from has to be released and used again. Two cycles are examinable, carbon and water, and microorganisms drive both by breaking down what is left behind.
Biology-only content. Decay is enzyme-controlled, so anything that affects the decomposers' enzymes affects the rate: temperature, water and oxygen. Required practical 10 measures that rate using lipase, milk and a colour change.
Biology-only and Higher Tier only. Three environmental changes shift where species can live, temperature, water availability and the composition of atmospheric gases, and the questions usually ask you to weigh up evidence rather than recall a fact.
Key definitions, in exam-ready wording.
The mistakes that come up again and again in this topic.
Draw food chain arrows from the eaten to the eater, they follow the food, not the hunter.
Count consumers, not boxes: in a four-organism chain the last organism is the tertiary consumer, not the fourth consumer of the chain including the plant.
On a predator-prey graph, identify the predator by which curve peaks second, not by which peaks are smaller.
When the quadrat is not 1 m², convert the mean to a number per m² before multiplying by the area, and remember a smaller quadrat means a bigger number per square metre.
A transect is deliberately not random: random sampling is for estimating abundance across a whole area, while a transect is for showing change along a line.
Decay does not release carbon dioxide by itself, it is the respiration of the decomposers that does. Say so.
Plants respire all the time, not only in the dark, so they release carbon dioxide as well as absorbing it.
Carbon enters an animal in its food, not through its lungs; breathing in supplies oxygen.
The water cycle provides fresh water, the word fresh is often the mark.
The nitrogen cycle is not on this specification, so do not spend revision time on it.
Enzymes are denatured, not killed, organisms are what get killed. Using the wrong word loses the mark.
"Warmer is always faster" is only true up to the optimum; past it the rate falls.
Rate = 1000 ÷ time, so a longer time means a slower rate; check your answer is the right way round.
Give the unit s⁻¹ when calculating rate, and watch for questions asking for three significant figures.
Naming a factor is only half of a "describe and explain" answer, say how the factor affects the decomposers or their enzymes.
This sub-point is Higher Tier only, and biology only, it is not on the combined science course.
"Evaluate" means give both sides and then a judgement; answers that only agree, or only criticise, rarely reach full marks.
Not every change is caused by humans, seasonal and geographic causes are named in the specification too.
Say which of the three changes you are using (temperature, water, atmospheric gases) rather than writing vaguely about "the environment".
172 questions on Organisation of an ecosystem, marked against the scheme. Anything you get stuck on will be scheduled to come back sooner.